Review Articles

One Lung Anesthesia

Roohina N. Baloch*, Erum Zeb, Summaiya Hafeez, Madiha Zaheer

Department of Anesthesiology, Pain management and Surgical Intensive Care Unit, Jinnah Postgraduate Medical Centre (JPMC), Rafique Shaheed Road, Karachi 75510 (Pakistan)

Correspondence: Prof. Roohina N. Baloch*, 39/1, 16th Street, Off Khayaban-e-Mujahid, Defence Phase-V, Defence Housing Authority, Karachi (Pakistan);Cell: +923008234442; E-mail: nrbaloch@gmail.com

ABSTRACT

Thoracic anesthesia with one lung ventilation is challenging. The anesthetist is faced with demands of establishing proper isolation of one lung from the other in order to facilitate good surgical exposure and prevent intraoperative complications. Due to advances in one lung ventilation (OLV) strategies and equipment, complex intrathoracic procedures are being performed with success. During one-lung ventilation, mismatch of perfusion leads to an increase in shunt and dead space. Hypoxemia is an inevitable adverse consequence of OLV. Prompt management is required. This may particularly occur with high airway pressure caused by malpositioned double lumen tube or endobronchial blocker causing incomplete lung ventilation and/or airway obstruction. Other causes may be bronchospasm, air trapping with dynamic hyperinflation pneumothorax of the ventilated lung and coughing due to inadequate muscle relaxation. One lung induced acute lung injury (ALI) must be recognized.

Acute lung injury (ALI) is a major cause of overall mortality after thoracic surgery. Protective ventilation strategies have been identified and recommended by researchers for implementation during OLV. This includes small tidal volumes based on ideal body weight, reducing the fraction of inspired oxygen (FiO2), use of positive end-expiratory pressure (PEEP) to the ventilated lung, and low peak and plateau airway pressures.

One-lung ventilation has to be managed from the start before beginning OLV till the end of OLV (in order to prevent complications like ALI). Extreme care is required during re-expansion of the lung towards the end of OLV. Noninvasive ventilation may be used during this period to improve oxygenation.

Keywords:     One-lung ventilation; Hypoxemia, Anesthesia; Thoracic surgery; Lung
protective ventilation; Endobroncheal devices

Citation: Baloch R N, Zeb E, Hafeez S, Zaheer M. One lung anesthesia. Anaesth Pain & Intensive Care. 2016;20 Suppl 1:S126-S135 

Received: 20 August 2016; Reviewed: 12 September 2016; Accepted: 23 September 2016

INTRODUCTION

Thoracic surgery is greatly facilitated with isolation of one lung from the other or by causing selective atelectasis of the lung requiring surgery (One-lung ventilation / anesthesia).1

Due to advances in one-lung ventilation complex intrathoracic procedures could be performed with success. This is particularly true for video-assisted thoracoscopic surgeries (VATS).2

In the last decade, the management of OLV has been influenced by two major changes – first, the recognition of one lung ventilation induced acute lung injury (ALI), which is a major cause of post lung-resection death, and video assisted thoracoscopic surgeries as it is critical to have good surgical exposure for the success of surgery.2

Hypoxemia has become less frequent due to advancement like better lung isolation and ventilation strategies and newer anesthetic agents.

In a review article on ventilator management of one-lung ventilation, Rocca et al identified that despite of the fact more comorbidities existed for intermediate to major thoracic surgical procedures, the operative mortality remained unchanged over a period of time.3

The mortality causes have shifted from cardiac and surgical towards infections and acute lung injury (ALI) syndrome. The incidence of post thoractomy ALI has not decreased over the last twenty years. It still remains between 2-4%.3,4

HISTORY

In 1931, Gale and Waters described an anesthetic technique for selective one-lung ventilation. They aimed to open the thorax and surgically manipulate the lungs. They used a single light tube and inserted into the right or left main stem bronchus.5

Following this, various lung separation techniques have been proposed to make it safer.6

Only the simplest intrathroacic operations were safe or feasible till 1930s. The methods for securing the airway, isolating the lungs and selectively ventilating either or both lungs have contributed to present situation.3

MANAGEMENT OF OLV

Indications to Contraindications

Thoracic anesthesia includes a variety of diagnostic and therapeutic procedures involving the lungs, airways and other intrathoracic structures. Two techniques are fundamental to anesthetic management for the majority of thoracic procedures: (a) lung isolation to facilitate surgical access within the thorax, and (b) management of one – lung anesthesia.7,8,9

Techniques on lung isolation are basically designed to facilitate one-lung ventilation (OLV) in patients undergoing cardiac, thoracic, mediastinal, vascular, oesophageal, or orthopedic procedures involving the chest cavity.10

There is indication of lung isolation from each other when collapse of one lung confers a critical benefit to the surgical performance e.g., pneumonectomy, upper lobectomy, repair of thoracic aortic aneurysms, examination of pleural space. Lung isolation is also used to protect the lung from soiling by the contralateral lung in such cases as bronchopleural fistula, pulmonary haemorrhage and whole-lung lavage. Lung isolation also provides differential patterns of ventilation in unilateral reperfusion injury cases e.g., after lung transplantation, pulmonary thromboendarterectomy or unilateral lung trauma.1 No true contraindication to lung isolation has been identified excepting inadequate lung function to support OLV.

Predictors of Hypoxemia during OLV – include right-sided surgery, low PaO2 during two lung ventilation preferential perfusion to the operative lung, supine position, vasodilator use, excessive volatile anesthesia (>>1MAC), sepsis and lung function abnormalities.11

EXECUTION OF OLV

Lung isolation can be achieved by three methods:7,8,12 (1) Placing a right or left endobronchial double lumen tube (DLT),13 (2) Placing a single lumen endotracheal tube in conjunction with a bronchial blocker (Arndt, Cohen, balloon-tipped luminal catheters) or utilizing a commercially available endotracheal tube with a built-in blocker (Univent),9,14,15 and (3) Placing a single lumen tube in the right or left main bronchus. DLT and bronchial blockers have been shown to be clinically equivalent in the provision of OLV.16,17 Lung isolation can also be done through a tracheotomy tube and is the most commonly done with a bronchial blocker8.

Double Lumen Tube (DLT): Physiology Principles

Double lumen tubes are designed to isolate ventilation. They have the added benefit over other techniques of facilitating deflation and suctioning, and administering continuous positive airway pressure (CPAP) to the deflated lung. The DLT consists of (a) a tracheal and bronchial lumens, (b) cuffs on both lumens, and (c) right or left sidedness. Because of the anatomical differences in the bifurcation of tracheobronchial tree, right and left DLTs are slightly different in their endobronchial lumen design. Right DLTs have a side opening (Murphy’s eye) close to the tip and surrounded by the cuff so that it can be positioned at the right upper lobe origin to assure its ventilation; the cuff of a left DLT may inadvertently block the right main bronchus.7,18

Proper placement of DLT can be technically challenging due to its larger diameter, requirement of positioning into a specific mainstream bronchus and complications like rupture of DLT cuff to potential life threatening disruption of tracheobronchial tree.19 In the event of a suspected (or unsuspected) difficult airway, it is better to place a single lumen tube and isolate the lung with a bronchial blocker. But in case DLT is necessary, a single lumen tube can be placed and a tube exchanger can be used to switch to a DLT.

The Robert-shaw is the most commonly used DLT. However, disposable plastic DLTs have replaced older red rubber Carlens, White and Robert-Shaw tubes. Sizes available range from 32F to 41F from various manufacturers and refer o the outer diameter.20 Disagreement exists on the method of selection for the most appropriate size of DLT.

Some studies suggest choosing the largest tube that fits in order to decrease the chance of migration and obstruction, which can result in hypoxemia, lung isolation failure, increased airway resistance, difficulty of passing a fiberoptic scope or tracheal obstruction20,21,22. Other propose inserting a smaller than conventional size to decrease trauma with insertion. One study demonstrated that choosing a smaller size did not influence the incidence of hypoxemia, need for repositioning or success of lung isolation.23 Other studies suggest that measurement of the tracheal size from available CT or MRI imaging films can be used as a guide to choose DLT size24. Tracheal width can be measured from patient’s chest X-ray and the size of left bronchus may then be estimated20. Some recommended use of complex mathematical formulas for selecting the appropriate size of DLT.25,26 In general, most male adults take a size 37F or 39F and most female adults take a size 35F or 37F7,21,27,28. Lung collapse and OLV for left sided thoracic surgery can be achieved by either a left or right DLT.19,29 If a left pneumonectomy is planned, the left DLT can be withdrawn back to the trachea before the bronchus is stapled. Right sided double-lumen tube placement is more challenging because of the early branching of the right upper lobe and proper positioning of the opening for ventilation (Murphy’s eye) of the endobronchial lumen at its origin.30 But Campos et al concluded after comparison that Rt-DLT presented no increased risk of complications for Lt-sided thoracic surgery.

Insertion

The tip of the tube is inserted just through the vocal cords and then immediately rotated 90° in the direction of the bronchus that one is aiming to intubate.12

Confirming Position

  • One technique of determining proper positioning DLT is auscultation of the lungs and manual ventilation while the tracheal and bronchial lumens are selectively clamped and unclamped. Breath sounds should be absent distally when clamping the desired lumen (tracheal, bronchial).31,32
  • Fiberoptic bronchoscopy is considered the Gold Standard for confirming proper positioning of DLT.23,33,34,35,36
  • Purohit et al identified in their review that the positioning of the lung isolation device could also be confirmed by ultrasonography of the chest with the intercostals approach.37

Campos et al (2008) concluded in a prospective randomized trial that the experience of Anesthesiologist and his knowledge of endoscopic bronchial anatomy played a major role in successful placement of lung isolation devices.38 Ng et al also concluded that the anesthetist must acquire additional training on the placement of lung isolation devices.35

Dunlop (1939) concluded in his review that, “Many agents and divergent methods are in common use for major thoracic operations and various centers. It is impossible to say what is best. The one thing that impresses is that, as yet, the experience of the anesthetist, his knowledge of the pathology presents in each case and of the difficulties which may arise and his preparedness to cope with them are the most important factors”.39

PHYSIOLOGY OF ONE LUNG VENTILATION

The functional residual capacity decreases as a consequence of loss of muscle tone in the chest wall and diaphragm after the induction of anesthesia. During two lung ventilation, the dependent lung compliance is reduced that shifts most of the tidal volume towards upper lung. Due to gravity, the pulmonary perfusion goes to the dependent lung.

After the lung is isolated, ventilation gets restricted to the dependent lung and any residual perfusion to the nondependent lung becomes true shunt flow.1,40

Blood flow is decreased to the non-ventilated lung due to hypoxic pulmonary vasoconstriction (HPV) but it is still being perfused leading to a decrease in arterial oxygen tension (PaO2). The PaO2 may get further decreased by anesthetic agents during OLV due to inhibition of HPV in the non-ventilated lung.1

Hypoxemia during OLV is basically due to obligatory shunt through the collapsed lung. The major determinant of degree of venous admixture is this distribution of perfusion. As the right lung receives a larger proportion of cardiac output (55% vs 45% to the left). Gravity in lateral decubitus position, retraction of the lung, hypoxic pulmonary vasoconstriction and surgical stimulation are the factors helping to reduce perfusion to the nondependent lung.1,11,41-46

Hypoxemia (i.e., arterial oxygenation <90%) caused by OLV can be understood by considerating oxygen storage, dissociation of oxygen from haemoglobin ventilation/perfusion relationship and factors that reduce the effect of hypoxic pulmonary vasoconstriction. There is reduction in arterial oxygen partial pressure during OLV along with permissive hypercarbia and respiratory acidosis. These changes cause rapid dissociation of oxygen from hemoglobin (Bohr effect).47

Hypoxic pulmonary vasoconstriction is a potent physiological process that helps to reduce blood flow to poorly ventilation / perfusion mismatch. Decrease in the alveolar partial pressure of oxygen (PaO2) results in rapid vasoconstriction with an initial plateau at fifteen minutes following lung isolation. After about four hours of lung isolation, maximal vasoconstriction is reached and this reduces shunt flow by 40%1,44,48,49.

Inhibition of hypoxic pulmonary vasoconstriction was seen with older inhalational anesthetics like halothane and nitrous oxide in a dose dependent fashion, but not with the newer agents like desflurane and sevoflurane1,6,45,50-54.

Hypoxic pulmonary vasoconstriction is also influenced by systemic and pulmonary vasodilators, acid / base imbalance1,46,49.

COMPLICATIONS

Hypoxemia – affected from 9% to 27% of patients undergoing one-lung ventilation. It is influenced by several factors6. In earlier days of thoracic anesthesia hypoxemia was seen in about 40% of cases with Carlen’s DLT and manual ventilation55.

Over the years this complication has reduced significantly due to better tube designs, improvement in lung isolation and with newer volatile anesthetics having lesser effects on hypoxic pulmonary vasoconstriction. Although hypoxemia as low as 1% have been reported21 patients and is a serious clinical concern11.

Failure in proper lung isolation is a major cause of hypoxemia. Hence there is strong recommendation of confirmation, placement and/or displacement during positioning with fiberoptic bronchoscopy33-36.

Acute Lung Injury (ALI) – is diagnosed as an acute reduction in PaO2/FiO2 ratio to <300 and bilateral pulmonary infiltrates on chest radiography in the absence of left heart failure2. ALI is a sequalae of different pathogenic triggers and shows a biphasic distribution: (a) primary form triggered within three days by perioperative factors, (b) delayed form triggered by postoperative complication (3-10 days post-operative).56

Risk factors for primary ALI in thoracic surgery are: (a) peak inspiratory pressure >40 cmH2O, (b) plateau pressure >29 cmH2O, (c) long duration of OLV, (d) excessive fluid administration, (e) pneumonectomy, (f) alcohol abuse, and severe pulmonary dysfunction44,57. The delayed form is due to a sequence of deleterious events leading to alveolar and capillary injury.

Diffuse alveolar damage is due to barotraumas (direct high pressure on lungs), repetivitive opening and closing of alveoli (atelectotrauma), lung over-distension (volutrauma), cytokines and local inflammatory mediators (biotrauma). Due to its high (25-40%) mortality rate, ALI is a major cause of overall thoracic surgical mortality60.

MONITORING

In procedures carrying significant risk of haemorrhage, haemodynamic instability, fluid shifts and complicated surgeries an arterial line access (apart from standard monitoring) will not only enable haemodynamic monitoring, but also arterial blood gas analysis to assess oxygenation and adequacy of ventilation. In OLV, there is increased end tidal and arterial pressure carbon dioxide (ETCO2-PaCO2) gradient. Continuous spirometry is important to appreciate: (a) changes in lung compliance because of recruitment / derecruitment, (b) to confirm any air leaks and air trapping.

Trans Oesophageal echocardiography TEE may be helpful in perioperative period to assess any right heart dysfunction. This can get aggravated because of increase in pulmonary vascular resistance due to initiation of OLV and clamping of pulmonary artery vessels.2

Carassiti et al studied the pressures exerted by different endobronchial devices and concluded that patients undergoing one lung ventilation may be frail. Hence monitoring of pressure exerted by the tracheal and bronchial cuffs on the endobronchial device used is extremely important. This is beneficial to guide the anesthetist to do optimal inflation of the cuff to minimize bronchial mucosal damage.61

ANESTHESIA REGIMEN

In an update published in 2013 of a Cochrane Review (earlier published in the Cochrane Library issue 2, 2008) to evaluate the effectiveness and safety of intravenous versus inhalation anesthesia for one-lung ventilation. Modolo et al identified that they included randomized controlled trials of intravenous e.g., propofol versus inhalation (e.g., isoflurane, sevoflurane, desflurane) anesthesia for one lung ventilation “No evidence indicated that the drug used to maintain anesthesia during one lung ventilation affected patient outcomes”.62

A recent multicenter randomized controlled trial by Beck-Schimmer B et al studied the effect of volatile versus intravenous anesthetic on major complications after lung surgery. They concluded that “No difference between two anesthesia regimens was evident”.63 Lohser et al compared inhaled versus intravenous anesthetics and concluded that patients had less adverse events with sevoflurane.64

Modolo et al in their review published in Cochrane Database Systematic Reviews, 2013 concluded that, “If researchers believe that the type of drug used to maintain anesthesia during one-lung ventilation is important, they should design randomized controlled trials with appropriate participant outcomes rather than report temporary fluctuation in physiological variables. Researchers should include outcomes that are important to participants when assessing the effects of anesthetic technique during one lung ventilation. These include adverse postoperative effects, death and intraoperative awareness”.62

At the time of Induction of Anesthesia to Start of OLV

Protective lung ventilation guidelines are recommended for two lung ventilation without lung injury65. In order to prevent derecruitment during protective ventilation, application of positive end-expiratory of pressure (PEEP) with low (4-6 ml/kg) tidal volume (Vt) based on ideal body weight (IBW) is recommended. Derecruitment occurs after induction particularly with high FiO2 and bronchoscopy during the confirmation of placement of lung separation device.59 Oxygenation and ventilation is improved during two lung ventilation with alveolar recruitment maneuvers (ARM).66,67,58,59

This alveolar recruitment if done before OLV, leads to improvement in compliance of lung. This is also minimizes tidal recruitment during one lung ventilation, which represents atelectotrauma (repetitive alveolar opening (closure) and is a causative factor of acute lung injury (ALI). The simplest form of ARM comprises of vital capacity maneuver with a breath hold at 30 cmH2O for 10-40 secs. On modern anesthetic ventilators, these formal cycling ARMs are incorporated. Peak inspiratory pressure to be increased stepwise and PEEP held for 5-10 breathes per step up to 40/20 cmH2O.68

Establishing and Maintaining OLV

In a cohort study, Licker et al compared patients undergoing lung surgeries from year 1998-2003 with those during 2003-3008. They identified that there was a decreased incidence of acute lung injury where protective lung ventilation was applied as a routine.57 Protective lung ventilation (PLV) in their cohort consisted of low tidal volume (mean 5.3 ml/kg of IBW compared with 7.1 ml/kg of IBW) during OLV, PEEP, pressure controlled ventilation and recruitment maneuvers frequently.

Licker et al performed a secondary analysis of an observational cohort in 2009 and concluded that implementation of protective lung ventilation protocol intraoperatively on patients undergoing lung cancer surgery showed better respiratory outcomes postoperatively. There was significantly reduced ALI and atelectasis incidence, and reduced ICU requirements60,69.

Yung et al compared 102 patients in a randomized trial, having elective lobectomy to protective ventilation or conventional ventilation (F1O2 1.0 Vt 10 ml/kg, volume control ventilation and PEEP. They found that there was significantly lower pulmonary dysfunction in lung protective group as compared to conventional (4% vs 22%)70.

Positive End-Expiratory Pressure (PEEP)

Studies support the role of PEEP in protective ventilation for lung injury prevention, although optimal PEEP settings are controversial67,69,71-73..

Ferrando et al in their study showed improved oxygenation and lung mechanics with setting of external PEEP so as to achieve optimal dynamic lung compliance. They used PEEP decrement trial after an ARM74. Patients would most likely benefit from PEEP if their pulmonary function is normal.

Auto PEEP

Auto PEEP develops when tidal volume cannot be evacuated during the allocated expiratory time. Factors that pre-dispose auto-PEEP generation (single or in combination) are:
(a) respiratory mechanics of patient (e.g., COPD), (b) ventilator settings (short expiratory time), (c) resistance of artificial airways including ventilator tubings, heat and moisture exchanges and DLT.75

Double lumen tubes (DLT used for OLV during thoracic anesthesia increases positive end-expiratory pressure (auto-PEEP) to a moderate extent.75,76

Inspired Oxygen Fraction (FiO2)

Ischemia reperfusion injury as a result of surgical manipulation and lung collapse initiates tissue hypoperfusion. This leads to oxygen toxicity during OLV. Reperfusion leads to lung damage due to re-expansion. This is due to formation of reactive oxygen species (ROS).

Respiratory failure, cardiac arrhythmias, and pulmonary hypertension are seen in one lung ventilations exceeding 120 minutes. This is due to oxidate stress.77 An FiO2 of 0.4 provides adequate oxygenation in the lateral decubitus position.78

Recruitment

Atelectasis must be avoided in the non operative lungs as the shunt fraction from lung isolation can get worsened. Oxygenation is improved with the application of alveolar recruitment at the beginning of or just before OLV. Improved oxygenation or lung compliance following an ARM show that an adequate level of PEEP was being used. An optimal level of ARM has not been established2.

Peak / Plateau Pressure

Protective ventilation lowers peak and plateau pressures, thus decreasing the associated lung injury. Peak pressures >40 cmH2O and plateau pressures >29 cmH2O are associated with ALI.60

Respiratory Rate/Permissive Hypercapnia

The respiratory rate (RR) should be slightly high at the beginning of OLV to avoid hypercapnia with protective Vt. Permissive hypercapnia is a part of lung protection strategy. This allows decreased ventilatory pressure and mechanical stress.2

Pressure Controlled vs Volume Controlled Ventilation

Pressure control ventilation (PCV) is associated with lower ventilatory pressures. This gives minimal differences in the intrabronchial pressure. PCV has been preferred over volume control ventilation (VCV) for OLV. Some studies have failed to identify benefit of PCV.58,79,80

PREVENTION OF HYPOXEMIA DURING OLV11

  • Improving Perioperative Lung Function – to decrease postoperative pulmonary complications and to improve oxygenation.
  • Monitoring Lung Separation – fiberoptic monitoring of DLTs required both after intubation and after patient positioning.
  • Good Ventilation Strategy in the Dependent Lung keeping in mind the following:
  1. Impediment of the expansion of the dependent lung due to mediastinal weight, pressure of the abdominal organs and cephalad displacement of the diaphragm and by the pressure, and noncompliance of the thoracic wall. This leads to atelectasis and alveolar collapse of the dependent lung leading to HPV.
  2. Avoid atelectasis development and keep dependent lung open.
  3. Poor ventilation strategy may lead to ALI.
  • Oxygen Administration to the Nondependent Lung – used (with or without CPAP) to treat hypoxemia during OLV and also for prevention.
  • Modulation of Perfusiong. with Nitric oxide.
  • Type of Aneshesia – all volatile anesthetics inhibit HPV in a dose dependent fashion. Type of anesthesia (inhalational vs total intravenous, epidural vs no epidural) by itself does not affect oxygenation during OLV.
  • Haemoglobin Levels – factors leading to decreased oxygenation of venous blood are increased oxygen extraction and low haemoglobin levels.

One Lung Ventilation to Extubation

Re-expansion of the Operative Lung

Lung recruitment to be done at the end of OLV for:

  1. a) Restoration of normal lung to re-establish pleural interface.
  2. b) Minimize post-operative pneumothorax.
  3. c) Optimize pulmonary function post-operatively.
  4. d) Restoration of V/Q mismatching in order to improve oxygenation.

Selective lung re-expansion with low FiO2 should be used when recruiting lung after prolonged collapse.

Lung re-expansion should be gradual to a lower plateau pressure in order to avoid ALI.

Lung re-expansion can worsen lung injury (ischemic re-perfusion injury) due to reactive oxygen species and oxidative stress81.

Two Lung Ventilation: Post OLV to Emergence

Lung function is impaired postoperatively due to General Anesthesia, lung edema, residual paralysis, manipulation, inflammation and residual atelectasis. At this particular time patients are at risk of lung injury. Patient should not be switched to unsupported spontaneous ventilation at this point, keeping in mind the high airflow resistance of DLT and changes of derecruitment. Hypoventilation and high FiO2 will lead to atelectasis. Pressure support ventilation with PEEP may maintain optimal lung volumes at the time of emergence.2

Post Extubation Period

Continuous positive airway pressure (CPAP) to be applied after emergence particularly, in high risk patients. Studies have identified that non-invasive ventilation has been shown the decrease mortality and need for re-intubation in cases of acute respiratory failure following lung resection2.

RECOMMENDATIONS ON MANAGEMENT OF OLV2

  1. Induction of Anesthesia to Start of OLV: (a) pre-oxygenate with 100% Oxygen and ventilate with FiO2 of 1.0 prior to OLV, (b) alveolar recruitment maneuvers after intubation and initial bronchoscopy, (c) gently ramp up pressure to a plateau of 30 cmH2O. Plateau to be maintained for 10 seconds or more, (d) low Vt (6-8 ml/kg of IBW) during TLV, (e) PEEP 3-10 cmH2O.
  2. Establishing and Maintaining One-Lung Ventilation:
  3. i) Tidal volume – Vt should be 4-6 ml/kg of IBW.
  4. ii) Positive End-Expiratory Pressure (PEEP) – set PEEP at 3-10 cmH2O during OLV. Titrate to the highest lung compliance, or consider auto PEEP.

iii)        Inspired Oxygen Fraction (FiO2) – titrate FiO2 to target SpO2 of 92-96%.

  1. iv) Recruitment: (a) do initial ARM following lung isolation, (b) gently ramp up pressure to a plateau of 30 cmH2 To be maintained for more than 10 seconds, (c) initiate ARM therapeutically when required. Follow with PEEP adjustment.
  2. v) Peak/Plateau Pressure: (a) peak and plateau inspiratory pressures to be minimized during OLV (peak pressure <30 cmH2O and plateau pressure <20 cmH2O).
  3. vi) Respiratory Rate/Permissive Hypercapnia: (a) respiratory rate be 12-16 breaths per minute, (b) PaCO2 40-60 mmHg, (c) routine 1:E ratio is 1:2, (d) for restrictive lung disease 1:E:1.1-2:1, (e) for obstructive lung disease 1:E – 1:14-1:6 in order to avoid intrinsic PEEP.

vii)       Pressure Control vs Volume Control: use either VCV or PCV.

viii)      Maintenance of anesthesia: maintain anesthesia with Sevoflurane or Desflurane.

  1. ix) Management of Hypoxemia: (a) increase FiO2, (b) confirm lung isolation, (c) ensure adequate cardiac output, (d) recruitment maneuvers to the ventilated lung, (e) if no improvement, apply continuous positive pressure (CPAP) to the operative lung or use intermittent TLV.82-85
  2. From OLV to Extubation: re-expansion of the operative lung: (a) perform unilateral
    re-expansion, (b) minimize recruitment pressure (30 cmH2O without resection and 20 cmH2O with lung resection), (c) recruitment pressure to be developed slowly and maintain until full re-expansion has occurred (30-60 secs), (d) FiO2 to be minimized.
  3. Two Lung Ventilation: Post-OLV to Emergence: (a) maintain protective lung ventilation post OLV (4-6 ml/kg of IBW with PEEP (in lung resection); 6-8 ml/kg of IBW with PEEP (without lung resection); FiO2 to be titrated to target SpO2 of 92-96%, (b) pressure support ventilation from chest closure until extubation.
  4. Post Extubation Period: (a) in high risk patients post extubation oxygenation can be improved with CPAP, and (b) Noninvasive ventilation improves outcome in respiratory failure after lung resection.

CONCLUSION

Recent advances in thoracic anesthesia techniques and equipment have improved the outcomes. Recommendations on protective ventilation strategies have shown improved safety in the intra and post operative period. However, some disagreements still exist on the method of selection for the most appropriate size of DLT. No specific guidelines exist amongst the researchers. The patient outcomes have not been studied on the drugs used for maintaining anesthesia during one-lung ventilation (OLV).

Controversies still exist on the use of PCV vs VCV for OLV. Hence, larger multicentered randomized controlled trials should be designed to address the above concerns.

Conflict of interest: Nil

Authors’ contribution: RNB—Concept, Manuscript writing; EZ – Manuscript editing; SH & MZ – Literature search

REFERENCES

  1. Benumof JL. One-Lung Ventilation and Hypoxic Pulmonary Vasoconstriction: Implications for Anesthetic Management.  Anesth Analg. 1985;64:821-33. [PubMed] [Free full text]
  1. Brassard CL, Lohser J, Donati F, Bussieres JS.  Step by step clinical management of one-lung ventilation: continuing professional development. Can J Anesth. 2014;61(12):1103-1121. [PubMed] [Free full text] doi: 10.1007/s12630-014-0246-2
  1. Della Rocca G, Coccia C. Acute lung injury in thoracic surgery. Curr Opin Anesthesiol. 2013;26(1):40-6. [PubMed] [Free full text] doi: 10.1097/ACO.0b013e32835c4ea2.
  1. Della Rocca G, Coccia C. Ventilatory management of one-lung ventilation. Minerva anestesiol. 2011;77(5):534-6. [PubMed] [Free full text]
  1. Gale JW, Waters RM. Closed Endobronchial Anesthesia in Thoracic Surgery: Preliminary Report. Anesth Analg. 1932;11(6):283-8. [Free full text]
  1. Ferreira HC, Zin WA, Rocco PRM. Physiopathology and clinical management of one-lung ventilation. Jornal Brasileiro de Pneumologia. 2004;30(6):566-73. [PubMed] [Free full text] doi.: 10.1590/S1806-37132004000600012
  1. Weiskopf RB, Campos JH. Current techniques for perioperative lung isolation in adults. Anesthesiology. 2002;97(5):1295-301. [PubMed] [Free full text]
  1. Campos JH. An update on bronchial blockers during lung separation techniques in adults. Anesth Analg. 2003;97(5):1266-74. [PubMed] [Free full text] doi: 10.1213/01.ANE.0000085301.87286.59
  1. Campos JH, Kernstine KH. A comparison of a left-sided Broncho-Cath® with the torque control blocker Univent and the wire-guided blocker. Anesth Analg. 2003;96(1):283-9. [PubMed] [Free full text] doi: 10.1213/00000539-200301000-00056
  1. Campos JH. Progress in lung separation. Thorac Surg Clin. 2005;15:71. [PubMed]
  1. Karzai W, Schwarzkopf K. Hypoxemia during One-lung Ventilation Prediction, Prevention, and Treatment. Anesthesiology. 2009;110(6):1402-11. [PubMed] [Free full text] doi: 10.1097/ALN.0b013e31819fb15d.
  1. Koshy T and Nair SG. Positioning of double-lumen endobronchial tubes: Correlation between clinical and bronchoscopic findings. Indian J Anesh. 2003;47(2):116-119. [Free full text]
  1. Bjork VO, and Carlens E. The prevention of spread during pulmonary resection by the use of a double lumen catheter. J Thorac Surg. 1950;20:151-7. [PubMed]
  1. Ransom ES, Carter SL, Mund GD. Univent tube: a useful device in patients with difficult airways. J Cardiothorac Vasc Anesth. 1995;9(6):725-7. [PubMed] [Free full text] doi:10.1016/S1053-0770(05)80238-1
  1. Takenaka I, Aoyama K, Kadoya T. Use of the Univent bronchial-blocker tube for unanticipated difficult endotracheal intubation. Anesthesiology. 2000;93(2):590-1. [PubMed]
  1. Campos JH. Which device should be considered the best for lung isolation: double-lumen endotracheal tube versus bronchial blockers. Curr Opin Anesthesiol. 2007;20(1):27-31. [PubMed] doi: 10.1097/ACO.0b013e3280111e2a
  1. Narayanaswamy M, McRae K, Slinger P, Dugass G, Kanellakos GW, Rosoe A, et al. Choosing a lung isolation device for thoracic surgery: A randomized trial of three bronchial blockers versus double-lumen tubes. Anesh Analg. 2009;108:1097-101. [PubMed] [Free full text] doi: 10.1213/ane.0b013e3181999339.
  1. Campos JH, Massa FC, Kernstine KH. The incidence of right upper-lobe collapse when comparing a right-sided double-lumen tube versus a modified left double-lumen tube for left-sided thoracic surgery. Anesth Analg. 2000;90(3):535-40. [PubMed] [Free full text] doi: 10.1097/00000539-200003000-00007
  1. Benumof JL, Partridge BL, Salvatierra C, Keating J. Margin of safety in positioning modern double-lumen endotracheal tubes. Anesthesiology. 1987;67(5):729-38. [PubMed]
  1. Brodsky JB, Fitzmaurice B. Modern anesthetic techniques for thoracic operations. World J Surg. 2001;25(2):162-6. [PubMed] [Free full text] doi: 10.1007/s002680020014
  1. Brodsky JB, Lemmens HJM. Left double-lumen tubes: clinical experience with 1,170 patients. J Cardiothorac Vasc Anesth. 2003;17:289-98. [PubMed] [Free full text] doi: 10.1016/S1053-0770(03)00046-6
  1. Fitzmaurice BG, Brodsky JB. Airway rupture from double-lumen tubes. J Cardiothorac Vasc Anesth. 1999;13(3):322-9. [PubMed] [Free full text] doi:10.1016/S1053-0770(99)90273-2
  1. Amar D, Desiderio DP, Heerdt PM, Kolker AC, Zhang H, Thaler HT. Practice patients in choice of double-lumen tube size for thoracic surgery. Anesth Analg. 2008;106(2):379-383. [PubMed] [Free full text] doi: 10.1213/ane.0b013e3181602e41
  1. Brodsky JB, Macario A, Mark JB. Tracheal diameter predicts double-lumen tube size: a method for selecting left double-lumen tubes. Anesth Analg. 1996;82(4):861-4. [PubMed] [Free full text]
  1. Hannallah MS, Benumof JL, and Ruttimann UE. The relationship between left mainstem bronchial diameter and patient size. J Cardiothorac Vasc Anesth. 1995;9:119-21. [PubMed] [Free full text] doi:10.1016/S1053-0770(05)80180-6
  1. Brodsky JB, Lemmens HJ. Tracheal width and left double-lumen tube size: a formula to estimate left-bronchial width. J Clin Anesth. 2005;17(4):267-70. [PubMed] [Free full text] doi: 10.1016/j.jclinane.2004.07.008
  1. Slinger P. Lung isolation in thoracic anesthesia, state of the art. Can J Anesth. 2001;48(1):R13-R5. [PubMed] [Free full text] doi: 10.1007/BF03028172
  1. Wilson WC, Benymoff JL. Anesthesia for Thoracic Surgery. In Miller RD (ed). Anesthesia. Philadelphia, Churchill Livingstone. 2005 p 1877.
  1. Lewis JW, Serwin JP, Gabriel FS, Bastanfar M, Jacobsen G. The utility of a double-lumen tube for one-lung ventilation in a variety of noncardiac thoracic surgical procedures. J Cardiothorac Vasc Anesth. 1992;6(6):705-10. [PubMed]
  1. Campos JH, Massa FC, Kernstine KH. The incidence of right upper-lobe collapse when comparing a right-sided double-lumen tube versus a modified left double-lumen tube for left-sided thoracic surgery. Anesth Analg. 2000;90(3):535-40. [PubMed] [Free full text] doi: 10.1097/00000539-200003000-00007
  1. Alliaume B, Coddens J, Deloof T. Reliability of auscultation in positioning of double-lumen endobronchial tubes. Can J Anesth. 1992;39(7):687-690. [PubMed] [Free full text] doi: 10.1007/BF03008231
  1. Alliaume B, Coddens J, Deloof T. Reliability of auscultation in positioning of double-lumen endobronchial tubes. Can J Anesth. 1992;39(7):687-690. [PubMed] [Free full text] doi: 10.1007/BF03008231
  1. de Bellis M, Accardo R, Di Maio M, Lamanna C, Rossi GB, Pace MC, et al. Is flexible bronchoscopy necessary to confirm the position of double-lumen tubes before thoracic surgery? Eur J Cardiothorac Surg. 2011;40(4):912-8. [PubMed] [Free full text] doi: 10.1016/j.ejcts.2011.01.070
  1. Ng A, and Swanevelder J. Hypoxaemia associated with one-lung anesthesia: new discoveries in ventilation and perfusion. Br J Anesth. 2011;106(6):761-3. [PubMed] [Free full text] doi: 10.1093/bja/aer113
  1. Klein U, Karzai W, Bloos F, Wohlfarth M, Gottschall R, Fritz H, et al. Role of Fiberoptic Bronchoscopy in Conjunction with the Use of Double-lumen Tubes for Thoracic Anesthesia A Prospective Study. Anesthesiology. 1998;88(2):346-50. [PubMed] [Free full text]
  1. Benumof JL. The position of a double-lumen tube should be routinely determined by fiberoptic bronchoscopy. J Cardiothorac Vasc Anesth. 1993;7(5):513-4. [PubMed]
  1. Purohit A, Bhargava S, Mangal V, Parashar VK. Lung isolation, one-lung ventilation and hypoxaemia during lung isolation. Indian J Anesth. 2015;59(9):606. [PubMed] [Free full text] doi: 10.4103/0019-5049.165855

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4613408/

  1. Campos JH, Hallam EA, Van Natta T, Kernstine KH. Devices for Lung Isolation Used by Anesthesiologists with Limited Thoracic ExperienceComparison of Double-lumen Endotracheal Tube, Univent® Torque Control Blocker, and Arndt Wire-guided Endobronchial Blocker®. Anesthesiology. 2006;104(2):261-6. [PubMed] [Free full text]
  1. Dunlop JG. Anesthetic Practices in Thoracic Surgery. Anesth Analg. 1939;18(6):301-11. [Free full text]
  1. Brown DR, Kafer ER, Roberson VO,Wilcox BR, Murray GF. Improved Oxygenation during thoracotomy with selective PEEP to the dependent lung. Anesth Analg. 1977;56(1):26-31. [PubMed] [Free full text]
  1. Schwarzkopf K, Schreiber T, Preussler N-P, Gaser E, Hüter L, Bauer R, et al. Lung perfusion, shunt fraction, and oxygenation during one-lung ventilation in pigs: the effects of desflurane, isoflurane, and propofol. J Cardiothorac Vasc Anesth. 2003;17(1):73-5. [PubMed] [Free full text] doi: 10.1053/jcan.2003.13
  1. Watanabe S, Noguchi E, Yamada S, Hamada N, Kano T. Sequential changes of arterial oxygen tension in the supine position during one-lung ventilation. Anesth Analg. 2000;90(1):28-34. [PubMed] [Free full text] doi: 10.1097/00000539-200001000-00007
  1. Ishikawa S, Nakazawa K, Makita K. Progressive changes in arterial oxygenation during one‐lung anesthesia are related to the response to compression of the non‐dependent lung. Br J Anesth. 2003;90(1):21-6. [PubMed] [Free full text] doi: 10.1093/bja/aeg017
  1. Lohser J. Evidence-based management of one-lung ventilation. Anesthesiol Clin. 2008;26(2):241-72. [PubMed] doi: 10.1016/j.anclin.2008.01.011
  1. Lohser J, Ishikawa S. Physiology of the lateral decubitus position, open chest and one-lung ventilation. Principles and Practice of Anesthesia for Thoracic Surgery: Springer; 2011. p. 71-82.
  1. Hakim T, Lisbona R, Dean G. Gravity-independent inequality in pulmonary blood flow in humans. J Appl Physiol. 1987;63(3):1114-21. [PubMed] [Free full text]
  1. Ng A, Swanevelder J. Hypoxaemia during one-lung anesthesia. Continuing Education in Anesthesia Critical Care & Pain: Br J Anesth. 2010;10(4):117-122. [PubMed] [Free full text] doi: 10.1093/bja/aer113
  1. Benumof JL. Intermittent hypoxia increases lobar hypoxic pulmonary vasoconstriction. Anesthesiology. 1983;58:399-404. [PubMed]
  1. Benumof JL, Wahrenbrock EA. Blunted hypoxic pulmonary vasoconstriction by increased lung vascular pressures. J Appl Physiol. 1975;38(5):846-50. [PubMed].
  1. Fradj K, Samain E, Delefosse D, Farah E, Marty J. Placebo-controlled study of inhaled nitric oxide to treat hypoxaemia during one-lung ventilation.  Br J Anesth.  1999;82:208-12. [PubMed] [Free full text] doi: 10.1093/bja/82.2.208
  1. Slinger P, Scott W. Arterial Oxygenation during One-lung Ventilation. A Comparison of Enflurane and Isoflurane. Anesthesiology. 1995;82(4):940-6. [PubMed] [Free full text]
  1. Groh J. Effects of isoflurane on regional pulmonary blood flow during one-lung ventilation. Br. J Anesth. 1995;74:209-16. [PubMed] [Free full text]
    doi: 10.1093/bja/74.2.209
  1. Marshall C, Lindgren L, Marshall BE. Effects of halothane, enflurane and isoflurane on hypoxic pulmonary vasoconstriction in rat lungs in vitro.   1993;79:1348-53. [PubMed]
  1. Bjernaes LJ. Hypoxia induced pulmonary vasoconstriction in man: inhibition due to diethyl ether and halothane anesthesia. Acta Anesthesiol Scand. 1978;22:578.[PubMed] [Free full text]
  1. Tarhan S, Lundborg RO.  Blood gas  and  pH  studies  during  use  of  Carlens  Can Anesth Soc J. 1968;15:458-67. [PubMed] [Free full text] doi: 10.1007/BF03003730
  1. Della Rocca G, Coccia C. Acute lung injury in thoracic surgery. Curr Opin Anesthesiol. 2013;26(1):40-6. [PubMed] [Free full text] doi: 10.1097/ACO.0b013e32835c4ea2.
  1. Licker M, de Perrot M, Spiliopoulos A, Robert J, Diaper J, Chevalley C, et al. Risk factors for acute lung injury after thoracic surgery for lung cancer. Anesth Analg. 2003;97:1558-65. [PubMed] [Free full text] doi: 10.1213/01.ANE.0000087799.85495.8A
  1. Lohser J, Ishikawa S. Clinical management of one-lung ventilation. In Slinger P (Ed). Principle sand Practice of Anesthesia for Thoracic Surgery: Springer Science and Business Media: 2011;83-101. [Free full text]
  1. Moloney ED, Griffiths MJ. Protective ventilation of patients with acute respiratory distress syndrome. Br J Anesth. 2004;92:261-70. [PubMed] [Free full text] doi: 10.1093/bja/aeh031
  1. Licker M, Diaper J, Villiger Y, Spiliopoulos A, Licker V, Robert J, et al. Impact of intraoperative lung-protective interventions in patients undergoing lung cancer surgery. Crit Care. 2009;13(2):1-10. [PubMed] [Free full text] doi: 10.1186/cc7762
  1. Carassiti M, Mattei A, Pizzo CM, Vallone N, Saccomandi P, Schena E. Bronchial blockers under pressure: in vitro model and ex vivo model. Br J Anesth. 2016;117(S1):192-196. [PubMed] [Free full text] doi: 10.1093/bja/aew120
  1. Modolo NSP, Modolo MP, Marton MA, Volpato E, Monteiro AV, do Nascimento JP et al. Intravenous versus inhalation anesthesia for one-lung ventilation. Cochrane Database Syst Rev. 2013;11(7). [PubMed] [Free full text] doi: 10.1002/14651858.CD006313.pub3
  1. Beck-Schimmer B, Bonvine JM, Braun J, Seeberger M, Neff TA, Risch TJ, et al. Which Anesthesia Regimen is best to reduce morbidity and mortality in lung surgery? A multicenter randomized controlled trial. Anesthesiology. 2016;125(2):313-21. [PubMed] doi: 10.1097/ALN.0000000000001164.
  1. Lohser J. Managing hypoxemia during minimally invasive thoracic surgery. Anesthesiol Clin. 2012;30:683-97. [PubMed] doi: 10.1016/j.anclin.2012.08.006
  1. Schultz MJ, Haitsma JJ, Slutsky AS, Gajic O. What tidal volumes should be used in patients without acute lung injury? Anesthsiology. 2007;106:1226-31. [PubMed] [Free full text] doi: 10.1097/01.anes.0000267607.25011.e8
  1. Jung JD, Kim SH, Yu BS, Kim HJ. Effects of a preemptive alveolar recruitment strategy on arterial oxygenation during one-lung ventilation with different tidal volumes in patients with normal pulmonary function test. Korean J Anesthesiol. 2014 67(2):96-102. [PubMed] [Free full text] doi: 10.4097/kjae.2014.67.2.96
  1. Choi YS, Bae MK, Kim SH, Park JE, Kim SY, Oh YJ. Effects of Alveolar Recruitment and positive End-Expiratory Pressure on Oxygenation during One-Lung Ventilation in the supine position. Yonsei Med J. 2015 56(5):1421-7. [PubMed] [Free full text] doi:  3349/ymj.2015.56.5.1421
  1. Tusman G, Bohm SH, Sipman FS, Maisch S. Lung recruitment improved the efficiency of ventilation and gas exchange during one-lung ventilation anesthesia. Anesth Analg. 2004;98:1604-9. [PubMed] [Free full text] doi: 10.1213/01.ANE.0000068484.67655.1A
  1. Slinger P, Jr Hogue CW. Pro: Tidal Volume is indicated during one-lung ventilation. Anesth Analg. 2006;103(2):268-270. [PubMed] [Free full text] doi: 10.1213/01.ane.0000223701.24874.c8
  1. Yung M, Ahn HJ, Kim K, Kim JA, Yi CA, Kim Mj, et al. Does a protective ventilation strategy reduce the risk of pulmonary complications after lung cancer surgery?: a randomized controlled trial. Chest. 2011;139:530-7. [PubMed] doi: 10.1378/chest.09-2293.
  1. Hoftman N, Canales C, Leduc M, Mahajan A. Positive and expiratory pressure during one-lung ventilation: selecting ideal patients and ventilator settings with the aim of improving arterial oxygenation. Ann Card Anesth. 2011 14(3):183-7. [PubMed] [Free full text] doi: 10.4103/0971-9784.83991
  1. Kim SH, Jung KT, An TH. Effects of tidal volume and PEEP on arterial blood gases and pulmonary mechanics during one-lung ventilation. J Anesth. 2012 26(4):568-73. [PubMed] [Free full text] doi: 10.1007/s00540-012-1348-z
  1. Eldawlatly A, Turkistani A, Shelley B, El-Tahan M, Macfie A, Kinsella J. Anesthesia for thoracic surgery: A survey of middle eastern practice. Saudi J Anesth. 2016;6(3):192-196. [PubMed] [Free full text] doi: 4103/1658-354X.101196
  1. Ferrando C, Mugarra A, Guitierrez A, Carbonell JA, García M, Soro M, et al. Setting individualized positive end-expiratory pressure level with a positive end-expiratory pressure decrement trial after a recruitment maneuver improves oxygenation and lung mechanics during one-lung ventilation. Anesth Analg 2014;118:657-65. [PubMed] [Free full text] doi: 10.1213/ANE.0000000000000105
  1. Bardoszky G, d’Hollandere A, Yernault JC, Van Meuylem A, Moures JM, Rocmans P. On-line expiratory flow-volume curves during thoracic surgery: occurrence of auto-PEEP. Br J Anesth. 1994;72:25-28. [PubMed] [Free full text] doi: 10.1093/bja/72.1.25

http://bja.oxfordjournals.org/content/72/1/25.short

  1. Spaeth J, Ott M, Karzai W, Grimm A, Wirth S, Schumann S, Loop T. Double-lumen tubes and auto-PEEP during one-lung ventilation. Br J Anesth. 2016;116(1):122-30. [PubMed] [Free full text] doi: 10.1093/bja/aev398
  1. Misthos P, Katsaragakis S, Theodorau D, Milingos N, Skottis I. The degree of oxidative stress is associated with major adverse effects after lung resection: a prospective study. Eur J Cardiothorac Surg. 2006;29:591-5. [PubMed] [Free full text] doi: 10.1016/j.ejcts.2005.12.027
  1. Bardoczky GI, Szegedi LL, d’Hollander AA, Moures JM, de Francquen P, Yernault JC. Two lung and one lung ventilation in patients with chronic obstructive pulmonary disease: the effects of position and F(IO)2. Anesth Analg. 2000;90:35-41. [PubMed] [Free full text] doi: 10.1097/00000539-200001000-00008
  1. Unzueta MC, Casas JI, Moral MV. Pressure controlled versus volume controlled ventilation during one lung ventilation for thoracic surgery. Anesth Analg. 2007;104(5):1029-1033. [PubMed] [Free full text] doi: 10.1213/01.ane.0000260313.63893.2f
  1. Tuğrul M, Camci E, Karadeniz H, Sentürk M, Pembeci K, Akpir K. Comparison of volume controlled with pressure controlled ventilation during one-lung anesthesia. Br J Anesth. 1997;79(3):306-10. [PubMed] [Free full text] doi: 10.1093/bja/79.3.306
  1. Bruin G. Lung re-inflation after one-lung ventilation for thoracic surgery: importance of clamping the dependent lung. Can J Anesth. 2014;61(11):1061. [PubMed] [Free full text] doi: 10.1007/s12630-014-0222-x
  1. Yadav R, Chaturvedi A, Rath GP, Goyal K. Application of indigenous continuous positive airway pressure during one lung ventilation for thoracic surgery. Saudi J Anesth. 2011;5(4):438-439. [PubMed] [Free full text] doi: 10.4103/1658-354X.87279
  1. Yasuuji M, Kusunoki S, Hamada H, Kawamoto M. Intermittent reinflation is safe to maintain oxygenation without alteration of extravascular lung water during one-lung ventilation. J Clin Anesth. 2014;26(3):177-83. [PubMed] [Free full text] doi: 10.1016/j.jclinane.2013.10.006
  1. Kim YD, Ko S, Kim D, Lim H, Lee JH, Kim MH. The effects of incremental continuous positive airway pressure on arterial oxygenation and pulmonary shunt during one-lung ventilation. Korean J Anesthesiol. 2012;62(3):256-259. [PubMed] [Free full text] doi: 10.4097/kjae.2012.62.3.256
  1. Badner NH, Goure C, Bennett KE, Nicolaou G. Role of continuous positive airway pressure to the non-ventilated lung during one-lung ventilation with low tidal volumes. HSR Proc Intensive Care Cardiovasc Anesth. 2011;3(3):189-194. [PubMed] [Free full text]