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Tests of pulmonary function before thoracic surgery
Thoracic
Tests of pulmonary function before thoracic surgery
Respiratory mechanics Spirometry Spirometry involves the measurement of the forced expiratory volume in 1 second (FEV1) and the forced vital capacity (FVC), which is the largest volume of gas that can be forcibly exhaled from the total lung capacity (TLC). The patient breathes into a mouthpiece while wearing nose clips and performs the FVC, generating a volume–time curve from which the FEV1 is derived. Percentage predicted values should be reported to avoid bias against patients who are older, of smaller stature or women, all of whom may tolerate lower levels of lung function. Assessment of bronchodilator response in order to determine the degree of airflow-limitation reversibility is normally carried out as part of the test. An increase in FEV1 and/or FVC of more than 12% of the control or 200 millilitres constitutes a positive response. In normal individuals, the FEV1 and FVC are greater than 80% of the predicted values, and the FEV1/FVC ratio is greater than 70% of the predicted value. Table 1 shows the characteristic changes in flow rates and lung volumes associated with obstructive and restrictive pulmonary diseases.
Ving Yuen See Tho Jonathan Mackay
Abstract Pulmonary function tests form part of the comprehensive preoperative assessment of patients undergoing thoracic surgery. They aim not only to assess the severity and nature of the underlying lung pathology, but also to determine whether a patient will be able to tolerate a pulmonary resection. Tests which assess respiratory mechanics evaluate the mech anical delivery of oxygen to the alveoli and these include spirometry, measurement of lung volumes and flow–volume analysis. The forced ex piratory volume in 1 second (FEV1) and the predicted postoperative FEV1 (ppoFEV1) in particular are useful predictors of postoperative respiratory complications. Parenchymal function refers to the ability of the lung to exchange oxygen and carbon dioxide between the pulmonary blood and the alveoli, and this is assessed by determining the diffusing capacity of carbon monoxide and arterial blood gas analysis. Cardiopulmonary interaction, which is important in ensuring adequate cellular respira tion in skeletal muscle, is assessed using exercise tests that include the formal cardiopulmonary exercise test and other surrogates such as the 6-minute walk test, shuttle walk test and stair-climbing. Other tests including ventilation perfusion scintigraphy and split-lung function tests are also briefly mentioned.
Lung volumes Spirometry can also be used to measure lung volumes, excluding the residual volume (RV) and any capacity which includes RV in its definition. The functional residual capacity (FRC) is meas ured using helium dilution, nitrogen washout, body plethysmography or imaging techniques. Measurement of lung volumes may be more sensitive than the FEV1/FVC ratio in differentiating between obstructive and restrictive disorders. In restrictive lung disease, TLC, FRC and RV are all decreased. In obstructive disease, the FVC may be reduced owing to airway closure at high lung volume, giving rise to a normal FEV1/FVC ratio. The RV will, however, be abnormally high and this will confirm an obstructive defect.
Keywords exercise test; preoperative assessment; pulmonary diffusing capacity; respiratory function tests; spirometry; thoracic surgery
Flow–volume analysis Flow–volume loops are performed using spirometry, and are useful not only to detect obstructive and restrictive ventilatory defects but also to identify any fixed or variable intra- and extrathoracic airway obstruction by a tumour mass which would have an impact on the anaesthetic management of these patients (Figure 1).
Pulmonary function tests play an essential role in the preoperative assessment of patients presenting for thoracic surgery. There is no one single test of respiratory function which can accurately evaluate a patient’s fitness for surgery and predict outcome, and a combination of tests is often required. The ‘three-legged stool’ of pre-thoracotomy respiratory assessment comprises tests which target three areas of lung function, namely respiratory mechanics, parenchymal function and cardiopulmonary interaction.
Changes in flow rates and lung volumes associated with obstructive and restrictive pulmonary diseases
Ving Yuen See Tho is an Associate Consultant Anaesthetist at the Singapore General Hospital. She qualified from the Royal College of Surgeons, Ireland, Dublin, and trained in cardiothoracic anaesthesia at Papworth Hospital, Cambridge, UK. Her current interest is in perioperative transoesophageal echocardiography. Conflicts of interests: none declared.
Obstructive disease
Restrictive disease
FEV1 FVC FEV1/FVC TLC RV
Decreased Decreased or normal Decreased Normal or increased Increased
Normal or increased Decreased Normal or increased Decreased Decreased
FEV1, forced expiratory volume in 1 second; FVC, forced vital capacity; TLC, total lung capacity; RV, residual volume.
Jonathan Mackay, MRCP, FRCA, is a Consultant Anaesthetist at Papworth Hospital, Cambridge, UK. His special interests are cardiothoracic anaesthesia and resuscitation. Conflicts of interest: none declared.
ANAESTHESIA AND INTENSIVE CARE MEDICINE 9:12
Measurement
Table 1
523
© 2008 Elsevier Ltd. All rights reserved.
Thoracic
Flow–volume loops Expiration
b
c
Flow rate Q
a
Volume V
V
V
Inspiration
e
f
Flow rate Q
d
V
Volume V
V
a Normal lung. b Obstructive defect. Flow rate is low in relation to lung volume and a concave appearance of the expiratory limb is seen following the point of maximal flow. c Restrictive defect. Maximal flow rate and lung volume are reduced. d Fixed large airway obstruction, e.g. secondary to a tumour or foreign body in a bronchus. Inspiratory and expiratory flow rates are reduced. e Variable intrathoracic large airway obstruction. f Variable extrathoracic large airway obstruction
Figure 1
The single-breath carbon monoxide diffusing capacity (DLCO) is the most commonly used technique, and involves the patient taking a single vital capacity inspiration from residual volume of a mixture of 0.3% carbon monoxide and 10% helium, followed by a 10-second breath-hold and exhalation. Carbon monoxide is used because its affinity for Hb is about 400 times greater that that for oxygen, thus permitting carbon monoxide to move rapidly across the alveolar membrane without any ‘back pressure’. The rate of transfer of carbon monoxide is said to be diffusion limited. The calculation of DLCO is based on Fick’s law of diffusion, and the derived formula is as follows:
Spirometric assessment is very dependent on patient effort and cooperation, and this often results in an underestimation of FEV1 and FVC. Despite this, they remain essential modalities in the assessment of patients before lung resection. In particular, the predicted postoperative FEV1 (ppoFEV1) is a significant independent predictor of post-thoracotomy respiratory complications, and represents the most valid test to date.
Lung parenchymal function Diffusing capacity for carbon monoxide The diffusing capacity of the lung refers to the overall ability of the lung to transfer gas between the alveoli and the pulmon ary capillary blood. It is not only affected by the gas-diffusion properties of the alveolar–capillary membrane, but also by factors which affect pulmonary capillary blood volume and reaction rates of the gas with haemoglobin (Hb).
ANAESTHESIA AND INTENSIVE CARE MEDICINE 9:12
DLCO = Vco/Palvco where Vco is the rate of carbon monoxide transfer across the alveolar membrane, and Palvco is the partial pressure of carbon monoxide in the alveoli. 524
© 2008 Elsevier Ltd. All rights reserved.
Thoracic
Interpretation of DLCO involves comparing results with reference values generated from studies on healthy populations. Various physiological factors also affect DLCO, including age, sex, height, exercise, body position and altitude, and these factors are taken into account when interpreting results. A healthy 45-year-old male who is 175 cm tall should have a DLCO of 24 ml/min/mm Hg at sea-level, and this increases by two- to threefold with exercise. Current studies show that DLCO and its corresponding predicted postoperative value are well-recognized predictors of major morbidity and mortality following lung resection. DLCO is indicated if the perioperative risk is not clear based on spirometry results.
The interaction among the pulmonary, cardiovascular and skeletal muscle systems during exercise Muscle activity
CO2 production QCO
2
Arterial blood gas analysis A baseline arterial blood gas (ABG) analysis forms part of the preoperative respiratory assessment, particularly in patients with significant respiratory disease. Evidence of hypoxaemia and/or hypercapnoea should prompt further evaluation of the patient’s respiratory function. An arterial blood sample is collected in a heparinized syringe and passed through a blood gas analyser. The arterial pH, partial pressure of oxygen (Pao2) and carbon dioxide (Paco2) are directly measured using a glass pH electrode, polarographic (Clark) oxygen electrode and Severinghaus carbon dioxide electrode respectively. The actual and standard bicarbonate and base excess are calculated from the pH and Paco2 using the Siggard-Anderson nomogram.
O2 flow
Expired VCO
2
Creatine PO4
Muscle QO
Heart blood
Lungs
Pyruvate–Lactate
VO
2
2
O2 consumption
CO2 flow
Inspired
Mitochondrion VA, ideal alveolar ventilation/time; VD, physiologic dead space ventilation/time; VE, total ventilation during expiration/time; QO2, O2 consumption; QCO2, CO2 production; VO2, O2 uptake; VCO2, CO2 output; creatine PO4, creatine phosphate. Courtesy of Wasserman
Figure 2
between two cones set 10 metres apart in time to a set of auditory bleeps and the test is stopped when the patient is too breathless to keep up with the bleeps. The best of two results is taken, and anything less than 25 shuttles or 250 metres is associated with a Vo2max of <15 ml/kg/min. Exercise oximetry to detect desaturation is often simultaneously carried out during these tests. Stair-climbing lacks proper standardization, but is frequently performed because of its simplicity and ease. The patient walks up stairs at his or her own pace without stopping, and the number of flights he or she is able to ascend is recorded. There is no exact definition of a ‘flight’, but it is often taken as 20 steps at 15 centimetres per step. The ability to climb five flights indicates an FEV1 of >2 litres and a Vo2max of >20 ml/kg/min. Climbing three flights indicates an FEV1 of >1.7 litres and is associated with an average risk. A patient is considered very high risk if he or she is unable to climb one flight as this is associated with a Vo2max of < 10 ml/kg/min.
Cardiopulmonary interaction Exercise tests The aim of exercise testing is to assess the interaction among the pulmonary, cardiovascular and skeletal muscle systems during a period of exertion. In order to meet the demands of cellular respiration in exercising muscle, there must be adequate pulmonary ventilation and blood flow for oxygen and carbon dioxide transport, as illustrated by Wasserman (Figure 2). The formal cardiopulmonary exercise test (CPET) is a computerized breath-by-breath analysis of respiratory gas exchange at rest and during a period of exercise. Despite the need for complex equipment and trained personnel, it remains the ‘gold standard’ for assessment of cardiopulmonary function. A variety of parameters are recorded, and the measured maximal oxygen consumption (Vo2max) has been found to be a significant predictor of post-thoracotomy outcome. The calculation of Vo2max is based on the Fick equation as follows:
Other tests Ventilation perfusion (V/Q) scintigraphy measures what percentage the portion of lung to be resected contributes to overall ventilation and perfusion. If the diseased region is minimally functioning, it is reasonable to predict that there will be little impact on postoperative lung function. This test is particularly useful in patients undergoing a pneumonectomy, and should also be considered in any patient who has a ppoFEV1 of <40%. Split-lung function tests aim to simulate respiratory conditions after lung resection. Unilateral exclusion of a lung or lobe with a double-lumen tube or bronchial blocker is not done today because it is poorly tolerated by the awake patient and is difficult to perform. Unilateral occlusion of a pulmonary artery using a balloon catheter aims to estimate the effects of pulmonary ligation on pulmonary artery pressures. Owing to a lack of sufficient
VO2max = (SVmax × HRmax) × (CaO2max − CvO2max) where SV is the stroke volume, HR is the heart rate, Cao2 is the arterial oxygen content, and Cvo2 is the mixed venous oxygen content. Any factor which affects any one or more of the four variables in the Fick equation will have an impact on the Vo2max, for example a reduction in SV in heart failure or a reduction in blood oxygen content in pulmonary disease. The 6-minute walk test (6MWT) and the shuttle walk test (SWT) are surrogate tests for CPET, but the data on the value of these tests in predicting Vo2max are limited. The 6MWT involves measuring the maximum distance a patient can walk at his or her own pace in 6 minutes, and a distance of <600 metres correlates with a Vo2max of <15 ml/kg/min. The SWT requires the patient to walk ANAESTHESIA AND INTENSIVE CARE MEDICINE 9:12
O2 and CO2 Ventilation · · · transport (VA + VD = VE) Peripheral Pulmonary circulation circulation
525
© 2008 Elsevier Ltd. All rights reserved.
Thoracic
predictive value on postoperative outcome, it is not performed as part of the routine respiratory assessment. ◆
Colice GL, Shafazand S, Griffin JP, et al. Physiologic evaluation of the patient with lung cancer being considered for resectional surgery. ACCP evidence-based clinical practice guidelines (2nd edn). Chest 2007; 132(3 Suppl): 161S–77S. Slinger PD, Johnston MR. Preoperative assessment for pulmonary resection. Anesthesiol Clin North Am 2001; 19: 411–33. Thys D. Textbook of cardiothoracic anaesthesiology. Columbus, OH: McGraw-Hill Professional, 2001. Wasserman K. Diagnosing cardiovascular and lung pathophysiology from exercise gas exchange. Chest 1997; 112: 1091–101. West JB. Pulmonary pathophysiology: the essentials, 7th edn. Philadelphia: Lippincott Williams & Wilkins, 2007.
Further reading Albouaini K, Egred M, Alahmar A, Wright DJ. Cardiopulmonary exercise testing and its application. Heart 2007; 93: 1285–92. British Thoracic Society, Society of Cardiothoracic Surgeons of Great Britain and Ireland Working Party. BTS guidelines: guidelines on the selection of patients with lung cancer for surgery. Thorax 2001; 56(2): 89–108.
ANAESTHESIA AND INTENSIVE CARE MEDICINE 9:12
526
© 2008 Elsevier Ltd. All rights reserved.
Tests of pulmonary function before thoracic surgery
Respiratory mechanics Spirometry Spirometry involves the measurement of the forced expiratory volume in 1 second (FEV1) and the forced vital capacity (FVC), which is the largest volume of gas that can be forcibly exhaled from the total lung capacity (TLC). The patient breathes into a mouthpiece while wearing nose clips and performs the FVC, generating a volume–time curve from which the FEV1 is derived. Percentage predicted values should be reported to avoid bias against patients who are older, of smaller stature or women, all of whom may tolerate lower levels of lung function. Assessment of bronchodilator response in order to determine the degree of airflow-limitation reversibility is normally carried out as part of the test. An increase in FEV1 and/or FVC of more than 12% of the control or 200 millilitres constitutes a positive response. In normal individuals, the FEV1 and FVC are greater than 80% of the predicted values, and the FEV1/FVC ratio is greater than 70% of the predicted value. Table 1 shows the characteristic changes in flow rates and lung volumes associated with obstructive and restrictive pulmonary diseases.
Ving Yuen See Tho Jonathan Mackay
Abstract Pulmonary function tests form part of the comprehensive preoperative assessment of patients undergoing thoracic surgery. They aim not only to assess the severity and nature of the underlying lung pathology, but also to determine whether a patient will be able to tolerate a pulmonary resection. Tests which assess respiratory mechanics evaluate the mech anical delivery of oxygen to the alveoli and these include spirometry, measurement of lung volumes and flow–volume analysis. The forced ex piratory volume in 1 second (FEV1) and the predicted postoperative FEV1 (ppoFEV1) in particular are useful predictors of postoperative respiratory complications. Parenchymal function refers to the ability of the lung to exchange oxygen and carbon dioxide between the pulmonary blood and the alveoli, and this is assessed by determining the diffusing capacity of carbon monoxide and arterial blood gas analysis. Cardiopulmonary interaction, which is important in ensuring adequate cellular respira tion in skeletal muscle, is assessed using exercise tests that include the formal cardiopulmonary exercise test and other surrogates such as the 6-minute walk test, shuttle walk test and stair-climbing. Other tests including ventilation perfusion scintigraphy and split-lung function tests are also briefly mentioned.
Lung volumes Spirometry can also be used to measure lung volumes, excluding the residual volume (RV) and any capacity which includes RV in its definition. The functional residual capacity (FRC) is meas ured using helium dilution, nitrogen washout, body plethysmography or imaging techniques. Measurement of lung volumes may be more sensitive than the FEV1/FVC ratio in differentiating between obstructive and restrictive disorders. In restrictive lung disease, TLC, FRC and RV are all decreased. In obstructive disease, the FVC may be reduced owing to airway closure at high lung volume, giving rise to a normal FEV1/FVC ratio. The RV will, however, be abnormally high and this will confirm an obstructive defect.
Keywords exercise test; preoperative assessment; pulmonary diffusing capacity; respiratory function tests; spirometry; thoracic surgery
Flow–volume analysis Flow–volume loops are performed using spirometry, and are useful not only to detect obstructive and restrictive ventilatory defects but also to identify any fixed or variable intra- and extrathoracic airway obstruction by a tumour mass which would have an impact on the anaesthetic management of these patients (Figure 1).
Pulmonary function tests play an essential role in the preoperative assessment of patients presenting for thoracic surgery. There is no one single test of respiratory function which can accurately evaluate a patient’s fitness for surgery and predict outcome, and a combination of tests is often required. The ‘three-legged stool’ of pre-thoracotomy respiratory assessment comprises tests which target three areas of lung function, namely respiratory mechanics, parenchymal function and cardiopulmonary interaction.
Changes in flow rates and lung volumes associated with obstructive and restrictive pulmonary diseases
Ving Yuen See Tho is an Associate Consultant Anaesthetist at the Singapore General Hospital. She qualified from the Royal College of Surgeons, Ireland, Dublin, and trained in cardiothoracic anaesthesia at Papworth Hospital, Cambridge, UK. Her current interest is in perioperative transoesophageal echocardiography. Conflicts of interests: none declared.
Obstructive disease
Restrictive disease
FEV1 FVC FEV1/FVC TLC RV
Decreased Decreased or normal Decreased Normal or increased Increased
Normal or increased Decreased Normal or increased Decreased Decreased
FEV1, forced expiratory volume in 1 second; FVC, forced vital capacity; TLC, total lung capacity; RV, residual volume.
Jonathan Mackay, MRCP, FRCA, is a Consultant Anaesthetist at Papworth Hospital, Cambridge, UK. His special interests are cardiothoracic anaesthesia and resuscitation. Conflicts of interest: none declared.
ANAESTHESIA AND INTENSIVE CARE MEDICINE 9:12
Measurement
Table 1
523
© 2008 Elsevier Ltd. All rights reserved.
Thoracic
Flow–volume loops Expiration
b
c
Flow rate Q
a
Volume V
V
V
Inspiration
e
f
Flow rate Q
d
V
Volume V
V
a Normal lung. b Obstructive defect. Flow rate is low in relation to lung volume and a concave appearance of the expiratory limb is seen following the point of maximal flow. c Restrictive defect. Maximal flow rate and lung volume are reduced. d Fixed large airway obstruction, e.g. secondary to a tumour or foreign body in a bronchus. Inspiratory and expiratory flow rates are reduced. e Variable intrathoracic large airway obstruction. f Variable extrathoracic large airway obstruction
Figure 1
The single-breath carbon monoxide diffusing capacity (DLCO) is the most commonly used technique, and involves the patient taking a single vital capacity inspiration from residual volume of a mixture of 0.3% carbon monoxide and 10% helium, followed by a 10-second breath-hold and exhalation. Carbon monoxide is used because its affinity for Hb is about 400 times greater that that for oxygen, thus permitting carbon monoxide to move rapidly across the alveolar membrane without any ‘back pressure’. The rate of transfer of carbon monoxide is said to be diffusion limited. The calculation of DLCO is based on Fick’s law of diffusion, and the derived formula is as follows:
Spirometric assessment is very dependent on patient effort and cooperation, and this often results in an underestimation of FEV1 and FVC. Despite this, they remain essential modalities in the assessment of patients before lung resection. In particular, the predicted postoperative FEV1 (ppoFEV1) is a significant independent predictor of post-thoracotomy respiratory complications, and represents the most valid test to date.
Lung parenchymal function Diffusing capacity for carbon monoxide The diffusing capacity of the lung refers to the overall ability of the lung to transfer gas between the alveoli and the pulmon ary capillary blood. It is not only affected by the gas-diffusion properties of the alveolar–capillary membrane, but also by factors which affect pulmonary capillary blood volume and reaction rates of the gas with haemoglobin (Hb).
ANAESTHESIA AND INTENSIVE CARE MEDICINE 9:12
DLCO = Vco/Palvco where Vco is the rate of carbon monoxide transfer across the alveolar membrane, and Palvco is the partial pressure of carbon monoxide in the alveoli. 524
© 2008 Elsevier Ltd. All rights reserved.
Thoracic
Interpretation of DLCO involves comparing results with reference values generated from studies on healthy populations. Various physiological factors also affect DLCO, including age, sex, height, exercise, body position and altitude, and these factors are taken into account when interpreting results. A healthy 45-year-old male who is 175 cm tall should have a DLCO of 24 ml/min/mm Hg at sea-level, and this increases by two- to threefold with exercise. Current studies show that DLCO and its corresponding predicted postoperative value are well-recognized predictors of major morbidity and mortality following lung resection. DLCO is indicated if the perioperative risk is not clear based on spirometry results.
The interaction among the pulmonary, cardiovascular and skeletal muscle systems during exercise Muscle activity
CO2 production QCO
2
Arterial blood gas analysis A baseline arterial blood gas (ABG) analysis forms part of the preoperative respiratory assessment, particularly in patients with significant respiratory disease. Evidence of hypoxaemia and/or hypercapnoea should prompt further evaluation of the patient’s respiratory function. An arterial blood sample is collected in a heparinized syringe and passed through a blood gas analyser. The arterial pH, partial pressure of oxygen (Pao2) and carbon dioxide (Paco2) are directly measured using a glass pH electrode, polarographic (Clark) oxygen electrode and Severinghaus carbon dioxide electrode respectively. The actual and standard bicarbonate and base excess are calculated from the pH and Paco2 using the Siggard-Anderson nomogram.
O2 flow
Expired VCO
2
Creatine PO4
Muscle QO
Heart blood
Lungs
Pyruvate–Lactate
VO
2
2
O2 consumption
CO2 flow
Inspired
Mitochondrion VA, ideal alveolar ventilation/time; VD, physiologic dead space ventilation/time; VE, total ventilation during expiration/time; QO2, O2 consumption; QCO2, CO2 production; VO2, O2 uptake; VCO2, CO2 output; creatine PO4, creatine phosphate. Courtesy of Wasserman
Figure 2
between two cones set 10 metres apart in time to a set of auditory bleeps and the test is stopped when the patient is too breathless to keep up with the bleeps. The best of two results is taken, and anything less than 25 shuttles or 250 metres is associated with a Vo2max of <15 ml/kg/min. Exercise oximetry to detect desaturation is often simultaneously carried out during these tests. Stair-climbing lacks proper standardization, but is frequently performed because of its simplicity and ease. The patient walks up stairs at his or her own pace without stopping, and the number of flights he or she is able to ascend is recorded. There is no exact definition of a ‘flight’, but it is often taken as 20 steps at 15 centimetres per step. The ability to climb five flights indicates an FEV1 of >2 litres and a Vo2max of >20 ml/kg/min. Climbing three flights indicates an FEV1 of >1.7 litres and is associated with an average risk. A patient is considered very high risk if he or she is unable to climb one flight as this is associated with a Vo2max of < 10 ml/kg/min.
Cardiopulmonary interaction Exercise tests The aim of exercise testing is to assess the interaction among the pulmonary, cardiovascular and skeletal muscle systems during a period of exertion. In order to meet the demands of cellular respiration in exercising muscle, there must be adequate pulmonary ventilation and blood flow for oxygen and carbon dioxide transport, as illustrated by Wasserman (Figure 2). The formal cardiopulmonary exercise test (CPET) is a computerized breath-by-breath analysis of respiratory gas exchange at rest and during a period of exercise. Despite the need for complex equipment and trained personnel, it remains the ‘gold standard’ for assessment of cardiopulmonary function. A variety of parameters are recorded, and the measured maximal oxygen consumption (Vo2max) has been found to be a significant predictor of post-thoracotomy outcome. The calculation of Vo2max is based on the Fick equation as follows:
Other tests Ventilation perfusion (V/Q) scintigraphy measures what percentage the portion of lung to be resected contributes to overall ventilation and perfusion. If the diseased region is minimally functioning, it is reasonable to predict that there will be little impact on postoperative lung function. This test is particularly useful in patients undergoing a pneumonectomy, and should also be considered in any patient who has a ppoFEV1 of <40%. Split-lung function tests aim to simulate respiratory conditions after lung resection. Unilateral exclusion of a lung or lobe with a double-lumen tube or bronchial blocker is not done today because it is poorly tolerated by the awake patient and is difficult to perform. Unilateral occlusion of a pulmonary artery using a balloon catheter aims to estimate the effects of pulmonary ligation on pulmonary artery pressures. Owing to a lack of sufficient
VO2max = (SVmax × HRmax) × (CaO2max − CvO2max) where SV is the stroke volume, HR is the heart rate, Cao2 is the arterial oxygen content, and Cvo2 is the mixed venous oxygen content. Any factor which affects any one or more of the four variables in the Fick equation will have an impact on the Vo2max, for example a reduction in SV in heart failure or a reduction in blood oxygen content in pulmonary disease. The 6-minute walk test (6MWT) and the shuttle walk test (SWT) are surrogate tests for CPET, but the data on the value of these tests in predicting Vo2max are limited. The 6MWT involves measuring the maximum distance a patient can walk at his or her own pace in 6 minutes, and a distance of <600 metres correlates with a Vo2max of <15 ml/kg/min. The SWT requires the patient to walk ANAESTHESIA AND INTENSIVE CARE MEDICINE 9:12
O2 and CO2 Ventilation · · · transport (VA + VD = VE) Peripheral Pulmonary circulation circulation
525
© 2008 Elsevier Ltd. All rights reserved.
Thoracic
predictive value on postoperative outcome, it is not performed as part of the routine respiratory assessment. ◆
Colice GL, Shafazand S, Griffin JP, et al. Physiologic evaluation of the patient with lung cancer being considered for resectional surgery. ACCP evidence-based clinical practice guidelines (2nd edn). Chest 2007; 132(3 Suppl): 161S–77S. Slinger PD, Johnston MR. Preoperative assessment for pulmonary resection. Anesthesiol Clin North Am 2001; 19: 411–33. Thys D. Textbook of cardiothoracic anaesthesiology. Columbus, OH: McGraw-Hill Professional, 2001. Wasserman K. Diagnosing cardiovascular and lung pathophysiology from exercise gas exchange. Chest 1997; 112: 1091–101. West JB. Pulmonary pathophysiology: the essentials, 7th edn. Philadelphia: Lippincott Williams & Wilkins, 2007.
Further reading Albouaini K, Egred M, Alahmar A, Wright DJ. Cardiopulmonary exercise testing and its application. Heart 2007; 93: 1285–92. British Thoracic Society, Society of Cardiothoracic Surgeons of Great Britain and Ireland Working Party. BTS guidelines: guidelines on the selection of patients with lung cancer for surgery. Thorax 2001; 56(2): 89–108.
ANAESTHESIA AND INTENSIVE CARE MEDICINE 9:12
526
© 2008 Elsevier Ltd. All rights reserved.