Objective Methods for Preoperative Assessment of Functional Capacity - BJA Education
Highlights
- 6-min walk distance (6MWD) poorly predicts disability-free survival and 12-month mortality after major non-cardiac surgery
- An incremental shuttle walk distance (ISWD) < 250 m provides fair prediction of major complications and 30-day mortality after major colorectal surgery
- V˙ o2 peak > 14–15 ml kg−1 min−1, anaerobic threshold (AT) > 11 ml kg−1 min−1 and V˙e/V˙co2 at AT ≤ 41–42 predict a low likelihood of cardiorespiratory complications, in-hospital and 30-day mortality after non-cardiopulmonary surgery.
- V˙ o2 peak, AT and V˙e/V˙co2 at AT below these thresholds poorly identify individuals at high risk of postoperative complications.
- Stress echocardiography, using either exercise stress echocardiography (ESE) or dobutamine stress echocardiography (DSE), may identify inducible ischaemia and thus the presence of significant coronary artery disease. These tests have very good ability to exclude coronary artery disease and identify individuals at low risk of postoperative cardiac complications (negative predictive value >0.95)
- However, DSE is not a direct measure of functional capacity, and both ESE and DSE have modest sensitivity for identifying individuals at increased risk of 30-day postoperative cardiovascular complications (sensitivity of 0.68 and 0.79, respectively)
- The most widely researched variables for the purposes of functional capacity assessment and perioperative risk stratification are: (i) V˙ o2 peak (ii) Anaerobic threshold (iii) Ventilatory efficiency at the AT. These variables are derived directly from CPET
- V˙ o2 peak < 15 ml kg−1 min−1 denotes increased risk of perioperative complications
- AT < 11 ml kg−1 min−1 denotes increased risk of perioperative complications
- V˙ e/ V˙ co2 at AT > 34 predicts postoperative morbidity and mortality
- Oxygen uptake, or the rate of oxygen consumption (V˙o2 per unit weight per unit time), is one of the most important variables in CPET. V˙o2 peak is the peak rate of oxygen uptake when the individual exercises up to symptom limitation or fatigue. V˙o2 peak reflects an individual's ‘best effort’ and is distinct from V˙o2 max. V˙o2 max, a physiological endpoint, refers to maximum cardiac output and oxygen extraction and is identified by a plateau in oxygen uptake, despite an increase in workload.9 Most individuals do not reach their V˙o2 max during CPET, which limits the utility of V˙o2 max in clinical practice.
New highlights added August 25, 2022 at 12:28 AM
- V˙ o2 peak > 20 ml kg−1 min−1 was associated with low risk of postoperative complications and mortality
- V˙ o2 peak < 12 ml kg−1 min−1 was associated with significantly increased risk of postoperative pulmonary complications and mortality
- V˙ o2 peak of 15 ml kg−1 min−1 corresponds to 4.28 metabolic equivalents, which is considered to approximate the metabolic cost of major surgery
- During strenuous exercise, muscle oxygen requirements eventually exceed the capacity of the cardiopulmonary system to deliver oxygen, beyond which anaerobic metabolism takes place. The AT is the oxygen uptake ( V˙V˙ o2) at which anaerobic metabolism occurs.
- AT < 11 ml kg−1 min−1 has been accepted as the threshold associated with increased perioperative risk in individuals undergoing non-cardiac surgery
- Ventilatory efficiency is described by the V˙e/V˙co2 relationship, the ratio of minute ventilation to carbon dioxide production. This ratio refers to the volume of air that needs to be ventilated to exhale 1 L of carbon dioxide, per unit time. Therefore, V˙e/V˙co2 is a measure of the efficiency of gas exchange during exercise. In the assessment of dyspnoea, V˙e/V˙co2 at AT is often measured, as this identifies the point when ventilatory drive increases relative to workload.
- V˙ e/V˙co2 at AT ratio > 34 defines ventilatory inefficiency.4Whereas V˙o2 peak is influenced by subject motivation, both AT and V˙e/V˙co2 at AT are effort-independent, and therefore are more reproducible measures of exercise capacity.
- The 6MWT measures how far a patient can walk up and down a flat, 30 m corridor in 6 min. The measurements obtained include the 6-min walk distance (6MWD), oxygen saturation, heart rate, modified Borg dyspnoea scale and leg fatigue
- The median 6MWD for healthy individuals is between 500 and 600 m
- Normal values for the ISWD vary with age, but are usually between 560 and 820 m in healthy individuals
- During CPET, the patient is required to perform incremental exercise on an upright cycle ergometer, whilst breathing through a mouthpiece. The patient follows a standardised protocol up to limitation by symptoms. The test provides assessment of the integrative exercise responses involving the cardiovascular, pulmonary and musculoskeletal systems. Data obtained during CPET include heart rate, non-invasive blood pressure, 12-lead ECG, oxygen uptake (V˙o2 ), carbon dioxide production (V˙co2 ), oxygen saturation, gas flow rates and work rate.
- The role of CPET includes measurement of exercise capacity in individuals with unknown or suspected poor exercise tolerance, differentiation of the cause for exercise limitation and risk stratification to assist with perioperative planning and appropriate allocation of resources
- for non-cardiopulmonary surgery, CPET may not accurately predict postoperative complications in individuals with poor functional capacity.
- Overall, the following thresholds have been established for CPET-derived variables for prediction of in-hospital surgical morbidity, duration of stay in hospital, in-hospital mortality, 30-day mortality and 1-yr mortality
- 6MWD > 510 m was identified as the threshold that is associated with significantly increased chance of DFS
- CPET is thought to be particularly useful in individuals expected to have a postoperative forced expiratory volume in 1 s (FEV1) < 40% of predicted
- V˙ o2 peak < 10 ml kg−1 min−1 (or < 35% predicted) has been suggested as a prohibitive threshold for major lung resection
Objective Methods for Preoperative Assessment of Functional Capacity - BJA Education
Highlights
- 6-min walk distance (6MWD) poorly predicts disability-free survival and 12-month mortality after major non-cardiac surgery
- An incremental shuttle walk distance (ISWD) < 250 m provides fair prediction of major complications and 30-day mortality after major colorectal surgery
- V˙ o2 peak > 14–15 ml kg−1 min−1, anaerobic threshold (AT) > 11 ml kg−1 min−1 and V˙e/V˙co2 at AT ≤ 41–42 predict a low likelihood of cardiorespiratory complications, in-hospital and 30-day mortality after non-cardiopulmonary surgery.
- V˙ o2 peak, AT and V˙e/V˙co2 at AT below these thresholds poorly identify individuals at high risk of postoperative complications.
- Stress echocardiography, using either exercise stress echocardiography (ESE) or dobutamine stress echocardiography (DSE), may identify inducible ischaemia and thus the presence of significant coronary artery disease. These tests have very good ability to exclude coronary artery disease and identify individuals at low risk of postoperative cardiac complications (negative predictive value >0.95)
- However, DSE is not a direct measure of functional capacity, and both ESE and DSE have modest sensitivity for identifying individuals at increased risk of 30-day postoperative cardiovascular complications (sensitivity of 0.68 and 0.79, respectively)
- The most widely researched variables for the purposes of functional capacity assessment and perioperative risk stratification are: (i) V˙ o2 peak (ii) Anaerobic threshold (iii) Ventilatory efficiency at the AT. These variables are derived directly from CPET
- V˙ o2 peak < 15 ml kg−1 min−1 denotes increased risk of perioperative complications
- AT < 11 ml kg−1 min−1 denotes increased risk of perioperative complications
- V˙ e/ V˙ co2 at AT > 34 predicts postoperative morbidity and mortality
- Oxygen uptake, or the rate of oxygen consumption (V˙o2 per unit weight per unit time), is one of the most important variables in CPET. V˙o2 peak is the peak rate of oxygen uptake when the individual exercises up to symptom limitation or fatigue. V˙o2 peak reflects an individual's ‘best effort’ and is distinct from V˙o2 max. V˙o2 max, a physiological endpoint, refers to maximum cardiac output and oxygen extraction and is identified by a plateau in oxygen uptake, despite an increase in workload.9 Most individuals do not reach their V˙o2 max during CPET, which limits the utility of V˙o2 max in clinical practice.
- V˙ o2 peak > 20 ml kg−1 min−1 was associated with low risk of postoperative complications and mortality
- V˙ o2 peak < 12 ml kg−1 min−1 was associated with significantly increased risk of postoperative pulmonary complications and mortality
- V˙ o2 peak of 15 ml kg−1 min−1 corresponds to 4.28 metabolic equivalents, which is considered to approximate the metabolic cost of major surgery
- During strenuous exercise, muscle oxygen requirements eventually exceed the capacity of the cardiopulmonary system to deliver oxygen, beyond which anaerobic metabolism takes place. The AT is the oxygen uptake ( V˙V˙ o2) at which anaerobic metabolism occurs.
- AT < 11 ml kg−1 min−1 has been accepted as the threshold associated with increased perioperative risk in individuals undergoing non-cardiac surgery
- Ventilatory efficiency is described by the V˙e/V˙co2 relationship, the ratio of minute ventilation to carbon dioxide production. This ratio refers to the volume of air that needs to be ventilated to exhale 1 L of carbon dioxide, per unit time. Therefore, V˙e/V˙co2 is a measure of the efficiency of gas exchange during exercise. In the assessment of dyspnoea, V˙e/V˙co2 at AT is often measured, as this identifies the point when ventilatory drive increases relative to workload.
- V˙ e/V˙co2 at AT ratio > 34 defines ventilatory inefficiency.4Whereas V˙o2 peak is influenced by subject motivation, both AT and V˙e/V˙co2 at AT are effort-independent, and therefore are more reproducible measures of exercise capacity.
- The 6MWT measures how far a patient can walk up and down a flat, 30 m corridor in 6 min. The measurements obtained include the 6-min walk distance (6MWD), oxygen saturation, heart rate, modified Borg dyspnoea scale and leg fatigue
- The median 6MWD for healthy individuals is between 500 and 600 m
- Normal values for the ISWD vary with age, but are usually between 560 and 820 m in healthy individuals
- During CPET, the patient is required to perform incremental exercise on an upright cycle ergometer, whilst breathing through a mouthpiece. The patient follows a standardised protocol up to limitation by symptoms. The test provides assessment of the integrative exercise responses involving the cardiovascular, pulmonary and musculoskeletal systems. Data obtained during CPET include heart rate, non-invasive blood pressure, 12-lead ECG, oxygen uptake (V˙o2 ), carbon dioxide production (V˙co2 ), oxygen saturation, gas flow rates and work rate.
- The role of CPET includes measurement of exercise capacity in individuals with unknown or suspected poor exercise tolerance, differentiation of the cause for exercise limitation and risk stratification to assist with perioperative planning and appropriate allocation of resources
- for non-cardiopulmonary surgery, CPET may not accurately predict postoperative complications in individuals with poor functional capacity.
- Overall, the following thresholds have been established for CPET-derived variables for prediction of in-hospital surgical morbidity, duration of stay in hospital, in-hospital mortality, 30-day mortality and 1-yr mortality
- 6MWD > 510 m was identified as the threshold that is associated with significantly increased chance of DFS
- CPET is thought to be particularly useful in individuals expected to have a postoperative forced expiratory volume in 1 s (FEV1) < 40% of predicted
- V˙ o2 peak < 10 ml kg−1 min−1 (or < 35% predicted) has been suggested as a prohibitive threshold for major lung resection