Cardio Pulmonary Exercise Testing (CPEX/CPET)
Does your centre have one? We are lucky enough to say we do, and we find the test a very useful discriminator as to whether patients can physiologically ‘cope’ with the operation ahead of them (normally major surgery).
This is relevant to ICU, as there will often be a request for a higher level bed on the grounds of one of these tests. I much prefer a request to be backed up by this, as opposed to the occasional, ‘we just need one’, as defined by a reflex response from the surgeon’s booking secretary!!
To understand the basis of CPET, it is important to understand the basic physiology of exercise. With a constant increase in work rate/exercise (cycle ergometer with constant increase in work rate), there is increased oxygen consumption (VO2) in the muscles to produce ATP. This leads to increased production of CO2 as a result of metabolism.
To supply this increased oxygen requirement and to eliminate the increased carbon di-oxide production, there is increase in cardiac output (heart rate and stroke volume), and increase in minute ventilation (RR and TV).
However, with incremental exercise after a certain amount of time, aerobic metabolism (oxygen supply) is unable to sustain the required ATP production and will need anaerobic metabolism in addition to aerobic metabolism. Anaerobic metabolism is inefficient (12 times less ATP) and leads to lactic acid production. This lactic acid is buffered by body systems and is mainly eliminated as carbon dioxide.
The ability of body to sustain aerobic metabolism and continue to exercise with combination of aerobic and anaerobic metabolism forms the basis of CPET
This test looks at the efficiency of respiratory, cardiovascular and metabolic system of the body. If there is a restriction in oxygen supply (cardiac, respiratory), oxygen utilisation (metabolic) or CO2 elimination (cardiac, pulmonary) it becomes obvious with the test.
At the end of the test a 9 panel plot (below) with various parameters is produced from which a variety of inferences can be obtained.
Patient Information Video UCL
Essential Basics Video
Short explanatory video
Here is a simple explanation of some commonly used terms and their significance. A table explaining how patients are categorised is provided at the end.
Anaerobic threshold: signifies the point when anaerobic metabolism kicks in. There is an increase in blood lactate, which is seen as increased Co2 production signifying anaerobic metabolism. This is expressed as the VO2/ml/kg at that point of increased Co2 production.
VO2 Peak: A constant rate of change of work rate usually yields a constant rate of change of VO2 (usually linear). The rate of VO2/WR increase is usually between 9-11ml/min). It is low in cardiovascular disease (8ml/min/Watt or less)
The highest value of VO2 achieved with good subject effort is termed the peak VO2 (VO2 peak). The VO2 does not continue to increase with further increase in WR.
VE/VCo2: Minute Ventilation (VE) increases with exercise in response to pulmonary CO2 output (VCO2). The change is usually a linear function of VCO2 over a wide range of work rate. High values of VE/VCO2 (at anaerobic threshold) reflect either high VD/VT or a low PaCO2 or both. The causes of high VD/VT could be pulmonary, cardiac (CCF) or shallow breathing.
Oxygen pulse (VO2/HR): and heart rate response: There should be linear increase in cardiac output due to both increase in HR and stroke volume. If stroke volume increase is limited either due to CCF, IHD, valvular heart disease or cardiomyopathy, the increase in cardiac output will occur due to sudden increase in heart rate. This will be seen as a flat cardiac pulse trace.
The Key Values
Uses of CPEX
- Individualised objective risk stratification of patients undergoing major surgery
- Pre-optimisation of patients (cardiac, respiratory, haematology etc.)
- Pre-optimisation – Exciting new field of study where CPEX variables are used to improve patient’s aerobic capacity before surgery with programmed exercise regime
- Guidance of peri-operative management
- Allocation of appropriate beds/resources – the big one for us on ICU
Watch an Elite Triathlete bust the VO2 test!
Many thanks to my colleague Dr KK Ramaswamy (CPEX lead here) for the bulk of the information on this subject.
See our section here on some of the metabolic pathway re-caps!