The concept of ‘fitness’ is an important one for patients undergoing surgery. A such, the preoperative assessment of it has become an important part of the anaesthetic assessment.
Fitness can be considered as “the capacity for the body to cope with exercise (do work)”. Exercise involves an increase in the cellular activity of particular organs of the body (most commonly the muscles), often by very significant amounts. This requires significant physiological changes to support the ongoing increased cellular activity. Although there are other demands as well, a core requirement is the increased cellular demand for energy, in the form of the ‘energy currency’ ATP. Because of the very limited stores of ATP within tissues in the body, any prolonged activity requires a massive increase in the production of these molecules. This will be through the metabolism of fuels in the tissues, particularly carbohydrates. In humans, this is most optimally done through aerobic metabolism, although the anaerobic metabolism of fuel stores is also a viable, if less efficient and less sustainable, alternative.
As such, a key part of the concept of fitness is the body’s ability to support this energy production, and thus for the cardiorespiratory systems to provide adequate gas transport to the exercising tissues to allow this. The capacity for the body to do this is its ‘fitness’.
The concept of CPET testing is to gain an improved understanding of the patient’s fitness, prior to a major surgical procedure where this fitness will be important. This is because of the significant metabolic demand that is involved in major surgery due to the stress response. If the patient’s physiology is unable to meet this increased demand, then they are at high risk of complications arising. The demand for increased oxygen delivery may be as high as 50% after major surgery, and this requires a significant increase in cardiac output to achieve (this usually being the limiting step in the chain).
If we wanted to break down the physiological chain that supports this metabolic activity, we could see that it includes:
Some of these factors can be analysed in more specific details through different tests e.g. Hb level, but CPEX testing provides a global (and perhaps more useful) assessment of how the system functions as a whole.
Risk assessment for major surgery
Assessment and monitoring of cardiorespiratory disease
Assessment of risk for prospective mothers with adult congenital heart disease
The result of CPET testing provides objective measurements of cardiorespiratory fitness under stress. Certain values (as discussed below) have been demonstrated to be associated with a worse postoperative outcome, because of what they say about the patient’s ability to meet the increased metabolic demand of major surgery. It has some advantages over other assessments of fitness e.g. self reported exercise tolerance, because of this objectivity. Other advantages are that the results are reliable, repeatable and acquired in a safe and non-invasive manner.
The results can therefore be used to provide a degree of risk stratification of patients. This can in turn be used to:
Counsel the patient on the risks vs benefits of surgery
Alter anaesthetic or surgical technique
Guide perioperative care e.g. critical care admission
Provide advice on prehabilitation
The investigative process can also be diagnostic. Certain results can indicate specific respiratory or cardiac problems, and indeed ones that might have been previously asymptomatic or undiagnosed. These can be investigated further e.g. angiography, echocardiography, or allow treatment to be commenced e.g. bronchodilators.
These roles have resulted in CPET being used outside of perioperative medicine to help guide patient care. This has included:
Investigation of cardiorespiratory disease
Monitoring of heart failure treatment
Risk assessment for patients with adult congenital cardiac disease wishing to conceive.
There are some contraindications to performing CPET Absolute:
Patient refusal/unable to comply
Acute coronary syndrome
MI within 5 days
Severe symptomatic aortic stenosis
Acute thromboembolic disease
Uncontrolled medical conditions
Severe heart failure
Cardiac arrhythmias with CVS compromise
Moderate stenotic valvular heart disease
Left main stem coronary artery stenosis
The start of process involves a discussion and history with the patient. This will include an explanation of the process, and explanation of the benefits and (albeit small) risks. Additional aspects of the history need to be explored:
A health questionnaire will often be completed as part of this process.
Measurements are taken prior to the procedure:
Forced vital capacity (FVC)
Forced expiratory volume in 1 second (FEV1)
When ready the patient can be set up to perform the test. The equipment includes:
Ergometer e.g. a cycle
12 lead ECG
Non-invasive blood pressure
Inhaled/expired gas monitoring
The gas monitoring involves a tight fitting face mask. This will collect all the gas that is expired and control all the gas that is inspired by the patient. By measuring the concentrations of the gases (O2 and CO2) and by measuring the flow rates and volumes, the specific volumes of the gases over time can be known. The response time of the analysers is under 90ms so that breath by breath analysis can be done.
Whilst the ergometer is commonly an exercise bike, other options include an arm crank (e.g. for people with significant lower limb arthritis) or a treadmill. The cycle ergometer allows more control over the resistance and therefore work that the patient must do and there is generally less interference with monitoring e.g. ECG.
Unresisted cycling - about 60 revolutions per minute
Ramped increase in work
Warm down (after maximal effort has been reached) - unresisted cycling
At rest observation
The total test takes around 20 mins. A goal is to aim for around 10 minutes of ramped increase in workload. The patient should be encouraged to reach maximal exertion but can stop at any time.
The CPET process produces some useful information. Before using this, a few questions need to be asked to be satisfied with its reliability:
Were there any recording problems? E.g. poor fitting mask may interfere with gas analysis
Was maximal effort reached?
What stopped the patient? Did they complete the process or did breathlessness or fatigue stop them?
Were there any adverse events? E.g. Did the exertion lead to angina?
There are a number of values that are given by the CPET testing.
Oxygen consumption (VO2)
Carbon dioxide excretion (VCO2)
ECG changes e.g. ST segment
Work (in watts)
These can be used individually or combined to produce derived values. These values can then all be compared against predicted results, or their own independent value e.g. based on specific patterns. The results are classically displayed in a 9 panel plot.
With the results obtained from CPET, clinical interpretation can then be applied. Much of this will relate to what the results suggest about the cardiorespiratory reserve of the patient, and how they may cope with major surgery. As such there are specific values that are of interest:
Other variables not specific to the patients fitness may also arise during the test. For example, angina or significant ECG changes may identify significant coronary disease that may need to be considered separately from the patient’s fitness.
This refers to the volume of oxygen consumption. This is a measure of the amount of oxygen that is taken up by the body. As the amount taken up is matched to the metabolic demands of the body, it is related to the amount required to meet the demands placed on the body by exercise
It can be calculated by subtracting the volume of oxygen that is expired, from the volume that is inspired. This can in turn be calculated through knowing the inspired concentration of oxygen, and the minute ventilation of the patient, as all gas exchange will occur through the lungs.
For example: Breathing in an oxygen concentration of 21% at 12 x 500ml breaths per minute. Inspired = (12 x 500) x 0.21 = 1260ml/min of O2.
If we breathe out an oxygen concentration of 17%, then: Expired = (12 x 500) x 0.17 = 1020ml/min of O2
This is roughly how most people are at rest, using 250ml/min of oxygen to meet their basic metabolic demands. If we want to be more specific, we would work out a weight based calculation. If we assumed the above patient weighed 70kg, we could clearly calculate a basic oxygen consumption of about 3.5ml/kg/min. Note: there are a few assumptions and adjustments with the maths above
To help make this a more useful clinical measurement, this can be termed 1 Metabolic Equivalent (1 MET).
This is one of the measures taken during CPET assessment. Of particular note regarding the patient’s fitness, we are interested in the maximum amount of oxygen consumption that a patient can achieve. The VO2 max is the maximum level of cellular metabolic activity that the cardiorespiratory system can continue to provide oxygen for. This is a very unpleasant situation to be in, and few patients (i.e. people that aren’t athletes) are able to reach this level of physiological stress. Instead, the VO2 peak is used. This is the maximal level of O2 uptake that the patient achieved during the testing (usually at the point that it is terminated), and related to the degree of effort and motivation of the patient as well as physiological fitness.
As is well known, the aerobic metabolism of fuels is not the only chemical method for liberating energy from them. Energy can be released without oxygen being present, though the efficiency of this process is notably less. This will be through the glycolysis of carbohydrates (and also fatty acid breakdown), prior to Kreb’s cycle. The consequence of this is a build up of pyruvate, as this cannot be used in the oxidative pathways in the absence of oxygen. This excess can be converted into lactate, producing further ATP and hydrogen ions (catalysed by lactate dehydrogenase).
This excess hydrogen load is buffered by the body, particularly by the bicarbonate system. As can be appreciated, the equilibrium of this buffer will shift to the right, producing more CO2. Up until this point the CO2 production increased in proportion to that of O2 consumption. However, there will now be increased CO2 ‘production’ by the body from this buffering, and this can be detected by an increase in exhaled CO2.
The VO2 at this level is termed the anaerobic threshold (or lactate threshold). It represents the point where the body starts to supplement aerobic energy production with anaerobic energy production, in order to be able to meet the exercise demands. Fitter people can do higher levels of exercise before their physiology reaches this point.
Ventilatory Equivalent of CO2
Although this is less related to fitness, this concept is also measured at CPET and is related to postoperative outcome. The ventilatory equivalent of CO2 is the patient’s minute ventilation divided by the VCO2 at this time. It is effectively a marker of how well the lungs can facilitate CO2 transfer. As CO2 is so soluble, and quick to traverse the alveolar membrane, it is a marker of the bodies ability to maintain V/Q matching in the lungs.
It can be measured at any time during the testing, but the value of interest is that at the anaerobic threshold.