The curriculum asks us to understand "...measures of functional capacity [including cardiopulmonary exercise testing]".
In this vein, a CRQ question on CPET appeared in March 2022, with a 60% pass rate. Most marks were dropped on "the sections relating to the CPET variables".
CPET is an objective, dynamic, non-invasive and integrated measure of functional capacity.
One can get rather tangled in the minutiae of the nine-panel plot, so the time-poor candidate may wish to learn the three 'key' CPET variables in the table below, and call it quits.
Variable
Threshold Value
Consequence
Anaerobic threshold
≤11 mlO2/kg/min
Increased inpatient post-operative mortality
VO2 peak
≤15 mlO2/kg/min (or ≤60% predicted)
Increased post-operative complications
VE/VCO2
≤32 = normal
>35 = increased risk pulmonary complications
>50 = possible pulmonary HTN
Relates to gas exchange and V/Q matching
Not related to exercise efficiency
Hypothesis
The physiological stress of major surgery increases an individual's baseline O2 consumption (VO2)
Patients who are less physically fit are more likely to experience adverse perioperative outcomes
Therefore patients with insufficient cardiopulmonary capacity to increase O2 delivery perioperatively are more likely to experience organ dysfunction
Uses
CPET is used for patients undergoing major surgery from an increasing number of surgical specialties
These include:
Urological
Vascular e.g. elective AAA repair
Upper GI e.g. oesophagectomy
HPB e.g. liver transplant
CPET can help:
Inform patients of their individualised risks from surgery, facilitating shared decision making
Risk stratify patients, allowing appropriate prehabilitation exercise prescriptions based on in-CPET effort
Planning for perioperative management, inc. optimisation of comorbidities, and post-operative care
Process
Patient rides an electromagnetically-braked cycle ergometer for approx. 10mins
Monitoring
Rapid gas analyser (O2, CO2) and pressure-differential pneumotachograph (flowmeter)
12 lead ECG
SpO2
NIBP
Barometer and temperature measurement to ensure standardisation of temperature/pressure
Measurements taken at rest, pedalling without resistance, against increasing resistance and during the recovery phase
One should remember that CPET is not necessarily a benign intervention, itself carrying a mortality of 2 - 4/100,000 tests
There are a host of contraindications to CPET, categorised here in familiar fashion into 'absolute and relative':
Other medical conditions affecting performance e.g. syncope
Patient refusal
Patient inability to cooperate with the test e.g. due to communication difficulties
Relative
Cardiovascular disease not fitting the above criteria, such as:
Left main stem disease
Moderate valvular stenosis
Untreated, severe hypertension
HOCM
High degree AV nodal block
Pulmonary HTN
AAA >8cm
Advanced pregnancy
Electrolyte abnormalities
Work rate [Watts]
Gas exchange
Oxygen consumption (VO2)
Carbon dioxide production (VCO2)
Respiratory exchange ratio (RER)
I.e. VCO2 divided by VO2
A value >1.15 represents maximal effort on a CPET test
It is roughly equivalent to the respiratory quotient during steady state conditions
Ventilatory
SpO2
Minute ventilation (VE)
Tidal volume
Respiratory rate
Ventilatory equivalents of oxygen (VE/VO2)
Ventilatory equivalents of carbon dioxide (VE/VCO2)
This is a unitless measure
Higher numbers imply greater ventilation required for CO2 clearance i.e. inefficient gas exchange or poor V/Q matching
High VE/VCO2 does not indicate the cause of the gas exchange inefficiency, which may be due to heart failure, pulmonary fibrosis, fibrotic lung disease etc.
Oxygen pulse (VO2/HR)
Anaerobic threshold
Cardiovascular
Heart rate
Non-invasive blood pressure
ECG ST-segment changes
Maximal effort
Has the patient demonstrated maximal effort on the test? The answer is 'yes' if they've hit one of the following thresholds:
They've achieved >80% predicted maximum work rate (in Watts)
They've achieved >80% predicted maximum heart rate
They've achieved a RER >1.15
The other pertinent question here is: "why did the test stop?"
It might have been completed, or the patient may have suffered symptoms (chest pain, leg claudication, MSK pain)
or signs (dysrhythmia, ST-segment changes, hypotension) mandating early cessation of the test
VO2 peak
VO2 max is the maximum VO2 achievable by an individual performing exercise, but is often not achieved by the elderly, comorbid individuals undergoing CPET
VO2 peak is the highest VO2 recorded, and is usually the VO2 at the point when the test is terminated
Usually increases linearly in proportion to work
A VO2 peak of <15 mlO2.kg-1.min-1 is associated with greater risk of peri-operative complications
VO2 - work relationship
VO2 should increase linearly at 10 mlO2/min/W once resistance is added to the ergometer
A VO2/W <10 implies anaerobic respiration is required to generate the Watt of energy
Anaerobic threshold
The anaerobic threshold is the point at which the oxygen demand of the muscles exceeds the ability of the cardiopulmonary system to supply oxygen
Muscle will begin to generate ATP anaerobically, producing lactic acid, which is thus buffered by bicarbonate and extra carbon dioxide is produced
It can be found using the 'V-slope' method at the point where VCO2 increases disproportionally to VO2 using lines of best fit
Alternatively, it can be found at the point where VE/VO2 increases despite VE/VCO2 remaining constant
Mayn't be found in ~10% of tests
If the anaerobic threshold is found, the pertinent marker is the VO2 at this point of the test
A VO2 of <11 mlO2/kg/min at the anaerobic threshold are at increased risk of perioperative complications
Heart rate response
Normally, there is a linear response of HR to increasing exercise intensity
Immediately after cessation of exercise there is a rapid decrease in HR
This response may be blunted i.e. chronotropic incompetence, which can be due to rate-limiting medications
Oxygen pulse
Oxygen pulse = VO2/HR
It represents the product of stroke volume and arterial-venous oxygen difference (SV x [CaO2 - CvO2])
It is therefore a surrogate marker of stroke volume
It should increase at the start of exercise, before slowly plateauing
An early flattening off of the trajectory is a strong indicator of cardiac limitation and a sign of ischaemic heart disease
Ventilatory limitations
There should be a linear increase in VE up to AT, followed by a disproportionally higher increase in VE (due to extra CO2 production from anaerobic metabolism)
In healthy individuals, exercise is never limited by ventilation but in those with obstructive or restrictive lung disease ventilation may be a limiting factor
Static spirometry (FEV1 and FVC) is often also performed at the time of CPET
Maximum voluntary ventilation (MVV) is also calculated; it is the maximum volume of air that can be inhaled in one minute
Roughly equates to FEV1 x 40
Normally, VE does not exceed 80% of MVV
Saturations should ideally remain >95% throughout the test
ECG changes
Cardiac ischaemia may manifest as classic ECG changes during the test, e.g.:
1mm ST depression in two adjacent leads
2mm ST depression in a single lead
T-wave inversion
New BBB
Induced dysrhythmia
Excessive post-exercise oxygen consumption is termed oxygen debt
It is due to the processes which restores the body to its resting state and adapts the body to the exercise performed
It is primarily caused by restoration of:
ATP and phosphocreatine stores
Myoglobin oxygen stores
Muscle and liver glycogen stores
Normal intracellular electrolyte values
Dissipation of heat and repair/hypertrophy of myofibrils are other contributors