What does a Cardio Pulmonary Exercise Test tell you?

Firstly, on a physiological level, it is important to consider what is happening to the patient.

During an exercise test, the oxygen consumption rises in a linear fashion to the work applied, until the point where the oxygen delivered by the air in the lungs is insufficient to maintain the effort. At this point the body chemistry changes to deliver oxygen from lactate.

This change from Aerobic to Anaerobic metabolism can be referred to as the Anaerobic Threshold (AT), the Ventilatory Threshold (Vth), the Gas Exchange Threshold or, where blood samples are also collected as part of the data, as the Lactate Threshold.

So is it just the Anaerobic Threshold that we’re looking for?

The AT is commonly assessed by the V-slope analysis of Oxygen uptake (VO2) plotted against Carbon dioxide production (VCO2). By finding the greatest angle of departure, this can be defined as the AT.

Others will use the first crossing of the Ventilatory Equivalents of the Carbon Dioxide (VE/CO2) and Oxygen (VE/O2) to indicate the threshold.

This effectively is how hard you have to breathe in order to expire CO2 and inspire O2, low being good and high being bad. Values are increased with lung disease, pulmonary hypertension, ventilatory perfusion mismatch or heart failure.

However, it is the arrival of this point as a function of the maximal test that is a direct indicator of a subject’s state of fitness. Some authors of CPET guidelines also relate this point to the subject’s body weight as an uptake of oxygen per ml per kg. This can be misleading in exercise modes where the subject weight is not a factor of the actual loading.

Furthermore, the oxygen uptake maximum (VO2 max) shows the peak performance of the subject and, when looked at in relation to the Heart rate maximum, is a further indication of respiratory or cardiac impairment.

So the cardiac assessment is also useful then?

Very much so! An important feature of a CPX Test is the 12 Lead ECG recording which is simultaneous with the respiratory analysis.

It is important again that a baseline ECG recording is made prior to exercise to ascertain whether the patient has any existing arrhythmias and is fit to undertake the exercise protocol. This allows us to obtain a baseline V5 waveform which can then be superimposed against waveform measurements taken during the exercise Test. This will additionally reveal any ST Segment changes associated with Myocardial Ischemia.

Also, the Oxygen Pulse (ml/beat) reflects stroke volume, which should increase in a linear manner as the work during exercise increases.

A low O2 Pulse or a flattening of the O2 Pulse may reflect a reduce Cardiac Output. The system allows us to construct a line in order to plot O2 versus work to confirm this.

How can the data collected be interpreted?

No single number defines the outcome of a CPET, the key indices must be looked at together and graphical analysis with pattern recognition is the key. The Wasserman 9 plot is a common analysis tool for CPET; its graphical solutions allow rapid interpretation of the collected data. Break points, maximums and performance against predicted normal values all permit evaluation of the subject’s performance.

So what does the CPET tell us?

The important fact is the CPET does not decide if the patient should undergo surgery or not.

The CPET will provide data that can help those who are involved in the patient care pathway in advising the patient and their families of all the associated risks. This communication can provide additional confidence for the patient and makes aware to the family, the doctors and the nurses how the patient views the risks involved.

It can assist in the overall planning of both pre and post Operative Care - the major benefit to both Anaesthetist and Intensivist being the ability to address the risk versus the benefits of the elective surgery.