Sales Manager & Technical Director, Pulmolink
Q: Kevin, first of all, can you explain basically what is meant by CPX?
A: KH: CPX, or to use its more correct abbreviation, CPET stands for Cardio Pulmonary Exercise Test. It is a means of assessing the fitness of a subject or patient by exercising them in a controlled and quantifiable manner whilst gathering data. This embraces gas analysis, ventilation and cardiac function to assess the gas exchange and circulation of a subject to a given workload. The CPET brings together the functions of the respiratory, cardiac and circulatory systems in a complete diagnostic test. No other single test system provides such comprehensive information.
Q: So, Anthony, what equipment is required?
A: Typically a CPET (CPX) test system, such as our ERGOCARD system, comprises a trolley-mounted, computer-controlled 12 lead ECG module plus a gas analysis unit that measures the oxygen uptake and the carbon dioxide production of a patient by sampling their breathing whilst they exercise. Non-invasive blood pressure and oxygen saturation is also recorded, usually by an integrated patient monitoring module. The exercise is preferably performed on a static bicycle (Ergometer) although a Treadmill or Arm Ergometer (Handcrank) can also be used. These are connected to and controlled by the CPET system & its operator.
Q: Why does the Ergometer or treadmill need to be controlled?
A: Initially, the subject should be observed at rest, or pedalling with unloaded work, to obtain a baseline condition, without hyperventilation or hypoventilation as a result of anxiety or stress. Once this is achieved, a load or “ramp” needs to be applied progressively in a controlled manner, so that the subject has to work harder as the test progresses to achieve a peak performance in 10 – 12 minutes. This load or “ramp” needs to be set after prior discussion with the subject to understand their normal state of fitness and then a computer controlled protocol for the ergometer can be selected that will achieve the desired effect. This control allows for quantifiable and reproducible results.
Q: What else should be assessed before the test is performed?
A: KH: The lead up to performing CPET is to evaluate the patient’s ventilation function by means of performing spirometry and maximum voluntary ventilation (MVV). The latter is often substituted for an indirect measure using the FEV1 x 40 (35 in former publications) which is achieved using the volume measuring part of the equipment. The FEV1 is measured through the performance of a flow – volume loop and, if performed, the direct MVV by breathing hard and fast for 12 seconds. These measures of ventilatory capacity can be used in assessment of the exercise tolerance for ventilatory limitation. This is part of the diagnostic criteria used by Wasserman in his interpretation of CPET results.
Q: So what happens next?
A: ASP: The patient is fitted with a very lightweight facemask which incorporates a flow sensor to measure inspired oxygen & exhaled carbon dioxide. Once they are comfortable, they begin either cycling on the Ergometer or walking on the Treadmill and the data collection begins. As Kevin has explained, once the baseline has been established, the load is increased to make the patient progressively work harder.
Q: On a physiological level, what is happening to the patient?
A: KH: 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.
Q: So is it just the Anaerobic Threshold that we’re looking for?
A: KH: 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.
Q: So the cardiac assessment is also useful then?
A: ASP: 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.
Q: Kevin, can you explain how the data collected can be interpreted?
A: KH: 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.
Q: So what does the CPET tell us?
A: ASP: 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.
Q: In your experience, where should one site a CPET System?
A: ASP: A very good question with no simple answer. Historically CPET systems were developed for use in Respiratory Medicine but you will find that most Pulmonary Function Laboratories underutilise the equipment. This is mainly because historically the equipment has been used to differentiate a Respiratory disease after a previous Cardiac referral which proved negative. In order to take advantage of the potential benefits of CPET, the system would be best sited in a purpose built room in a Pre-Assessment Unit. The room should be well ventilated for obvious reasons with the availability of patient changing facilities. Importantly, as CPET is both demanding and potentially stressful for the patient, the availability of Resuscitation Equipment is essential. To extract the true benefits of CPET in the context of pre-operative assessment, it has often proved vital that the Anaesthetists have direct control over the use of the system in order that they can perform CPET whenever necessary.
Q: So, who should supervise a CPET session?
A: ASP: Another good question linked to the above. There is no doubt that an Anaesthetist should make him or herself available, in order to both safely guide the patient though and conduct the consultation and CPET processes. One of the important reasons for this is that again the patient needs to be supervised, in case of an emergency, by suitably qualified personnel. Agreement for support in conducting CPET can often be gained by prior discussion with both the Cardiac & Respiratory Medicine Groups, in order that either a Cardiac or Respiratory Physiologist can assist with the sessions. Ideally, from a reporting angle, it would be useful to enlist the skills of the Consultant Chest Physician in reaching an objective assessment of both the performance and outcome of the CPET Test.
Q: Kevin, what do you suggest be done to establish such a service in a hospital?
A: KH: Firstly, why not assess what services you already have working with your colleagues in Respiratory Medicine and Cardiology? See what facilities exist? Then build your business plan for the number of studies per year and the benefits to be gained by the hospital through this service. Then it just requires establishing the level of equipment that can support the service at the level you require, plus the clinic space and support or admin staff to deliver the service. None of these are obstacles that should not be overcome by discussion with hospital management teams, keen to see real savings and improved services.
Q: Can you expand on what these benefits might be?
A: ASP: Estimates show that cardio pulmonary exercise testing will allow Hospitals to reduce surgical patients’ length of stay (LOS) by a minimum of 1 day, as patients can be admitted on the day of surgery. This alone brings considerable savings. Evidence from other UK centres shows that CPET testing could well save the Hospital Trusts many Intensive Care bed days. Since some patients may be considered more likely to die from major surgery than from their underlying condition, the surgery may not take place. And if it does, their post-operative care can be better planned for and managed in advance, ensuring the likelihood of unexpected complications setting in several days later is significantly reduced.
Q: Kevin, can you give any advice how this service might expand and benefit a hospital?
A: KH: The initial pre-operative assessment itself will allow education of the patient to the very real risks they may be facing. Or it may give the confidence to proceed to a successful outcome, which thus may avoid un-necessary palliative care. However, it can also mark the commencement of a programme of fitness assessment, to encourage a patient to adopt a healthier lifestyle or fitness regime that will ultimately permit the intervention with an improved likelihood of a successful outcome. Post operatively, it can be used to continue monitoring the rehabilitation of a subject. Finally, it is worth considering that just one single CPET can yield a vast amount of data profiling the patient for a variety of hospital procedures. The test is relatively inexpensive in consumables and the capital cost can be regained in less than one month’s testing.
Q: So in summary, what do you feel will be gained?
A: KH: The greatest reward is positive outcome from interventions and less need of palliative care. Patients are better informed, educated and aware of risk and benefit. What is more, the hospital is more efficient and negative operations are minimised.
Over 20,000 patients die within a year following surgery. (Source: Modernising Care for Patients Undergoing Major Surgery, based on a report by the ‘Improving Surgical Outcomes Group’ in June 2005). Of those patients who die following surgery, the vast majority have other serious medical conditions at the time of surgery, although data from the National Confidential Enquiry into Patient Outcome and Deaths suggest that patients’ actual risk of death is not being fully appreciated prior to their surgery. The report indicates three areas of surgical care and decision making that will reduce peri-operative death, one of which points to the use Cardio-Pulmonary Exercise Testing (CPET or CPX).
Improved Pre-Operative Assessment, Triage and Preparation require objective evaluation prior to planned major surgery that can identify patients with an increased risk profile. For some, this may result in a decision not to proceed with major surgery at all; for others a change of lifestyle to improve their odds. But in all cases, the old adage applies, “if you fail to prepare, prepare for failure.”
Many involved in pre-operative assessment now find that this preparation can effectively be achieved by Pre-Operative Cardio-Pulmonary Exercise Testing.
We spoke with Anthony Silvio Phillips, Pulmolink’s specialist Sales Manager for Cardio Pulmonary exercise systems & his colleague, Kevin Hogben, Pulmolink’s Technical Director to find out more….
Pulmolink are one of the UK’s leading independent, specialist CPET & respiratory equipment suppliers.
Their Sales Manager, Anthony Silvio Phillips, has been intrinsically involved with the practicalities of running several popular CPX workshops at recent & imminent AAGBI meetings, as well as working closely with Consultant Anaesthetists up and down the country, setting up & equipping pre-op assessment clinics.
Technical Director, Kevin Hogben, has been involved with numerous respiratory function & exercise physiology training seminars in conjunction with the ARTP & BTS.
For more information please visit www.pulmolink.co.uk