Oxygen challenge testing or gas analysis are the names by which cardiopulmonary exercise testing (ergospirometry) is popularly known.
It has many similarities to a conventional stress test, but incorporates technology to measure the amount of air entering and leaving the lungs and the concentration of oxygen and carbon dioxide in the exhaled air.
What do these tests consist of?
The patient makes a progressive effort on a cycloergometer, a treadmill or a rowing machine, as in a common stress test. The duration will be approximately 5-15 minutes, depending on the time it takes the patient to reach fatigue.
As in conventional stress tests, the patient is monitored by electrocardiogram, blood pressure, symptoms, sensation of exertion and sometimes even a pulse oximeter is used.
The patient must wear a rubber mask that seals the nose and mouth. This mask makes it possible to measure the gases inhaled and exhaled by the patient from his own environment.
What are the differences between the test with and without oxygen consumption?
The stress test without gas analysis makes it possible to calculate the patient’s exercise capacity approximately, as well as to study the symptoms, the behavior of the heart rate, the blood pressure and the electrocardiogram signal while the patient is exercising. In some cases this is sufficient. However, if this information is combined with the gaseous exchange data, the potential of the test and the data that can be obtained increases exponentially.
What data is obtained through these tests?
One of the most important data that the test gives us is the maximum oxygen consumption (VO2 max), which refers to the maximum aerobic power that an individual can develop and is the clinical indicator that is most closely related to the survival of individuals. It is known as the “universal prognostic marker” and can only be accurately measured by this test.
In addition, it will also give us the thresholds of metabolic transition and the intensity of the effort that the person is developing and the amount and proportion of energy substrates (fatty acids and carbohydrates) required for a given intensity of effort.
We can also study the behavior of pulmonary ventilation, if air is retained with exercise, if the ventilatory capacity limit is reached, the efficiency of ventilatory effort….
All these data can be very important in some patients and it is not possible to know them from a conventional stress test.
What is this test used for and in which cases?
It can be said that the test has 2 main applications. The sports application and the clinical application.
Knowing the maximum aerobic power and metabolic transition thresholds can help athletes to know the effect of a training program or to program training loads in a precise way. It also gives them information on the proportion of energy they obtain from fats and carbohydrates when working at a given intensity.
In cardiac rehabilitation, as with athletes, it is also used to calculate training loads and to check improvement with the physical exercise regimen.
Tests that are clinically indicated are used for diagnostic and prognostic purposes and for objective functional assessment of patients with disease or symptoms.
The main indication for clinical ergospirometry is the functional and prognostic assessment of patients with heart failure. The information we obtain from the exercise test with oxygen consumption is equivalent in importance to that obtained with an echocardiogram. In other words, all patients with heart failure, in the same way that they have an echocardiogram, should also have a stress test of this type. This same indication can be transferred to patients with pulmonary hypertension, heart valve pathology (valvular heart disease), and congenital heart disease.
In the same way as with heart disease, it can also be used to evaluate patients with respiratory diseases (chronic bronchitis, pulmonary emphysema, pulmonary fibrosis, etc.).
In addition, it is very useful to evaluate the response to any type of treatment in all these pathologies.
The other major indication is the diagnosis of the causes of fatigue. It is a very useful test answering the question “how much a person gets tired” and “why”. In addition to giving us an accurate quantification of their functional capacity and the possibility of comparing the patient with their reference population, it also helps to identify possible causes. It can guide us towards a respiratory, cardiological or anemia cause, or even if it is only a subjective or functional limitation. This allows us to orient the subsequent study and to know the limiting factor in patients with different pathologies.
There are other indications, such as the preoperative functional assessment of patients with lung neoplasms. This indication is also being extended to patients undergoing bariatric or digestive system surgery and other major surgeries, in order to obtain information about surgical risk.
It can be said that the cardiopulmonary stress test is the technique that best characterizes the stress physiology of different types of patients.