How to determine exercise intensity - the tricks of cardiopulmonary exercise testing

With the increasing pressure of life, sitting for long periods of time, eating greasy food, staying up late, and reducing daily activities have gradually become common bad behaviors in all walks of life and different age groups, and the incidence of obesity, diabetes, metabolic syndrome, coronary heart disease, and cerebrovascular diseases has also increased. Statistics show that the prevalence of hypertension in my country has increased fourfold from 5.15% in 1958 to 23.2% in 2012, while the rate of dyslipidemia in people aged 18 and above has increased from 18.6% in 2002 to 40% in 2012, and the group obesity rate has increased from 4.8% in 2002 to 11.9% in 2012. The long-term treatment of these chronic diseases and the continuous management of sequelae will undoubtedly bring heavy burdens to patients' families and society.

According to the physical activity and sedentary behavior guidelines released by the WHO in 2020, regular and quantitative physical activity can effectively prevent and help control the occurrence of heart disease, type 2 diabetes and cancer , which account for nearly three-quarters of global deaths. At the same time, the guidelines indicate that all physical activity is beneficial and everyone can benefit from increasing physical activity and reducing sedentary behavior. However, although the guidelines have given different types and durations of exercise recommendations for people of different ages, health or illness, there are still many patients with chronic diseases who do not quite understand the so-called "low-intensity" and "moderate-intensity" exercise. Is it calculated by time or by changes in heart rate? For this reason, the editor will give you a simple explanation, hoping to help you understand exercise intensity in your daily exercise.

Strictly speaking, exercise intensity is a quantifiable indicator, and there are many clinical evaluation methods, including the Mets method, oxygen uptake method, heart rate method, and self-fatigue perception method . The same moderate intensity may mean different levels of oxygen uptake or target heart rate values ​​for different people. Therefore, it is recommended that patients with chronic diseases, especially those with coronary heart disease and cerebrovascular disease, consult professionals for a cardiopulmonary exercise test before going home to exercise after being discharged from the hospital, so as to understand their exercise capacity and exercise within a safe range to avoid unnecessary injuries while gaining benefits.

**Cardiopulmonary exercise testing (CPET)** is currently the gold standard for testing exercise capacity. It simultaneously evaluates the cardiovascular and respiratory system responses to the same exercise stress by measuring gas exchange in the airways. While measuring gas exchange, it is also possible to observe changes in the electrocardiogram, heart rate, and blood pressure in real time, and understand the cardiovascular and pulmonary circulation responses under different oxygen consumptions, i.e., exercise intensities, thereby evaluating the exercise capacity of the tester and providing a scientific quantitative basis for setting the appropriate exercise intensity. Figure 1 concisely and clearly reflects the relationship between pulmonary respiratory function (intake VO2 and VCO2) and cellular respiration (consumption QO2 and QCO2) coupled through the circulatory system.

Figure 1: Gas transport mechanism coupling cellular respiration (internal respiration) and pulmonary respiratory function (external respiration) (This figure is from "Principles and Interpretation of Cardiopulmonary Exercise Test")

Under normal circumstances, the human body's oxygen intake and oxygen consumption are in balance, and cardiopulmonary exercise testing breaks this dynamic balance through extreme exercise intensity stimulation, amplifying the abnormalities of various organs involved in the internal and external respiration processes. Therefore, in the early stages of exercise limitation caused by any reason, CPET can help identify the approximate cause or direction of intolerance, thereby greatly saving the time and cost of clinical examinations.

There are two commonly used test methods in clinical practice: the treadmill test and the bicycle test. The treadmill test is performed by having the test subject wear a mask connected to a gas sensor and walk, jog, or run at a set test speed and slope.

The bicycle test can accurately calculate the exercise power. The bicycle exercise can be performed while sitting or lying down. Through the gradual increase of the load, the patient's maximum exercise tolerance is gradually reached. These two commonly used test methods have their own advantages and disadvantages. In clinical practice, they can be selected according to the patient's condition.

Table 1: Comparison of characteristics of two maximal exercise tests

Cardiopulmonary exercise testing is used to guide exercise rehabilitation or daily exercise tolerance. The most common method is to develop an individualized exercise plan based on the test results, including maximum oxygen uptake (VO2max) and maximum heart rate (HRmax). According to the latest exercise guidelines for cardiovascular patients published by the European Society of Cardiology in 2020, low-intensity exercise refers to oxygen uptake during exercise below 40% VO2max, and moderate-intensity exercise refers to oxygen uptake during exercise maintained at 40-69% VO2max. An effective half-hour of moderate-intensity exercise recommended by the guidelines actually requires oxygen uptake to be maintained at 40-69% VO2max during half an hour of cycling or brisk walking. Such exercise under oxygen uptake monitoring usually requires wearing professional portable equipment to complete. So how to monitor and control the intensity of exercise for daily exercise after returning home? In fact, you can also use the heart rate monitoring method. Through the maximum heart rate obtained after CPET evaluation, set the target heart rate for low or moderate intensity exercise. During exercise, according to the changes in heart rate displayed on the bracelet, you can understand whether you have reached a certain intensity when walking, cycling or running.

Table 2: Intensity indicators for various types of exercise (refer to the 2020 European Cardiology Guidelines)

It can be seen that completing a cardiopulmonary exercise test can not only determine whether your cardiopulmonary and skeletal systems can safely complete exercise, but also determine the specific amount of exercise, so as to achieve effective exercise goals and gradually improve cardiovascular function and exercise endurance. In fact, this is just the tip of the iceberg of cardiopulmonary exercise testing. It can also be used to identify the cause of exercise intolerance, help determine the severity of heart disease, and perform preoperative risk assessment for surgical operations.

The popular science content of this platform has been funded by the China Association for Science and Technology's Science Popularization Department's 2022 National Science Literacy Action Project "National Society Science Popularization Capacity Improvement Project-Rehabilitation Science Popularization Service Capacity Improvement Action Plan"