Practical approaches to exercise testing

Presently, regular physical activity (PA) is considered to be an important part of treatment for the majority of cardiovascular diseases (CVDs). It decreases cardiovascular and total lethality rates [1]. At the same time, despite evident benefits, PA can provoke sudden cardiac arrest in people with CVDs, especially, in those who had a sedentary lifestyle before or had a high risk of cardiovascular events [2].

An exercise test (ET) allows specialists to evaluate any abnormal cardiovascular response (symptoms, electrocardiography (ECG) findings, arrhythmias, anomalous blood pressure (BP) response) that can occur during routine activities. Based on the test results, the doctor can choose the intensity, regimen, and duration of physical activities that are suitable for a particular patient [3]. ET is an integral part of examination of patients with cardiologic issues as a method of functional diagnostics [2]. According to the recent international guidelines, ET is performed as a cardiopulmonary exercise test (CPET) [4][5]. Stress testing with additional measurement of gas metabolism parameters is the most reliable diagnostic tool that provides significant diagnostic and prognostic information on the status of patients with cardiovascular and pulmonary diseases [6].

CPET is widely used in rehabilitation, exercise therapy, and fitness centers. However, the wide implementation of CPET into clinical practice of cardiologic departments is limited by such factors as complexity and cost of the equipment, and requirements for highly qualified and trained medical specialists. Often, the application of CPET complicates the procedure of testing with a physical load. Both cardiologists and patients understand that ET is an additional diagnostic tool that can only indirectly influence the treatment outcome. In the clinical practice of cardiologic departments, the significance of the CPET-provided data does not overweight the additional cost of the therapy. The exclusion of the pulmonology component from ET makes it more available, simpler, and more often used in clinical practice.

However, the guidelines of the leading cardiologic associations (European Society of Cardiology – ESC, European Association for Cardiovascular Prevention & Rehabilitation – EACPR, American Heart Association – AHA, American College of Cardiology – ACC) on ET issues, published within the past 20 years, include CPET. The only guideline published after 2020, which allows the specialists to perform cardiologic ET and not cardiopulmonary ET (considering evidence-based medicine), is the ACC/AHA 2002 Guideline for Exercise Testing [7]. Due to the recommendations of these guidelines, Russian doctors can use ET more often.

Presently, the following types of ET are used in clinical practice in Russia: (1) cycloergometric test (CET) and (2) treadmill test (TMT). CET and TMT supplemented with a gas analyzer is called CPET or ergospirometry [8]. Some ETs can be classified as screening tests: (3) Harvard step test; (4) Ruffieux test and other rarely used screening tests (Cooper, Flac, Dubovskiy, Crampton, Kushelevskiy, Kverg, Kotov-Demin, etc.), because during these tests, it is impossible to monitor the most important parameter of heart rate (HR) that is available on modern training equipment. Some ETs reviewed in Russian and foreign guidelines [2][5][9] are characterized as tests of certain conditions. In particular, it is a (5) six-minute walk test (6MWT) that is used for patients with verified heart failure [10] and conducted during ET with low loads [4]. ET with additional visualization (6) is performed in specialized cardiologic centers. The visualization of the results is provided by semi-invasive computed tomography, magnetic resonance imaging with gadolinium, and single-photon positron emission tomography. These methods are more complicated and expensive but more feasible in combination with pharmaceutical drugs (adenosine and dobutamine) [2].

It is necessary to consider contraindications to ET and indications for ET discontinuation. In the majority of guidelines, they are specified by ACC/AHA 2002 [7] with certain corrections according to ESC, 2016 and 2018 [5][11]. Absolute contraindications to ET are acute myocardial infarction (up to 2 days); unstable angina; uncontrolled ventricular arrhythmia with subjective symptoms and/or hemodynamics impairment; symptomatic severe stenosis of the aorta; decompensated heart failure; pulmonary artery thromboembolism; acute myocarditis or pericarditis; and acute aortic dissection. Relative contraindications to ET include the stenosis of the left main coronary artery; moderate stenosis aortic valve stenosis; electrolytes disorders; severe arterial hypertension (180/110 mmHg); tachycardia/bradycardia; hypertrophic cardiomyopathy, and other forms of blood outflow disorders; psychic or physical impairments that lead to exercise performance failure, and atrioventricular block of high degree.

During ET, it is important to consider not only target parameters (HR) but also the patient’s perceived exertion by the Borg scale [12] and indications for ET discontinuation. Absolute indications for ET discontinuation include a decrease in sBP by > 10 mmHg from the baseline; angina attack, neurologic symptoms (ataxia, dizziness, fainting); signs of hypoperfusion (cyanosis or paleness); the patient’s will to stop the test (Borg score >16); sustained ventricular tachycardia; the rise of ST (≥1.0 mm) in the leads without diagnostic waves Q. Relative indications for ET discontinuation include changes in ST or QRS, depression of ST (>2 mm, horizontal or descending) or a significant shift in the axis; arrhythmias different from sustained ventricular tachycardia, including multifocal ventricular extrasystoles, supraventricular tachycardia, heart block or bradycardia; fatigue, dyspnea, rales, leg cramps or limping; His bundle blockade or intraventricular conduction abnormalities; increasing chest pain; hypertonic response (sBP >250 and/or dBP >115 mmHg).

Thus, for the evaluation of physical endurance/exercise tolerance, it is important to monitor the dynamics of such parameters as ECG, HR, BP, and patient’s health status.

ECG parameters during ET are often discussed in literature and barely changed from 2002. The most important ECG data are depression and rise in the ST segment. The most frequently used definition for the interpretation of the positive result of ET is ≥1 mm of the horizontal or descending depression or increase in the ST segment for at least 60–80 ms after QRS [3][13]. As a rule, for the dynamic control of ischemic manifestations, rhythm disturbances, and heart conduction, it is sufficient to monitor continuously ECG at the level of the 2nd standard lead with ECG readings records in 12 leads after submaximal HR. It is necessary to consider the patient’s perceived exertion. Subjective sensations of the patient should be interpolated on the Borg scale. For patients with established CVD or high/very high cardiovascular risk, 14–16 points are sufficient at the level of speaking test (physical exercises are difficult for the patient but speaking is comfortable). The specified level corresponds to HR 140–160 bpm, which is a submaximal HR at the level of 75–90% in the majority of cases.

Sinus tachycardia with HR >90 bpm accompanies physical load (in the lack of pharmacotherapy) according to the level of physical development of a certain patient. However, there are safe limits for an increase in HR that are identified in patients with high/very high cardiovascular risk >140 bpm and HR >160 bpm in patients with low cardiovascular risk [14]. It should be noted that there are no sex-related differences in the calculated maximal HR (HRmax) [2][5]. It should be highlighted that the calculated HRmax does not necessarily coincide with individual HRmax. For patients with established CVD or high/very high cardiovascular risk, HRmax is achieved during ECG monitoring considering the Borg scale (17–18). Further, it is important to calculate pulse reserve (pHR) [rHR = maxHR – rest HR] [2]. ET with HR 75–90% of the calculated HRmax is called submaximal. Maximal ET is possible in people with suspected CVD, professional sportsmen, and low/moderate cardiovascular risk (SCORE). The achievement of the calculated HRmax [HRmax = 220 – age] is possible only together with ECG monitoring (19 points by the Borg scale). At the same time, an increase in HR > 160 bpm is not recommended.

An increase in the BP during TE is a normal response of the organism to physical load. The mean increase in sBP during progressive ET is around 10 mmHg/MET (1 MET – level of metabolism in the resting state, which is 1 kcal/kg/h). After the maximum load, sBP usually decreases due to a quick reduction of the cardiac output. It reaches the level of the resting state or lower within 6 minutes and even remains lower than level observed before the load for several hours. In the cases of “abnormal” types of reactions to ET, the normalization period is delayed [15]. A hypertensive reaction should be considered pathological in healthy people during ET when sBP increases ≥20 mmHg and/or dBP increases ≥115 mmHg, or when sBP increases ≥20 mmHg and/or there is any increase in dBP that led to worsening of the patient’s condition and discontinuation of ET. In patients with hypertensive disease (HD) and those who have high/very high cardiovascular risk, an increase in BP to the level of “severe HD” (degree III of BP increase) during ET is pathological or critical. According to the recommendations of ACC/AHA 2002, which often provide a basis for Russian guidelines, this level of BP is 200/110 mmHg, while since 2018, the level of BP ≥180/ 110 mmHg is considered pathological and should be an indication for ET discontinuation [11].

In Russian literature, there are up to 5 types of organism reactions to ET [8][13]. At the same time, ESC 2016 distinguishes only 3 types of BP reactions during ET [5].

Hemodynamic reaction during ET (ESC, 2016) can be of three types: normal reaction – increase in the sBP by 10–20 mmHg and insignificant increase in dBP; hypertensive reaction – significant increase in sBP ≥ 20 mmHg and/or increased dBP mmHg; and hypotensive reaction – unchanging or decreased sBP.

Considering the fact that recommendations of ESC-2020 on sports cardiology do not define ET test as a diagnostic test for HD, this significantly simplifies the interpretation of ET. ESC-2020 on sport cardiology considers arterial hypertension in ET a parameter of cardiovascular risk and indication for ET discontinuation [2].

Interpretation of absolute and relative levels of BP as a criterion of cardiovascular reaction to ET is constantly changing. At the same time, presently, guidelines are becoming more subjective. It is believed that it is important to consider not only absolute levels of BP but also the patient’s perceived exertion. The main reason that allows for the interpretation of hypertensive reaction as pathological is the sensation of the patient who stopped ET. It should be mentioned that ESC, 2020 recommends postponing ET to the normalization of BP in patients with degree II of HD (in patients with sBP in the resting state >160 mmHg) [2]. The highest cardiovascular risk to PA is a hypotensive reaction. It should be mentioned that earlier, hypotensive response was defined as the lack of sBP increase minimum by 20–30 mmHg [3][16], lack of increase in sBP more than 120 mmHg [5], or progressive decrease in sBP during PA [15]. Presently, a hypotonic response is defined as an unchanged level of BP that led to worsening of the patient’s condition and discontinuation of PA [2][5].

The last obligatory component considered during ET is the perceived exertion of the patient. The universal evaluation inventory used for this purpose is the Borg Rating of Perceived Exertion. It should be mentioned that any subjective rating scales are imperfect and non-absolute, which led to different modifications and updates of the Borg scale.

The WHO’s recommendations in ET (2020) [1] highlight the modified variant of the scale – Borg CR10 (Table 1) [17][18].

Table 1

It should be noted that the modified variant of PA measurement presented in Table 1 is more suitable not for specialists but for the general population and TMT, wherein MET is more often used.

However, experience showed that it is better to use the Rating of Perceived Exertion Scale, Borg RPE Scale® [15][16] because the application of the modified and added scale only misleads specialists.

The main ETs that can monitor the main parameters of physical load tolerance (ECG, HR, BP, and perceived exertion) without significant time and cost input are CET and TMT.

CET is one of the main exercise load tests that allows for dosing of ET in Watts (W). CET can be step-increasing, continuous, and mixed. During continuous CET, the load starts from 1 W/kg of body weight, every 2 minutes it increases by 1 W/kg until the aims of the testing are achieved. In the mode of step-increasing testing, PA increases discretely with a break for rest or without them. Constantly step-increasing mode is primarily used with a 3-minute step. “Pedalling” is performed at the rate of 60 rpm. Usually, the load increases by 25 W at each step starting with 50 W. When ECG is recorded from 12 leads (modified Mason-Likar leads), electrodes from arms are placed in the area of clavicles, and electrodes from legs – in the area of iliac bones [3].

TMT allows specialists to dose physical load by changing the speed and angle of inclination of the moving platform. It is dosed in MET and does not have significant advantages in comparison with CET. CET is used only in step mode without intervals with a gradual increase in the load. By different approaches, the number of steps varies from 7 to 15 with the duration of steps from 1 to 3 minutes. The standard Bruce’s protocol or modified Bruce’s protocol is based on 3-minute steps of PA. TMT is dosed to achieve the target values (manifestations of the signs of intolerance or HRmax) [9][19]. Working efficiency is defined as “double product” (Robinson,1967) = HR · sBP. The criteria of adequate TMT include the achievement of stage IV (13 MET), double project ≥20,000, and HRmax 85% at normal ECG. In clinical conditions, the double product is the equivalent of oxygen consumption by the myocardium. In healthy men, this index is 29,000–31,000, in patients with ischemic heart disease, it decreases (in some guidelines, this index is divided by 100 to simplify the calculations).

It should be noted that for cardiovascular testing, the type of ET (TMT, CET, etc.) is not so important as the achievement of the target HR. Modern TMT and CET allow for continuous monitoring of HR by the patient or medical personnel.

What is the difference between CPET and CET or TMT?

First, it differs by the selection of patients for ET. During CPET, a significant difference in the results is observed in patients with cardiovascular or bronchopulmonary diseases associated with pulmonary hypertension. Second, it differs by several parameters that might affect the interpretation of ET.

What are the main parameters that are evaluated during CPET that distinguish it from CET and TMT?

The most frequently used CPET parameter in sports cardiology is VO₂ that shows the peak (maximum) levels of oxygen consumption or aerobic capacity (VO₂max) [2][5][20][21].

Below is the table proposed by the guidelines “Sports cardiology” (Table 2).

Table 2

It should be noted that the parameters presented in the Table target aerobic loads and this is CET and TMT. Further, it can be seen that the respective VO₂max is used not only with the percent of HRmax but also the Borg scale. Besides, “Sports cardiology” 2020 ESC specifies that in the majority of cases, rHR [HRmax – HR rest] can replace the parameter VO₂max in CPET [2].

Conclusion

Even though CPET is a modern diagnostic method that provides a significant volume of additional information, it should be indicated differentially. In particular, it should be performed in patients with bronchopulmonary and some CVDs associated with the development of pulmonary hypertension (for example, in patients with congenital heart disease or severe chronic heart failure). In patients without the specified diseases, CPET is not feasible and only complicates ET. CET and TMT provide all the necessary ET data for cardiologic patients.