*2.2. Cardiopulmonary Exercise Test*

## 2.2.1. Rationale

Exercise tolerance is driven by three factors: pulmonary gas exchange, cardiovascular performance, and skeletal muscle metabolism [23]. For functional exercise testing, the Fick equation is fundamental, stating that oxygen uptake is a product of cardiac output and the arteriovenous oxygen difference at peak exercise [24].

The cardiopulmonary exercise test (CPET) utilizes a symptom-limited exercise test on a treadmill or cycle ergometer combined with respiratory gas exchange analysis—i.e., it incorporates measurements of respiratory oxygen uptake (VO2), carbon dioxide production (VCO2), and ventilatory measures. Hence, CPET is of

great value for an assessment of cardiovascular, respiratory, and metabolic changes in response to exercise.

2.2.2. Indications

Indications for CPET include [25]:


Table 13 exhibits the American Thoracic Society (ATS) and American College of Chest Physicians (ACCP) indications for CPET [26].


**Table 13.** Indications for cardiopulmonary exercise testing.

Abbreviations: AHA—American Heart Association. Source: Adapted from [26].

CPET is widely applied in the functional assessment of patients with heart failure to determine the severity of the disease (American College of Cardiology/American Heart Association recommendation Class IIa, Level of Evidence C), to identify candidates for cardiac transplantation (American College of Cardiology/American Heart Association Recommendation Class IIa, Level of Evidence B), and to facilitate the exercise prescription (American College of Cardiology/American Heart Association Recommendation Class I, Level of Evidence C) [27].

### 2.2.3. Technical Aspects

Patients should be instructed appropriately to attain the best possible effort. Prior to each cardiopulmonary exercise test, all CPET systems should be calibrated. This should include the calibration of airflow, volumes, and both the oxygen and carbon dioxide analyzers. It should be followed by spirometry to assess resting pulmonary function: forced expiratory volume in 1 s, forced vital capacity, and peak expiratory flow. CPET starts with 1–2 min registration, followed by warm-up (2–3 min). CPET should be performed as symptom-limited, and the optimal test duration is between 8 and 12 min. Thereby, patient–staff communication techniques during the test are essential for test safety and should be discussed before, including hand signs regarding symptom scoring—e.g., the Borg scale [26]. Both cycle ergometer and treadmill protocols can be used. The initial workload on a cycle ergometer for patients with heart failure is usually 20–25 W, which is increased by 15–25 W every 2 min until maximal exertion is attained. Optionally, ramp protocol (e.g., 10 W/min) can be executed. The modified Naughton protocol is recommended for treadmill exercise testing in patients with heart failure in order to increase the workload by approximately 1 MET for each 2 min stage [24,26]. Treadmill exercise testing has advantages over cycle ergometry, as patients, particularly those who are untrained, usually terminate cycle exercise because of quadriceps fatigue at an oxygen uptake that is on average 10% to 15% lower than oxygen uptake from a treadmill [28]. Furthermore, walking is a more familiar activity than cycling and involves a larger muscle mass. Electrocardiogram and blood pressure is monitored during the test, and BP should be recorded at rest and every 2 min or during the final 2 min of each stage (in the case of non-ramp protocol). Contraindications to cardiopulmonary exercise testing and criteria for terminating the exercise test are the same as those described earlier.

### 2.2.4. Parameters

CPET allows for cardiac and ventilatory parameter assessment. The main parameters assessed in CPET include [29]:

### Peak Oxygen Uptake

Peak oxygen uptake is the volume of oxygen extracted from the air and inhaled over time, calculated in mL/min, and often normalized for body weight (mL/kg/min). One metabolic equivalent (MET) is the resting oxygen uptake in a sitting position and is equal to 3.5 mL/kg/min [5]. As oxygen uptake increases

linearly with work, achieving a clear plateau in oxygen uptake has traditionally been used as the best evidence of maximal oxygen uptake [26]. Hence, maximal oxygen uptake is the best index of aerobic capacity and the gold standard for cardiorespiratory fitness assessment. In practice, this plateau may not be achieved before symptoms, with the termination of exercise. Consequently, peak oxygen uptake is often used as an estimate for maximal oxygen uptake (maximal and peak oxygen uptake are used interchangeably). Maximal oxygen uptake depends on age, sex, weight, height, fitness level, and ethnic origin. It can also be affected by training and patient motivation. A normal value for oxygen consumption is considered to be >20 mL/kg/min, and <10 mL/kg/min value is linked to poor prognosis [30,31]. Peak oxygen uptake has been demonstrated to predict prognosis in patients with heart failure in many studies. In a prospective study of 114 ambulatory patients with heart failure referred for cardiac transplantation, an oxygen consumption of <14 mL/kg/min has been used as a cutoff for the acceptance for cardiac transplantation [32].

### Respiratory Exchange Ratio (RER)

RER is the ratio of carbon dioxide output/oxygen uptake (VCO2/VO2) and is the best non-invasive indicator of exercise intensity. RER increases during exercise due to either buffered lactic acid or hyperventilation. Resting RER is about 0.8, and RER > 1.0 indicates maximal exercise effort [33].

## Anaerobic Threshold (AT) or Lactate Threshold

During light- to moderate-intensity incremental exercise, aerobic metabolism dominates and the blood lactates level remains stable (or is only marginally elevated). This initial, aerobic phase of CPET, lasts until 50–60% of peak oxygen uptake is attained, with expired ventilation (VE) showing a linear increase with oxygen uptake. As mentioned, at this phase the blood lactate level is relatively stable, as muscle lactic acid production is insignificant. The following period of exercise, however, with increasing effort intensity, is characterized by anaerobic metabolism, since oxygen supply is insufficient with the increasing metabolic requirements of the exercising muscles. Significant increases in lactic acid production in the muscles and in its concentration in blood are observed during this phase, and greater reliance on anaerobic metabolism is needed for continued energy production [25,26]. The oxygen uptake at the onset of blood lactate accumulation is called the first ventilatory threshold or the anaerobic threshold, above which blood lactate and pH start to increase and decrease, respectively, and ventilation accelerates to eliminate the excess carbon dioxide produced during the conversion of lactic acid to lactate [34]. The first ventilatory threshold is typically attained at 60–70% of maximal oxygen uptake [35,36]. Hence, AT estimates the onset of metabolic acidosis, is related to the

oxygen uptake at which it occurs and should be expressed as a percentage of the predicted value of maximal oxygen uptake.

Several methods allow for the identification of the anaerobic threshold. AT can be determined both invasively (lactic acid) and noninvasively (ventilatory equivalent for oxygen and carbon dioxide methods: VE/VO2, VE/VCO2, V-slope, and the modified V-slope method).

In practice, the main non-invasive executed methods for estimating the anaerobic threshold are [24]:

The V-slope method: The anaerobic threshold is defined as the oxygen uptake at which the rate of increase in VCO<sup>2</sup> relative to VO<sup>2</sup> increases in the absence of hyperventilation. The AT determined by this method is a more reproducible estimate.

The ventilatory equivalents method: The AT is the VO<sup>2</sup> at which the ventilatory equivalent for O<sup>2</sup> (VE/VO<sup>2</sup> ratio) and end-tidal oxygen tension (PET O2) begin to increase systematically without an immediate increase in the ventilatory equivalent for CO<sup>2</sup> (VE/VCO2) and end-tidal CO<sup>2</sup> tension (PET CO2).

AT usually occurs at 47% to 64% of the measured maximal oxygen uptake in a healthy individual. The value of AT is important for exercise prescription, as the heart rate value at AT is crucial [25]. Optimally, the exercise training heart rate should be 5% lower than the heart rate at anaerobic threshold (10% lower in heart failure individuals).

Exercise Oscillatory Breathing or Exercise Oscillatory Ventilation (EOV)

EOV is present in heart failure and has been described as the regular alteration of tidal volume with a crescendo–decrescendo pattern. The presence of EOV is a marker of significant hemodynamic impairment and is associated with worse clinical prognosis. If present, EOV should persist for 60% of the exercise test, with an amplitude of >15% [37].

A summary of the CPET variables is presented in Table 14.


**Table 14.** Cardiopulmonary exercise test variables [38].

Abbreviations: BP—blood pressure; CPET—cardiopulmonary exercise test; HR—heart rate; PET CO2—end-tidal carbon dioxide partial pressure; RER—respiratory exchange ratio; VCO2—carbon dioxide output; VE—minute ventilation; VE/VCO2—ventilatory equivalent of carbon dioxide; VE/VO2—ventilatory equivalent of oxygen; VO2—oxygen uptake. Source: Adapted from [38].

### 2.2.5. Interpretation

Maximal effort is assumed in the presence of one or more of the criteria listed below [31,35]:

• Oxygen uptake and/or heart rate fail to increase with further increase in work rate;


Among these, a plateau in the relationship of oxygen uptake versus work rate during incremental exercise is the gold standard for the determination of maximal effort.

A CPET report should include [23]:


Norms for peak oxygen uptake for women and men at different ages:

For 20–29 years: women 36 ± 6.9 VO2, men 43 ± 7.2 VO2. For 30–39 years: women 34 ± 6.2 VO2, men 42 ± 7.0 VO2. For 40–49 years: women 32 ± 6.2 VO2, men 40 ± 7.2 VO2. For 50–59 years: women 29 ± 5.4 VO2, men 36 ± 7.1 VO2. For 60–69 years: women 27 ± 4.7 VO2, men 33 ± 7.3 VO2. For 70–79 years: women 27 ± 5.8 VO2, men 29 ± 7.3 VO2.

The severity of the functional capacity impairment during incremental treadmill testing in heart failure (Weber–Janicki classification) is shown in Table 15 [39].


**Table 15.** Functional classification of patients with congestive heart failure.

Abbreviations: AT—anaerobic threshold; mL/kg/min—milliliters per kilogram per minute; L/min/m2—liters per minute per square meter; VO2—oxygen uptake. Source: Adapted from [39].

### *2.3. Walking Tests*

### 2.3.1. Six-Minute Walk Test

The six-minute walk test (6 MWT) measures the distance that a patient can walk quickly on a flat, firm surface within six minutes (Figure 2). The test should be performed in a minimally trafficked area, optimally in an enclosed corridor, but can be also performed outdoors [40]. A six-minute walk test on a treadmill is not recommended, as patients will be unfamiliar with the machinery and attain a significantly lower walk distance [41]. The six-minute walk test is a simple and safe method of approximate functional capacity assessment. As most patients do not achieve their maximal exercise capacity when walking at their own pace, the results of the 6 MWT should be considered complementary to conventional exercise testing [42,43]. The 6 MWT is not suitable for use in exercise risk stratification, as even brisk walking at a speed of 6 km/h (i.e., at an intensity of 4 METs) will not elicit an adequate intensity threshold—i.e., 5 MET—to guide exercise risk assessment [44]. In practice, 6 MWT has been used for exercise training qualification in patients following incomplete revascularization or early after cardiac surgery. On the other hand, as walking is a natural activity, the walking test is well tolerated and easy to administer. The 6 MWT has clear advantages over treadmill walking, reflecting the real situation, and can be more suitable for the elderly and patients with musculoskeletal disorders—e.g., with knee arthrosis. Furthermore, middle-aged patients and the elderly can consider the 6 MWT as a moderate- to high-intensity test. In numerous studies, a strong correlation has been found between the metabolic equivalent estimated from the 6 MWT and conventional treadmill exercise tolerance tests [45,46]. A strong correlation between peak oxygen uptake and distance covered during the 6 MWT was achieved by adding the terminal rating of perceived exertion [47].

Figure 2. The six-minute walk test. Source: Photo by authors. **Figure 2.** The six-minute walk test. Source: Photo by authors.

2.3.2. The Two-Minute Step Test Test requirements [2]:


and iliac crest when standing [56]. Similarly, like in the 6 MWT, the Appropriate patient preparation [48]:

	- Patients should sit at the starting point ten minutes prior to the testing;

Optimally, two tests should be performed due to the patient improving during the second one (learning curve effect). An increase in 6 MWT distance by 27 min on the second walk has been documented in a study including over 1500 patients with chronic pulmonary obstructive disease [49]. The six-minute walk test is considered safe; however, testing personnel should be certified in basic life support, and immediate access to emergency equipment should be provided. Absolute contraindications for 6 MWT include <7–10 days from primary angioplasty due to myocardial infarction and <24 h from elective coronary angioplasty. Relative contraindications include resting heart rate of >120 bpm, systolic blood pressure of >180 mmHg, and diastolic blood pressure of >100 mmHg [50].

Reasons for test termination include:


The test report should include:


The 6 MWT distance can be affected by many factors—e.g., BMI 25 kg/m<sup>2</sup> and age ≥75 years were found to be independent predictors of poor performance (i.e., <300 m) in a study by Pepera that included patients with heart failure. Furthermore, patients with heart failure have been found to have a shorter step length and to walk more slowly than controls during the 6 MWT [51].

Gibbons et al. attempted to determine the best 6-min walk distance from several repetitions in healthy volunteers. The reference value they found for 6 MWT was 698 ± 96 m [52].

Enright and Sherill developed reference equations for women and men to utilize the percentage of predicted 6 MWT distances [53]:


For patients after coronary artery bypass surgery and valve surgery, two formulas have been proposed; these include sex, the presence of diabetes, and atrial fibrillation [54]. The translation of the 6 MWT results into exercise prescription has been suggested, with 80% of the average 6 MWT speed corresponding to a high but tolerable exercise intensity and resulting in training benefits [55].

### 2.3.2. The Two-Minute Step Test

The two-minute step test (2 MST) was introduced in 1999 by Rikli and Jones as part of the Senior Fitness Test. The test requires that tested individuals march in place as fast as possible for 2 min while lifting their knees to a height midway between their patella and iliac crest when standing [56]. Similarly, like in the 6 MWT, the patients can slow down or stop the test if necessary. The norm for the 2 MST is 65 steps or more [57]. Rikli and Jones have demonstrated good inter-test reliability [58].

### 2.3.3. The Incremental Shuttle Walking Test

The incremental shuttle walk test (ISWT) is a symptom-limited, externally paced test performed along a 10 m course and involving walking back and forth between two cones set 0.5 m from either end of a 10 m course. The initial walking speed, indicated by an audible signal, is increased each minute until the patient is too fatigued to continue or cannot maintain the required speed. The number of completed shuttles—i.e., the ISWT total distance covered—is the test outcome. Unlike in the 6 MWT, there seems to be no learning effect in the ISWT [59]. In addition, the turning maneuver has no impact on the test result in stable patients with cardiovascular disease unless reduced mobility due to orthopedic limitations is present [60].

A relationship between the number of shuttles completed and the maximum oxygen uptake has been demonstrated, supporting the potential role of ISWT as a valuable tool for assessing changes in patients' functional capacity during cardiac rehabilitation. Numerous studies have addressed variables affecting test performance, with the ISWT distance correlating most strongly with step length and

height [61]. As expected, the ISWT distance covered by men was further than that by women, with mean values of 395 and 269 m, respectively [62]. The ISWT results have been translated into exercise training intensity by the prescription of the walking exercise at an intensity equal to 70% of the peak ISWT speed [63].
