Clinical Usefulness of Response Profiles to Rapidly Incremental Cardiopulmonary Exercise Testing

The advent of microprocessed “metabolic carts” and rapidly incremental protocols greatly expanded the clinical applications of cardiopulmonary exercise testing (CPET). The response normalcy to CPET is more commonly appreciated at discrete time points, for example, at the estimated lactate threshold and at peak exercise. Analysis of the response profiles of cardiopulmonary responses at submaximal exercise and recovery, however, might show abnormal physiologic functioning which would not be otherwise unraveled. Although this approach has long been advocated as a key element of the investigational strategy, it remains largely neglected in practice. The purpose of this paper, therefore, is to highlight the usefulness of selected submaximal metabolic, ventilatory, and cardiovascular variables in different clinical scenarios and patient populations. Special care is taken to physiologically justify their use to answer pertinent clinical questions and to the technical aspects that should be observed to improve responses' reproducibility and reliability. The most recent evidence in favor of (and against) these variables for diagnosis, impairment evaluation, and prognosis in systemic diseases is also critically discussed.


Introduction
Cardiopulmonary exercise testing (CPET) provides a means of unraveling abnormal physiologic functioning which may not be apparent at rest [1,2]. The advent of microprocessed CPET systems [3] increased our technical capabilities in recording several variables throughout a single exercise bout-even of a relatively "short" duration of 10 minutes [4,5]. The response normalcy to rapidly incremental CPET is more commonly judged by comparing the observed values at discrete time points (e.g., at the estimated lactate threshold (LT) and at peak exercise) with those previously obtained in apparently healthy subjects [6,7]. It should be noted, however, that relying only in such discrete analysis leads to substantial loss of physiologic information given by the observation of the responses profiles during submaximal exercise and recovery [8][9][10][11].
The purpose of this brief review, therefore, is to emphasize the practical usefulness of analyzing the response profiles of selected variables during rapidly-incremental CPET. Special care is taken to physiologically justify their use to answer relevant clinical questions and to the technical details that  Figure 1: Noninvasive estimation of the lactate threshold by the -slope method (gas exchange threshold (GET), panel (a)) and the ventilatory method (ventilatory threshold (VT), panel (b)) in a normal subject. Note that the GET slightly precedes the VT as the later depends on the ventilatory response to the "extra-CO 2 " generated by buffering of H + associated with (lactate) increase. 1 and 2 refer to the two sequential slopes (before and after the GET) with 2 being characteristically steeper than 1 (i.e., slope inclination >1.) should be observed to improve responses' reproducibility and reliability. The response profiles to be discussed, however, are applicable to ramp-incremental [4] cycle ergometry, and the practitioner should be aware that different patterns of response can be anticipated if other ergometers (e.g., treadmill) and protocols (e.g., step-like) are used.

Estimated Lactate Threshold
2.1.1. Physiological Background. The rate at which arterial lactate anions [Lac − ] a and the associated proton (H + ) accumulate as exercise progresses is directly related to the ratio between lactic acid (LA) release as a final byproduct of muscle anaerobic glycolysis and LA clearance by metabolism and buffering [29][30][31]. Although there seems to exist a period of time-not a discrete time point-in which LA production exceeds its rate of clearance, the term LA "threshold" (LT) [32,33] is widely used. LA production increases as tissue O 2 delivery diminishes [34] though some LA can be produced without any evidence of tissue hypoxia [35]. This justifies the notion that LA release during exercise is a reasonably sensitive (albeit non-specific) [36] marker of tissue anaerobiosis.
LA dissociates fast in Lac − and H + in the physiological pH; that is, it is a strong acid. Plasma bicarbonate (HCO 3 − ) is the main buffer of lactic acidosis leading to the formation of carbonic acid (H 2 CO 3 ) which in turn dissociates into carbon dioxide (CO 2 ) and water; that is, Although this reaction has the advantage to turn a fixed acid into a volatile gas, the "extra-CO 2 " (approximately 22-26 mL of additional CO 2 is produced from each mEq decrease of [HCO 3 − ]) [31] derived from buffering of Lac −associated protons will not only accelerate CO 2 output (̇C O 2 ) relative to O 2 uptake (̇O 2 ) but also stimulate ventilation (̇E). These phenomena underlie the techniques for a noninvasive estimation of the LT.

Technical Considerations.
As LA is buffered by HCO 3 − , CO 2 increases (1) out of proportion oḟO 2 , and a plot between these variables will show a discernible breakpoint; that is, thėC O 2 -̇O 2 relationship evidences an increased slope at the point of [Lac − ] a increase. This is more commonly referred as the gas exchange threshold and determined by the V-slope method (Figure 1(a)) [37]. Increase iṅC O 2 will drivė E in its direct proportion leading the latter to increase faster thaṅO 2 . The consequent increase iṅE/̇O 2 (and the endtidal partial pressure for O 2 , ET O 2 ) with a stablėE/̇C O 2 (and ET CO 2 ) establishes the so-called ventilatory threshold (Figure 1(b)) [38]. It should be noted that despite reflecting the same phenomenon (LA buffering), the gas exchange threshold slightly precedes the ventilatory threshold (VT) (Figure 1). After the LT,̇E/̇C O 2 and ET CO 2 remain stable for a variable period of time during the "isocapnic buffering". However, as more H + is released with further increases in work rate,̇E eventually increases out of proportion tȯC O 2 at the respiratory compensation point (RCP) thereby leading to alveolar hyperventilation and progressive reductions in ET CO 2 towards the end of the test (Figure 1(b)).
Irrespective of the denomination, the following technical aspects for the LT estimation should be noted: (1) automatic estimations (by the CPET software) should  be viewed with caution and routinely double-checked  with manually determined values; (2) if an unitary tangent is used to estimate the LT in the -slope plot, the range oḟO 2 anḋC O 2 values should be the same as any discrepancy would invalidate its underlying mathematical (and physiological) principles [37] (Figure 1(b)); (3) use of discrete R (̇C O 2 /̇O 2 ) values (i.e., > 1 from tabular data) as indicative of the LT might lead to erroneous estimations; (4)̇O 2 at any particular WR during a ramp-incremental test is lower than the steady-statėO 2 value at that same WR due to a variablėO 2 kinetics delay. As a result, the WR corresponding tȯO 2 LT precedes the WR in which the LT was identified by approximately 30-45 s (or even more in patients) [4]. Accordingly, if one is interested in exercising a subject at thėO 2 LT, the selected WR should lead the WR-LT by this timeframe; (5) a given change iṅE has a greater effect on CO 2 release than O 2 uptake by the lungs; consequently, preexercise hyperventilation may deplete the amount of CO 2 stored in the body without major effects on O 2 stores [39]. As the body capacitance for CO 2 increases during the early phase of the ramp, repletion of the CO 2 stores slowṡC O 2 relative tȯO 2 ; that is,̇C O 2 -O 2 slope in this region becomes shallow (" 1 " in Figure 1(a)). As the body CO 2 reservoirs are filled in with exercise progression, the rate of CO 2 storage will decrease thereby acceleratinġC O 2 relative tȯ CO 2 [40]. This might mistakenly suggest the onset of lactic acidosis, that is, a "pseudo-LT" [41]. Precautions should therefore be taken to avoid hyperventilation prior to the noninvasive estimation of LT by theslope method; (6) LT should always be expressed relative to predicteḋ O 2 peak not to the attaineḋO 2 peak, especially in patient populations where the latter procedure might create a false concept of preserved (or even increased) O 2 LT, and (7)̇O 2 peak declines with senescence at a steeper rate thaṅO 2 LT; that is,̇O 2 LT (%̇O 2 peak) increases as a function of age in both genders [41][42][43].

Clinical Usefulness.
The physiologic changes associated with [Lac − ] a and H + accumulation (e.g., metabolic acidosis, impaired muscle contraction, hyperventilation, and altereḋ O 2 kinetics) are important to document clinically as they are associated with reduced cardiopulmonary performance. An early LT is a marker of impaired aerobic metabolism [44][45][46][47][48][49] due to insufficient O 2 delivery, increased recruitment of fast-twitch type II fibers which are metabolically less efficient than the slow-twitch type I fibers (i.e., have a greater O 2 /ATP ratio), and/or mitochondrial enzymatic dysfunction. The isolated analysis of the LT does not allow the differentiation of cardiovascular limitation from sedentarity though a severely decreased LT (e.g., <40% predicteḋO 2 peak) [6] is more frequently found in patients. A low LT has been found useful to predict an increased risk of post-operatory complications in the elderly [50,51], worse prognosis in chronic heart failure (CHF) [52], and disease severity in pulmonary arterial hypertension (PAH) [53]. On the other hand, improvements in LT after pharmacological and nonpharmacological interventions have been associated with increased functional performance in a range of clinical populations [54][55][56][57][58][59][60][61][62][63][64][65][66][67][68][69]. Although there is lack of evidence that training at (or above) thėO 2 LT is essential to improve exercise capacity in patients with CHF, coronary artery disease (CAD), and chronic obstructive pulmonary disease (COPD), training at higher intensities elicits larger physiological adaptations in less severe patients who are able to tolerate such regimens [54,70,71]. Training at thėO 2 LT also seems to reduce the risk of complications during early phases of cardiac rehabilitation [72,73]. In patients with COPD, however, LT cannot always be identified (even using the V-slope method), and when identified it varies widely as expressed iṅO 2 % peak [74]. In fact, important subjective improvements after rehabilitation can be found despite the lack of measurable physiological effects [75] which casts doubt on its usefulness to target exercise training intensity in these patients.

Δ Oxygen Uptake (̇2)/Δ Work Rate (WR)
2.2.1. Physiological Background. From a relatively constant value of 500 mL/min at unloaded pedaling,̇O 2 increases linearly as exercise progresses during a rapidly-incremental exercise test [4]. The slope of the Δ̇O 2 /ΔWR relationship, therefore, is an index of the overall gain of thėO 2 response, and normal values would indicate adequate metabolic cost for the production of a given power output [4,8].

Technical Considerations.
For an accurate calculation of the Δ̇O 2 /ΔWR slope, any delay iṅO 2 increase at the start of the ramp or any eventual plateau near the end of exercise should be discarded (Figures 2(a) and 4). Considering that the LT can potentially distort the response's linearity [157][158][159][160], it is advisable to check if there is an inflection point in the Δ̇O 2 /ΔWR at the LT. If this is discernible, the slope should be calculated over the sub-LT range.

Interpretative Issues. Δ̇O
2 /ΔWR is not significantly influenced by the training status, ageing, or gender ( Figure 3(a)) [2,10,[12][13][14]. A shallow Δ̇O 2 /ΔWR over the entire range of values and/or a shift from a linearly increasing profile to a shallower rate of change has been shown to be indicative of circulatory dysfunction [77][78][79][80] ( Figure 4) and severe impairment in mitochondrial function [81]. The latter pattern of response has been found to enhance ECG sensitivity to detect myocardial ischemia [82][83][84][85][86], and some studies suggested that it might be useful to unravel early abnormalities in the coronary microcirculation [87,88].     Figure 3: The submaximal relationships depicted in Figure 2 as a function of age in males (left panels) and females (right panels). Regression lines are shown with their respective 95% confidence intervals for those relationships in which the variables were influenced by age. Regression coefficients and intercepts of the linear prediction equations are depicted with their respective standard error of the estimate (SEE). (Modified with permission from [10]).

Physiological Background.̇E increases curvilinearly relative tȯO
2 in response to a ramp-incremental exercise test. At least in theoretical grounds, several variables known to interfere with botḣE anḋO 2 would bear an influence in this relationship; that is, it is deemed to be modulated by cardiovascular, pulmonary, and muscular factors [161][162][163][164][165][166][167][168]. Most authors have expressed thėE-̇O 2 relationship witḣ O 2 as the dependent variable [89,165,169]. In this construct, higheṙO 2 values (or steeper rates of change) for a giveṅE would indicate a more "efficient" O 2 uptake by the lungs. It should be emphasized, however, that exercisėE is more closely related tȯC O 2 thaṅO 2 [170] which makes the concept oḟO 2 efficiency prone to misinterpretation (see Section 2.3.3). [165] proposed a logarithmic transformation oḟE over the entire exercise period to "linearize" this relationship, the so-calleḋ : Oxygen uptake (̇O 2 )/work rate (WR) relationship during ramp-incremental CPET before and after pulmonary endarterectomy in a 21-year-old male with thromboembolic occlusion of the left pulmonary artery. Note that after the surgery, peak̇O 2 increased not only due to a higher peak WR but also owing to a large improvement in Δ̇O 2 /ΔWR. et al. [89,169] expressed the OUE as a ratio (̇O 2 /̇E in mL/L) over time which, as expected, gives a mirror image of the ventilatory equivalent for O 2 . The authors proposed the term OUE plateau (OUEP) to the 90 s-average of the highest con-secutivėO 2 /̇E measurements; that is, the values just before the LT ( Figure 5(b)). Although they reported that OUEP was more reproducible than OUES, this was not yet independently confirmed. It has been claimed that both relationships are independent of interobserver variability and effort [90,164,[171][172][173]. However, Williamson et al. [173] recently found that there was a significant increase in OUES as exercise moved from low to moderate intensity with a peak value at an RER value of 1.0. Oscillatory breathing (see Section 3.3) has been found to interfere little with OUE estimations [89]. It should be recognized that both OUES and OUEP require separate computation though some commercially available CPET systems allow logarithmic transformations for OUES calculation.

Interpretative Issues.
It is well established that exercise hyperpnea is under stronger influence of a CO 2 and pH a (rather than a O 2 ) [170]. As detailed later (Section 3.1), changes in CO 2 set-point and ventilatory "efficiency" control the rate of CO 2 clearance. This brings substantial uncertainty on the exact physiological meaning of a disturbed relationship betweeṅE anḋO 2 . Nevertheless, the literature pertaining to the clinical usefulness of OUES is rather vast in CHF [90,164,165,167,171,172], and interest in this relationship has been spread to other populations (cystic fibrosis, and surgical candidates) [174,175]. A number of studies have found that OUES is strongly correlated witḣO 2 peak [90,164,165,167,171,172,176,177] and may hold prognostic value in CHF [18,[89][90][91][92][93][94]. However, the prognostic advantage of OUES over Δ̇E/Δ̇C O 2 slope remains unclear [178,179]. In the pediatric group, mixed results were reported and at least one study found that OUES determined at different WRs differed significantly within patients with cystic fibrosis and correlated only moderately witḣO 2 peak and VT [180]. Interestingly, OUES showed to be more sensitive to the effects of training than Δ̇E/Δ̇C O 2 slope in patients with CHF [96], a finding correlated with enhanced cerebral and muscle hemodynamics in another study [95]. On a single investigation from the group which proposed OUEP, this relationship either on isolation or in combination with oscillatory breathing was prognostically superior to traditional key CPET parameters in CHF [89]. Predicting equations for OUES and OUEP have been recently published [169].

Physiological Background.
After ramp-incremental exercise,̇O 2 does not decline immediately towards the resting level. The traditional view is that there would be a "debt payment" of energy deficit contracted at the start of effort (O 2 deficit). Indeed, the time course oḟO 2 recovery after a moderate, constant test has been found to track the rate of phosphocreatine resynthesis [181]. At early recovery, replenishment of local O 2 sources in muscles (oxymyoglobin and dissolved O 2 ) and reloading of haemoglobin are also needed [182]. At later stages, lactate metabolism (oxidation or gluconeogenesis) and increased cathecolamines and temperature also interfere with the dynamics oḟO 2 decrease [183,184].
sleep apnea [139]. Impairment in cardiovascular responses to exercise as indicated by a delayed recovery of cardiac output was closely associated with slower off-exercisėO 2 kinetics in CHF [200]. Improvements in O 2 delivery might be expected to speed the rate of O 2 recovery in cardiovascular diseases ( Figure 6) [201].  produced CO 2 . ExercisėE for a giveṅC O 2 is inversely related to the prevailing level at which a CO 2 is regulated (the CO 2 "set-point") and the dead space ( D )/tidal volume ( T ) ratio; that is,̇Ė

Ventilatory Responses
Consequently, the largesṫE/̇C O 2 values will be found in those who chronically hyperventilate (low CO 2 "set-point") and have the large D coupled with a low T [202][203][204][205][206]. In the clinical literature, an increased slope of thėE-̇C O 2 relationship has been termed ventilatory "inefficiency" though it could be argued that there is no "inefficiency" when increaseḋ E results from alveolar hyperventilation. "Excess exercise ventilation" seems therefore a more appropriated description of a greater-than-expected ventilatory response to metabolic demand [205].

Technical Considerations.
There are a number of alternatives to express thėE-̇C O 2 relationship during progressive exercise: (1) as a ratio (̇E/̇C O 2 ) at peak exercise, at the VT (Figure 1(b)), and as the lowest (nadir) value and (2) as a slope oḟE versuṡC O 2 from the beginning of exercise to the RCP (Δ̇E/Δ̇C O 2(rest-RCP) ) ( Figure 2(c)) or, alternatively, up to peak exercise (Δ̇E/Δ̇C O 2(rest-PEAK) ) ( Figure 7) [26]. However,̇E/̇C O 2 might not decline at all during early exercise in some patients with severe cardiopulmonary disease (Figure 8) which might preclude LT identification. a CO 2 is relatively constant up to the RCP, and, as described (2), a steeper-than-normal Δ̇E/Δ̇C O 2(rest-RCP) can be explained by a higher D / T and/or a low CO 2 set point. Δ̇E/Δ̇C O 2(rest-PEAK) is expected to be even steeper than Δ̇E/Δ̇C O 2(rest-RCP) (Figure 7(a)) because the former adds a component of hyperventilation to lactic acidosis and/or to other sources oḟE stimuli at near maximum exercise [26,207]. It should be emphasized, however, that there are interpretational pitfalls of using Δ̇E/Δ̇C O 2(rest-PEAK) as a single linear characterization of a relationship which is characteristically curvilinear (Figure 7). E /̇C O 2nadir is equal to Δ̇E/Δ̇C O 2(rest-RCP) when the slope has an -intercept of zero. However, Δ̇E/Δ̇C O 2(rest-RCP) has a positive -intercept in normal subjects [208] which explains whẏE/̇C O 2VT is usually greater than the slope.̇E/̇C O 2VT will also exceed the slope if the VT is a low value (i.e., in less fit subjects) [10]. On the other hand, a very steep Δ̇E/Δ̇C O 2(rest-RCP) would produce a negative -intercept thereby making it greater thaṅE/̇C O 2VT [205]. [25,26]; however, it increases with age probably as a result of larger D / T in older subjects [10,11]. Females have lower T for a giveṅ E than males independent of senescence which might explain their higher Δ̇E/Δ̇C O 2(rest-RCP) across all age ranges (Figure 3(c)) [10,11]. There is plenty of evidence that Δ̇E/Δ̇C O 2(rest-RCP) is clinically useful as a prognostic marker in CHF [52,108,109,163,[209][210][211][212] and, more recently, in PAH [97,98,213] with more discriminatory information thaṅ O 2 peak. The prognostic value in CHF persisted in patients on -blockers [99,100]. Interestingly, Δ̇E/Δ̇C O 2(rest-PEAK) has been found better than Δ̇E/Δ̇C O 2(rest-RCP) to predict 1-year cardiac mortality and hospitalization in these patients [207]. As expected, composite scores adding Δ̇E/Δ̇C O 2 to other cardiopulmonary variables improved even further their prognostic value [211]. A single study found that coexistence of COPD tends to "normalize" Δ̇E/Δ̇C O 2 in CHF patients which casts doubt on its prognostic usefulness in this specific subpopulation [214].

Interpretative Issues. Δ̇E/Δ̇C O 2(rest-RCP) in healthy young males is approximately 30
In patients with PAH, Δ̇E/Δ̇C O 2 anḋE/̇C O 2 (at rest, VT, and peak) are higher compared to CHF [215]. E /̇C O 2VT > 37 plus ET CO 2VT < 30 mmHg increased the probability of pulmonary vascular disease [111]. In those with idiopathic PAH, higher Δ̇E/Δ̇C O 2 anḋE/̇C O 2 (VT and nadir) were related to clinical [53] and hemodynamic impairment [104]. Importantly, these indexes improved with specific treatment [104,105] and after pulmonary endarterectomy [106]. Although to date there is a lack of evidence that indices  Figure 9: Time course of end-tidal partial pressure for carbon dioxide ( ET CO 2 ) during incremental exercise and early recovery in a healthy control (panel (a)) and five patients with pulmonary arterial hypertension of progressing severity (panels (b) to (f)). Note that ET CO 2 becomes lower and even fails to increase as disease progresses. Moreover, it frequently increases (instead of diminishing) during recovery. Panel (f), in particular, depicts a severely impaired patient showing abrupt and sustained decrease in ET CO 2 concomitant with the opening of a forame ovale (Figure 8). Unl is unloaded pedaling.
of excess exercise ventilation in PAH hold the same prognostic importance as in CHF, Deboeck et al. recently described thaṫE/̇C O 2VT (and the 6-min walking distance) were independent predictors of death [98]. Oudiz et al., however, found thaṫE/̇C O 2 was valuable to prognosis assessment only when exercise-induced right-to-left shunt ( Figure 8) was absent [119]. AlthougḣE/̇C O 2 is particularly disturbed in chronic thromboembolic pulmonary hypertension (CTEPH) (Figure 7(b)), thrombotic vessels occlusion increases D / T and excess exercise ventilation to levels which may not be proportionately related to hemodynamic impairment [216].
In patients with other chronic respiratory diseases, Δ̇E/Δ̇C O 2(rest-RCP) > 34 increased the risk of post-operative complications after lung resection surgery with better prediction power thaṅO 2 peak and predicted post-operativė O 2 peak [110]. It could also be empirically expected that a loẇE/̇C O 2VT would be rarely associated with increased D / T whereas the opposite would be likely at very higḣ E /̇C O 2VT . In fact, Roman and coworkers recently described that wheṅE/̇C O 2VT was ≤28 and within 29-32, 96% and 83% of subjects had normal D / T . On the other hand, D / T was abnormal in 87% of the cases wheṅE/̇C O 2VT was ≥39. Unfortunately, intermediate values were not useful to dis-criminate the underlying mechanisms. Interestingly, 95% of the patients with an obstructive ventilatory defect (FEV 1 / FVC < 0.7) anḋE/̇C O 2VT ≥ 39 had increased D / T [217].

Physiological Background.
Expired CO 2 concentration increases as air from the serial ("anatomic") D is progressively enriched with CO 2 from the gas exchanging areas. Consequently, the largest partial pressures for CO 2 are found at the end of tidal expiration ( ET CO 2 ). However, ET CO 2 is influenced not only by the metabolic rate (i.e., the rate of increase in mixed venous CO 2 ) but also by the deepness of the previous inspiration (i.e., VT) and the duration of the exhalation. ET CO 2 reflects poorly a CO 2 , (ideal alveolar) as there are significant regional variations in alveolar CO 2 ( A CO 2 ) anḋA-to-perfusion ratios-even in normal subjects [2,16]. It should also be recognized that ET CO 2 becomes systematically greater than a CO 2 during incremental exercise as the first is the peak of the intrabreath oscillation of A CO 2 and a CO 2 measured in peripheral arterial blood is an average of the oscillation over several breaths [2,16].

Technical
Considerations. ET CO 2 increases from rest to LT (which is proportional to decrease iṅE/̇C O 2 ) in this time range, followed by a stable phase during the isocapnic buffering period, and then a fall after the RCP (Figures 1(b) and 9(a)). As mentioned, a CO 2 underestimation by ET CO 2 is roughly proportional to D / T ; consequently, computing D / T using ET CO 2 instead of a CO 2 overestimates D / T in normal subjects and underestimates it in patients [218].

Interpretative
Issues. ET CO 2 differs from a CO 2 as a result of ventilation-to-perfusion inhomogeneities, right-toleft shunt, and changes in breathing pattern [2,16]. However, arterial blood gases are not routinely measured during clinical CPET. Consequently, interpretation of a reduced ET CO 2 is complex in the absence of a CO 2 measurements as it might be related to abnormal gas exchange, alveolar hyperventilation, or a tachypneic and shallow pattern of breathing. Regardless of the exact mechanism, abnormally low values at the LT have been found useful for the diagnosis of pulmonary vascular diseases in patients with unexplained dyspnea [111]. There is now established evidence that ET CO 2 at rest [112][113][114], LT [115], and peak exercise [116] are valuable for prognosis estimation and disease severity assessment in CHF [219,220]. Low ET CO 2 values have also been found in PAH (see also later) [97,111,117,118]. Decreased ET CO 2 at rest and during exercise seems to track the blunted cardiac output response to exercise in cardiovascular disease [219,221]. Accordingly, exercise training after acute myocardial infarction increases both ET CO 2 and cardiac output [120]. In addition to reduced cardiac output, an augmented ventilatory drive may also account for a reduction in ET CO 2 whereas altered breathing pattern seems to have a minor role in CHF [204]. ET CO 2 is typically lower in PAH than CHF [111,219]. In fact, Yasunobu and co-workers suggested that observation of an unusually low ET CO 2 at the LT (<30 mmHg or, in particular, <20 mmHg) in a patient with exertional dyspnea of unknown cause without evidence of acute hyperventilation (ie, normal R) should prompt the hypothesis of pulmonary vasculopathy [111]. ET CO 2 response profile is also informative as failure to increase below the LT or progressive decreases from the start of exercise are associated with worsening clinical and hemodynamic impairment (Figures 9(b) to 9(e)) [111] and are rarely found in CHF [112][113][114][115][116]. Based on (2), it might be expected that if ET CO 2 changed parallel to A CO 2 , a hyperbolic relationship between  Figure 12: Change in Δ heart rate (HR)/Δ oxygen uptake (̇O 2 ) (arrow) slope (arrow) during incremental CPET in a patient with severe cardiovascular limitation to exercise (panel (a)). Note that this led to a plateau in O 2 pulse (̇O 2 /HR ratio) as the -intercept becomes zero; that is, the relationship passes through its origin (panel (b)). Unl is unloaded pedaling. E /̇C O 2 and ET CO 2 at the LT would result. As this was observed by Yasunobu et al. [111] and confirmed by others [104,216], it seems that alveolar hyperventilation is an important contributing mechanism to the excess exercise ventilation in PAH. Moreover, sharp decreases in ET CO 2 may indicate exercise-induced intracardiac shunt, a finding with ominous consequences (Figures (8) and 9(f)) [119]. Additionally, an abnormal increase in ET CO 2 during early recovery has been described in PAH (Figure 9(c)), even in mildly-impaired patients [111].

Technical
Considerations. Different criteria for EOV might help explaining why its prevalence has been found to vary from 12% to 50% in CHF [123,124,[236][237][238]. A widely used definition is as follows ( Figure 10): (1) three or more regular oscillations (i.e., clearly discernible from inherent data noise); (2) standard deviation of three consecutive cycle lengths (time between 2 consecutive nadirs) within 20% of the average; (3) minimal average amplitude oḟE oscillation of 5 L/min (peak value minus the average of two in-between consecutive nadirs) [27]. Alternative definitions require: (i) criteria for persistence of the EOV pattern (three or more consecutive cyclic oscillations) for at least 60% of exercise at an amplitude ≥ 15% of the average resting value [122,[239][240][241] or (ii) 3 or more consecutive cyclic fluctuations with amplitude exceeding 30% of meaṅE and oscillatory cycle within 40 to 140 s in 3 or more gas exchange/ventilatory variables [124].

Clinical Usefulness.
There is now well-established evidence that EOV holds important negative prognostic implications in patients with CHF [27,124,222,236,239], being related to worsening clinical status [121,122,124], severe hemodynamic dysfunction [123], and reduced functional capacity [125,126]. Unfortunately, EOV may preclude an adequate identification of the LT by either the -slope or the ventilatory equivalent methods [242]. EOV is highly reproducible regardless of the CHF aetiology [121]. Interestingly, several interventions including inotropics [237], exercise and inspiratory muscle training [243][244][245], and transplantation [237] lessened of even abolished EOV. Future larger trials should establish whether EOV might add independent information to commonly used outcomes for interventional studies in CHF. HR as a function oḟO 2 during ramp-incremental exercise [3,24,25] though departs from linearity might occur at higher exercise intensities (Figure 2(b)) [247]. According to the Fick principle, reduced stroke volume (SV) and/or diminished C(a-v)O 2 would lead to a steeper ΔHR/Δ̇O 2 slope. Consequently, cardiac dysfunction, decreased arterial O 2 content (anemia and hypoxemia), and impaired muscle aerobic capacity (e.g., deconditioning, mitochondrial dysfunction) can potentially increase ΔHR/Δ̇O 2 . On the other hand, training has a flattening effect on ΔHR/Δ̇O 2 ( Figure 11).

Technical Considerations.
AlthougḣO 2 is the appropriate dependent variable, this relationship has been traditionally described with HR on the -axis [3,24,25] Figure 14: Heart rate (HR) response after incremental exercise in a healthy control and a patient with pulmonary arterial hypertension (PAH) of same age and gender (both females aged 31). Note the delayed HR recovery (HRR) up to the 5th minute after-exercise in the patient compared to the control. HRR 1 min ≤ 18 bpm after cycle ergometer exercise test has recently been found an independent predictor of mortality in these patients [28].
the slope should be calculated only over the initial linear phase response (Figure 2(b)). As detailed later, pronounced changes in linearity may hold important clinical implications.

Clinical
Usefulness. ΔHR/Δ̇O 2 increases with age being consistently higher in females than males (Figure 3(b)) [10]. As expected, cardiovascular and muscular diseases which are known to impair O 2 delivery and/or utilization have been found to increase both the slope and the intercept of the ΔHR/Δ̇O 2 relationship [127][128][129][130]. Some specific conditions, however, may prevent HR to increase even in the presence of disease: (a) patients under -blocker therapy [248], (b) ischemic involvement of the sinusal node artery [249], and (c) advanced CHF [250]. The so-called O 2 pulse (̇O 2 /HR ratio) is a commonly used derivation of ΔHR/Δ̇O 2 . As the primarẏO 2 -HR relationship has a negative -intercept, O 2 pulse increases hyperbolically [16] towards an asymptotic value at end-exercise (Figure 13(a)) which might reflect the SV response [131]. However, all pathologic conditions known to increase ΔHR/Δ̇O 2 (including desaturation, anemia, and impaired O 2 extraction) will also diminish peak O 2 pulse. Moreover, early exercise termination due to symptom limitation (including breathlessness in patients with COPD) (Figure 13(b)) and/or submaximal effort would decrease peak O 2 pulse in the absence of cardiovascular limitation. In these cases, however, a normal ΔHR/Δ̇O 2 is reassuring. A more clinically useful pattern of response relates to abrupt increases in ΔHR/Δ̇O 2 slope to an extent that the relationship goes through its origin or becomes with a negative -intercept; that is, O 2 pulse turns flat ( Figure 12) or even decreases (Figure 13(d)). This suggests that the HR response became the sole mechanism for cardiac output increase due to a severely impaired SV response. In practical grounds, there is limited evidence that as myocardial perfusion is reduced in patients with coronary artery disease, there is reversible left ventricle dysfunction thereby steepening ΔHR/Δ̇O 2 (Figure 12(a)) and flattening (Figure 12(b)) (or even decreasing) (Figure 13(d) [246]. After effort cessation, vagal reactivation (with opposition of the sympathetic drive) is primarily responsible for the return to baseline conditions [251], especially during the first 30 seconds of recovery [252]. Consequently, autonomic imbalance (increased sympathetic stimuli and/or impaired parasympathetic activity) might slow post-exercise HR decay.

Technical
Considerations. HRR is the difference between peak HR and HR at selected time points after exercise (e.g., 30 sec and every minute thereafter). HRR analysis may be performed independent of the mode of exercise (treadmill [134,135,140,152,253], cycle ergometer [28,[254][255][256], or field tests [257]), and a cool-down period at the end of maximal effort seems not to interfere with its prognostic value [28,134,150].
Although a considerable lack of information on the individual diagnostic and prognostic value of the dynamic submaximal relationships still persists, the bulk of evidence is reassuring in relation to their practical usefulness. Largescale, multicentric studies, however, are urgently needed to validate the suggested cutoffs of abnormality (