Kinetics of oxygen uptake at onset of exercise related to cardiac output, but not to arteriovenous oxygen difference in patients with chronic heart failure

Cardiorespiratory mechanisms underlying the impaired oxygen uptake kinetics at exercise onset after stroke

BACKGROUND

Oxygen uptake (V˙O₂) kinetics at the onset of exercise is slower in patients with stroke than in healthy adults. However, little is known about the cardiorespiratory mechanisms underlying the impaired V˙O₂ kinetics.

OBJECTIVES

This study aimed to investigate the relative effect of impaired oxygen delivery and utilisation on V˙O₂ kinetics at the onset of submaximal exercise in patients with stroke by comparing the time constants of cardiac output (τCO) and V˙O₂ (τV˙O₂). In addition, we aimed to examine the association between the kinetics of cardiorespiratory variables and functional outcomes.

METHODS

We included 21 patients with stroke (15 males, mean [SD] age 58.7 [9.5] years, mean days post-stroke 67.9 [30.9]). A submaximal constant-load exercise test was performed to measure τV˙O₂, τCO, and the time constant of arterialvenous oxygen difference (τAVO₂diff). The ratio of τCO to τV˙O₂ was calculated to assess the matching of oxygen delivery and consumption. Fugl-Meyer lower-extremity motor scores, comfortable gait speeds, and Functional Independence Measure motor scores were used as functional variables.

RESULTS

Mean (SD) τAVO₂diff was markedly shorter than τV˙O₂ and τCO (26.1 [7.1] vs. 38.7 [10.2] and 46.6 [23.2 s], P<0.05), with no significant difference between τV˙O₂ and τCO (P=0.444). The greater ratio of τCO to τV˙O₂ was related to poorer motor function (rho=-0.484, P=0.026) and slower comfortable gait speed (r=-0.482, P=0.027).

CONCLUSIONS

An increase in CO was slower than that in AVO₂diff in patients with stroke. Therefore, V˙O₂ kinetics in patients with stroke appears to be affected by a delayed increase in CO rather than AVO₂diff. Furthermore, these patients with motor and gait impairments may have a poor matching of oxygen delivery and consumption during exercise onset.

Beyond the myocardium? SGLT2 inhibitors target peripheral components of reduced oxygen flux in the diabetic patient with heart failure with preserved ejection fraction

Recent cardiovascular outcome trials have highlighted the propensity of the antidiabetic agents, SGLT2 inhibitors (SGLT2is or -flozin drugs), to exert positive clinical outcomes in patients with cardiovascular disease at risk for major adverse cardiovascular events (MACEs). Of interest in cardiac diabetology is the physiological status of the patient with T2DM and heart failure with preserved ejection fraction (HFpEF), a well-examined association. Underlying this pathologic tandem are the effects that long-standing hyperglycemia has on the ability of the HFpEF heart to adequately deliver oxygen. It is believed that shortcomings in oxygen diffusion or utilization and the resulting hypoxia thereafter may play a role in underlying the clinical sequelae of patients with T2DM and HFpEF, with implications in the long-term decline of extra-cardiac tissue. Oxygen consumption is one of the most critical factors in indexing heart failure disease burden, warranting a probe into the role of SGLT2i on oxygen utility in HFpEF and T2DM. We investigated the role of oxygen flux in the patient with T2DM and HFpEF extending beyond the heart with focuses on cellular metabolism, perivascular fibrosis with endothelial dysfunction, hematologic changes, and renal effects with neurohormonal considerations in the patient with HFpEF and T2DM. Moreover, we give a commentary on potential therapeutic targets of these components with SGLT2i to gain insight into disease burden amelioration in patients with HFpEF and T2DM.

A predictor of aerobic threshold for patients with heart failure with reduced ejection fraction

[Purpose] The initial cardiopulmonary response to exercise is hypothesized to be a useful predictor of aerobic threshold in patients with heart failure. This study aimed to evaluate the correlation between aerobic threshold and cardiopulmonary responses to exercise onset by comparing patients with heart failure using preserved (≥50%) and reduced (<50%) left ventricular ejection fractions. [Participants and Methods] Twenty-eight males (age, 36-82 years; 12 with preserved and 16 with reduced left ventricular ejection fractions) underwent a progressive submaximal cardiopulmonary exercise test using a cycle ergometer. The aerobic threshold, time constant, and area under the oxygen uptake curve for the first 4 min (V̇O₂AUC) were determined. [Results] A significant association was observed between aerobic threshold and V̇O₂AUC in the reduced group but not in the preserved group. No significant correlations were found between time constant and V̇O₂AUC or between aerobic threshold and time constant in either group. [Conclusion] The results suggest that V̇O₂AUC measured from exercise onset to an initial 4-min period could provide an easily and safely obtained predictor to assess aerobic capacity in people with reduced left ventricular ejection fractions.

Oxygen uptake on-kinetics during six-minute walk test predicts short-term outcomes after off-pump coronary artery bypass surgery

PURPOSE

We aimed to investigate the ability of oxygen uptake kinetics to predict short-term outcomes after off-pump coronary artery bypass grafting.

METHODS

Fifty-two patients aged 60.9 ± 7.8 years waiting for off-pump coronary artery bypass surgery were evaluated. The 6-min walk test distance was performed pre-operatively, while simultaneously using a portable cardiopulmonary testing device. The transition of oxygen uptake kinetics from rest to exercise was recorded to calculate oxygen uptake kinetics fitting a monoexponential regression model. Oxygen uptake at steady state, constant time, and mean response time corrected by work rate were analysed. Short-term clinical outcomes were evaluated during the early post-operative of off-pump coronary artery bypass surgery.

RESULTS

Multivariate analysis showed body mass index, surgery time, and mean response time corrected by work rate as independent predictors for short-term outcomes. The optimal mean response time corrected by work rate cut-off to estimate short-term clinical outcomes was 1.51 × 10^-3 min²/ml. Patients with slower mean response time corrected by work rate demonstrated higher rates of hypertension, diabetes, EuroSCOREII, left ventricular dysfunction, and impaired 6-min walk test parameters. The per cent-predicted distance threshold of 66% in the pre-operative was associated with delayed oxygen uptake kinetics.

CONCLUSIONS

Pre-operative oxygen uptake kinetics during 6-min walk test predicts short-term clinical outcomes after off-pump coronary artery bypass surgery. From a clinically applicable perspective, a threshold of 66% of pre-operative predicted 6-min walk test distance indicated slower kinetics, which leads to longer intensive care unit and post-surgery hospital length of stay. Implications for rehabilitation Coronary artery bypass grafting is a treatment aimed to improve expectancy of life and prevent disability due to the disease progression; The use of pre-operative submaximal functional capacity test enabled the identification of patients with high risk of complications, where patients with delayed oxygen uptake kinetics exhibited worse short-term outcomes; Our findings suggest the importance of the rehabilitation in the pre-operative in order to "pre-habilitate" the patients to the surgical procedure; Faster oxygen uptake on-kinetics could be achieved by improving the oxidative capacity of muscles and cardiovascular conditioning through rehabilitation, adding better results following cardiac surgery.

Oxygen kinetics during 6-minute walk tests in patients with cardiovascular and pulmonary disease

BACKGROUND

The 6-Minute Walk Test (6MWT) is representative of daily-life activities and reflects the functional capacity of patients. The change of oxygen uptake (VO2) in the initial phase of low-intensity exercise (VO2 kinetics) can be used to assess submaximal exercise performance of patients.The objective of the following study was to analyse VO2 kinetics in patients with different pulmonary and cardiovascular diseases. In addition, we investigated the extent to which VO2 kinetics at the onset of the 6MWT were associated with exercise capacity, morbidity and mortality.

METHODS

VO2 kinetics of 204 patients and 16 healthy controls were obtained using mobile telemetric cardiopulmonary monitoring during a 6MWT. A new mean response time (MRT) index (wMRT) was developed to quantify VO2 kinetics by correcting MRT for work rate. The differences in wMRT between disease categories as well as the association between wMRT and patients' exercise capacity and outcome - time to hospitalization/death- were tested.

RESULTS

The assessment of a robust wMRT was feasible in 86% (244/284) patients. wMRT was increased in patients compared to healthy controls (p <0.001). wMRT was largest in patients with pulmonary arterial hypertension (PAH). There were significant associations between wMRT and exercise capacity in all patients. High wMRT was found to be associated with a high rate of death and re-hospitalization in patients with CHF (p = 0.024). In patients with pulmonary diseases and pulmonary hypertension wMRT was not associated with outcome (p = 0.952).

CONCLUSIONS

Submaximal exercise performance of patients is reduced. O2 kinetics at the onset of exercise are associated with exercise capacity in all patients. wMRT was found to be an important prognostic factor in patients with congestive heart failure (CHF), but not with pulmonary diseases.

Cardiac resynchronization therapy modulation of exercise left ventricular function and pulmonary O₂ uptake in heart failure

To better understand the mechanisms contributing to improved exercise capacity with cardiac resynchronization therapy (CRT), we studied the effects of 6 mo of CRT on pulmonary O(2) uptake (Vo(2)) kinetics, exercise left ventricular (LV) function, and peak Vo(2) in 12 subjects (age: 56 ± 15 yr, peak Vo(2): 12.9 ± 3.2 ml·kg(-1)·min(-1), ejection fraction: 18 ± 3%) with heart failure. We hypothesized that CRT would speed Vo(2) kinetics due to an increase in stroke volume secondary to a reduction in LV end-systolic volume (ESV) and that the increase in peak Vo(2) would be related to an increase in cardiac output reserve. We found that Vo(2) kinetics were faster during the transition to moderate-intensity exercise after CRT (pre-CRT: 69 ± 21 s vs. post-CRT: 54 ± 17 s, P < 0.05). During moderate-intensity exercise, LV ESV reserve (exercise - resting) increased 9 ± 7 ml (vs. a 3 ± 9-ml decrease pre-CRT, P < 0.05), and steady-state stroke volume increased (pre-CRT: 42 ± 8 ml vs. post-CRT: 61 ± 12 ml, P < 0.05). LV end-diastolic volume did not change from rest to steady-state exercise post-CRT (P > 0.05). CRT improved heart rate, measured as a lower resting and steady-state exercise heart rate and as faster heart rate kinetics after CRT (pre-CRT: 89 ± 12 s vs. post-CRT: 69 ± 21 s, P < 0.05). For peak exercise, cardiac output reserve increased significantly post-CRT and was 22% higher at peak exercise post-CRT (both P < 0.05). The increase in cardiac output was due to both a significant increase in peak and reserve stroke volume and to a nonsignificant increase in heart rate reserve. Similar patterns in LV volumes as moderate-intensity exercise were observed at peak exercise. Cardiac output reserve was related to peak Vo(2) (r = 0.48, P < 0.05). These findings demonstrate the chronic CRT-mediated cardiac factors that contribute, in part, to the speeding in Vo(2) kinetics and increase in peak Vo(2) in clinically stable heart failure patients.

Oxygen uptake kinetics in chronic heart failure: clinical and physiological aspects

One of the hallmark symptoms of patients with chronic heart failure (CHF) is exercise intolerance. Therefore, exercise testing has become an important tool for the evaluation and monitoring of heart failure. Whereas the maximal aerobic capacity (peak VO(2)) is a reliable indicator of the severity and prognosis of heart failure, submaximal exercise parameters may be more closely related to the ability to perform daily activities. As such, oxygen (O(2)) uptake kinetics, describing the rate change of O(2) uptake during onset or recovery of submaximal constant-load exercise (O(2) onset and recovery kinetics, respectively), have been shown to be useful parameters for objectively evaluating the functional capacity of CHF patients. However, their evaluation in this population is not a routine part of daily clinical practice. Possible reasons for this include a lack of standardisation of the assessment methodology and a limited number of studies evaluating the clinical use of O(2) uptake kinetics in CHF patients. In addition, the pathophysiological mechanisms underlying the delay in O(2) uptake kinetics in these patients are not completely understood. This review discusses the current literature on the clinical potency and physiological determinants of O(2) uptake kinetics in CHF patients and provides directions for future research. (Neth Heart J 2009;17:238-44.Neth Heart J 2009;17:238-44.).

Evaluation of two methods for continuous cardiac output assessment during exercise in chronic heart failure patients

The purpose of this study was to evaluate the accuracy of two techniques for the continuous assessment of cardiac output in patients with chronic heart failure (CHF): a radial artery pulse contour analysis method that uses an indicator dilution method for calibration (LiDCO) and an impedance cardiography technique (Physioflow), using the Fick method as a reference. Ten male CHF patients (New York Heart Association class II–III) were included. At rest, cardiac output values obtained by LiDCO and Physioflow were compared with those of the direct Fick method. During exercise, the continuous Fick method was used as a reference. Exercise, performed on a cycle ergometer in upright position, consisted of two constant-load tests at 30% and 80% of the ventilatory threshold and a symptom-limited maximal test. Both at rest and during exercise LiDCO showed good agreement with reference values [bias ± limits of agreement (LOA), −1% ± 28% and 2% ± 28%, respectively]. In contrast, Physioflow overestimated reference values both at rest and during exercise (bias ± LOA, 48% ± 60% and 48% ± 52%, respectively). Exercise-related within-patient changes of cardiac output, expressed as a percent change, showed for both techniques clinically acceptable agreement with reference values (bias ± LOA: 2% ± 26% for LiDCO, and −2% ± 36% for Physioflow, respectively). In conclusion, although the limits of agreement with the Fick method are pretty broad, LiDCO provides accurate measurements of cardiac output during rest and exercise in CHF patients. Although Physioflow overestimates cardiac output, this method may still be useful to estimate relative changes during exercise.

Reproducibility of onset and recovery oxygen uptake kinetics in moderately impaired patients with chronic heart failure

Oxygen (O₂) kinetics reflect the ability to adapt to or recover from exercise that is indicative of daily life. In patients with chronic heart failure (CHF), parameters of O₂ kinetics have shown to be useful for clinical purposes like grading of functional impairment and assessment of prognosis. This study compared the goodness of fit and reproducibility of previously described methods to assess O₂ kinetics in these patients. Nineteen CHF patients, New York Heart Association class II–III, performed two constant-load tests on a cycle ergometer at 50% of the maximum workload. Time constants of O₂ onset- and recovery kinetics (τ) were calculated by mono-exponential modeling with four different sampling intervals (5 and 10 s, 5 and 8 breaths). The goodness of fit was expressed as the coefficient of determination (R²). Onset kinetics were also evaluated by the mean response time (MRT). Considering O₂ onset kinetics, τ showed a significant inverse correlation with peak- \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document} $$ \ifmmode\expandafter\dot\else\expandafter\.\fi{V}{\text{O}}_{2} $$\end{document} (R = −0.88, using 10 s sampling intervals). The limits of agreement of both τ and MRT, however, were not clinically acceptable. O₂ recovery kinetics yielded better reproducibility and goodness of fit. Using the most optimal sampling interval (5 breaths), a change of at least 13 s in τ is needed to exceed normal test-to-test variations. In conclusion, O₂ recovery kinetics are more reproducible for clinical purposes than O₂ onset kinetics in moderately impaired patients with CHF. It should be recognized that this observation cannot be assumed to be generalizable to more severely impaired CHF patients.

The decreased oxygen uptake during progressive exercise in ischemia-induced heart failure is due to reduced cardiac output rate

We tested the hypothesis that the inability to increase cardiac output during exercise would explain the decreased rate of oxygen uptake (VO2) in recent onset, ischemia-induced heart failure rats. Nine normal control rats and 6 rats with ischemic heart failure were studied. Myocardial infarction was induced by coronary ligation. VO2 was measured during a ramp protocol test on a treadmill using a metabolic mask. Cardiac output was measured with a flow probe placed around the ascending aorta. Left ventricular end-diastolic pressure was higher in ischemic heart failure rats compared with normal control rats (17 +/- 0.4 vs 8 +/- 0.8 mmHg, P = 0.0001). Resting cardiac index (CI) tended to be lower in ischemic heart failure rats (P = 0.07). Resting heart rate (HR) and stroke volume index (SVI) did not differ significantly between ischemic heart failure rats and normal control rats. Peak VO2 was lower in ischemic heart failure rats (73.72 +/- 7.37 vs 109.02 +/- 27.87 mL min(-1) kg(-1), P = 0.005). The VO2 and CI responses during exercise were significantly lower in ischemic heart failure rats than in normal control rats. The temporal response of SVI, but not of HR, was significantly lower in ischemic heart failure rats than in normal control rats. Peak CI, HR, and SVI were lower in ischemic heart failure rats. The reduction in VO2 response during incremental exercise in an ischemic model of heart failure is due to the decreased cardiac output response, largely caused by depressed stroke volume kinetics.

Effect of acute blood pressure reduction on oxygen uptake kinetics at the onset of exercise in hypertensive patients

The aim of the present study was to investigate the adverse effects of hypertension on the cardiovascular system in daily activities and the effect of acute blood pressure reduction on oxygen (O(2)) uptake kinetics. Twenty hypertensive patients were included in the study group. Patients performed treadmill exercise tests (2.5 km/hour and 5 inclines) twice, before and after blood pressure reduction with sublingual captopril. In the control group, ten hypertensive patients underwent two tests one hour apart without blood pressure reduction brought about by drug therapy. The changes in O(2) kinetic values (O(2) deficit and mean response time [MRT]) between the two tests were investigated. In the study group, the O(2) deficit and MRT values measured during the first exercise testing were found to be 547 +/- 183 mL and 40 +/- 9 seconds, while those in the second exercise testing were 401 +/- 127 mL and 34 +/- 7 seconds, respectively. In the control group, the O(2) deficit and MRT values measured during the first exercise test were 491 +/- 217 mL and 42 +/- 16 seconds and 515 +/- 159 mL and 41 +/- 13 seconds in the second exercise test. The differences in O(2) deficit and MRT in the study group were considered to be statistically significant (P = 0.008 and P = 0.004, respectively). Based on our findings, there was a significant improvement in O(2) kinetic values with an acute reduction in blood pressure in hypertensive patients, most likely as a result of an improved response in cardiac output.

Oxygen-uptake (VO2) kinetics and functional mobility performance in impaired older adults

BACKGROUND

Measures of maximal oxygen uptake (VO(2max)) are limited in disabled older adults, and measures of submaximal oxygen uptake (VO(2)) may better predict functional mobility limitations. These measures may include oxygen-uptake kinetics at the onset of submaximal exercise or during recovery. We sought to determine whether the lag in oxygen uptake at the beginning of exercise (oxygen deficit) and excess oxygen uptake above rest following exercise (excess postexercise oxygen consumption) (a) predict physical performance in impaired older adults with decreased aerobic function, and (b) predict physical performance better than peak VO(2).

METHODS

Two groups of community-dwelling volunteers aged 65 or older were recruited according to their performance on a maximal graded exercise test. Using the Social Security Administration criterion of disability of a peak VO(2) <or = 18 ml/kg/min, we compared the performance of an impaired aerobic capacity group at a peak VO(2) < 18 ml/kg/min (Impaired, n = 20, mean +/- SEM age 82 +/- 1 years) with an unimpaired group at Peak VO(2) > 18 (Unimpaired, n = 21, mean +/- SEM age 76 +/- 1 years).

RESULTS

The mean +/- SEM peak VO(2) was 58% lower in the Impaired (14 +/- 1 ml/kg/min) than the Unimpaired (24 +/- 1 ml/kg/min) adults. The time constant for oxygen deficit, tc(deficit), was more than twice as high in the Impaired than the Unimpaired (p <.05), and the time constant for excess postexercise oxygen consumption, tc(EPOC), tended to be higher in the Impaired than the Unimpaired (by 43%, p =.09). Measures of submaximal oxygen-uptake kinetics were as strong or more strongly predictive of functional mobility performance than peak VO(2) in both Unimpaired and Impaired older adults. The major predictor of functional performance for the Unimpaired was a measure of oxygen deficit accruing during exercise (tc(deficit)), and for the Impaired, it was a measure of oxygen debt during recovery, tc(EPOC).

CONCLUSIONS

Measurement of submaximal oxygen-uptake kinetics may provide a more practical and relevant assessment of deconditioning in frail older adults, and may eventually supplant maximal (peak) oxygen uptake as a predictor of functional disability in older adults.

Comparison of oxygen uptake on-kinetic calculations in heart failure

PURPOSE

The analysis of oxygen (O(2)) uptake on-kinetics during steady-rate is gaining interest in the heart failure (HF) population. The rate change in O(2) at the initiation of exercise can be assessed via nonlinear regression time constant (TC) or an algebraic equation (mean response time [MRT]). These calculations are presumed to be interchangeable, but research supporting this claim is limited. This investigation compares and contrasts two of the more commonly used O(2) uptake on-kinetic calculations.

METHOD

Twenty-eight subjects diagnosed with compensated HF and 19 age, sex, and activity-matched controls underwent a symptom-limited exercise test and a steady-rate exercise session (6 min). Peak O(2) uptake, O(2) uptake at ventilatory threshold, the O(2) uptake TC (TC), and the O(2) uptake mean response time (MRT) were calculated for each subject.

RESULTS

O(2) uptake on-kinetics was significantly faster for the control group ( < 0.05) regardless of calculation method. There was a significant difference between the O(2) uptake TC and MRT for the HF group. All O(2) uptake on-kinetic calculations were significantly correlated with aerobic capacity.

CONCLUSIONS

O(2) uptake TC and MRT values may not be interchangeable in the HF population. All O(2) uptake on-kinetic calculations did produce a significant difference between experimental and control groups and correlated with indicators of aerobic capacity. The 10-s O(2) uptake on-kinetic calculations may be preferable secondary to expired gas fluctuations associated with breath-by-breath measures. Further work is, however, needed to determine which averaged O(2) uptake on-kinetic expression is optimal given the significant difference between TC and MRT. A mechanism for this difference may be the oscillatory ventilatory expired gas pattern demonstrated by some patients with HF.