Oxygen uptake kinetics during low intensity exercise: relevance for rate adaptive pacemaker programming

Prolonged mean VO2 response time in systolic heart failure: an indicator of impaired right ventricular-pulmonary vascular function

BACKGROUND

In patients with left ventricular systolic dysfunction (LVSD), the rate at which oxygen uptake (VO2) increases on initiation of exercise is inadequate to match metabolic demands. To gain mechanistic insights into delayed VO2 kinetics in LVSD, we simultaneously assessed hemodynamic measurements, ventilatory parameters, and peripheral oxygen usage during exercise.

METHODS AND RESULTS

Forty-two patients with symptomatic LVSD (age, 59±2 years [mean±SEM]; LV ejection fraction, 30±1%) and 17 controls (LV ejection fraction, 68±1%) underwent maximum upright cycle ergometry cardiopulmonary exercise testing. Hemodynamic monitoring and first-pass radionuclide ventriculography were performed at rest and during exercise. VO2 kinetics were quantified by mean response time (MRT), which was significantly longer in patients with LVSD compared with controls (64±3 versus 45±5 s; P=0.004). In LVSD patients, MRT was associated with higher biventricular filling pressures and reduced cardiac output during early exercise. LVSD patients with MRT ≥60 s, compared with LVSD subjects with MRT <60 s, demonstrated greater impairment in right ventricular-pulmonary vascular function during exercise as evidenced by lower right ventricular ejection fraction (35±2 versus 45±2%; P=0.03), steeper increment in transpulmonary gradient relative to cardiac output (3.7 versus 2.2 mm Hg/L; P<0.001), and increased ventilatory dead-space fraction (17±1 versus 12±2%; P=0.03). In contrast, MRT was not associated with LV ejection fraction (rest, exercise), PaO2, hemoglobin, or resting pulmonary function test results.

CONCLUSIONS

Delayed oxygen uptake on initiation of exercise (ie, MRT ≥60 s) in LVSD is closely related to impaired right ventricular-pulmonary vascular function and may represent an important surrogate for inability to augment RV performance during physical activity in patients with heart failure.

Validating optimal function of the closed loop stimulation sensor with high right septal ventricular electrode placement in 'ablate and pace' patients

PURPOSE

The study aim was to validate the closed loop stimulation (CLS) vs. accelerometer (ACC) rate-responsive sensors with electrodes placed in the right ventricular high septal (RVHS) or right ventricular apical (RVA) lead positions in patients following 'ablate and pace' therapy for persistent atrial fibrillation.

METHODS

'Ablate and pace' patients were randomised to either RVHS or RVA electrode placement with a dual sensor device. A double-blind crossover study comparing CLS vs. ACC rate-response pacing modes was undertaken. Subjects undertook cardiopulmonary testing with constant workload light exercise followed by a ramp protocol in addition to activity of daily living assessments.

RESULTS

Twenty subjects (14 male; age, 74 +/- 8 years) were studied. Heart rate increase was greater from lying to sitting with ACC. With mental stress, heart rate increase was greater with CLS. Peak heart rates were similar for stair ascent and descent in ACC mode. With CLS mode, however, the peak heart rate was significantly lower for stair descent. There was no difference between modes in mean response time, oxygen deficit, peak VO(2), VO(2) at anaerobic threshold, peak heart rate, total exercise time and total workload. CLS function was equally optimal at both electrode sites.

CONCLUSIONS

CLS rate adaptive pacing is appropriate for 'ablate and pace' patients, and this sensor functions equally well using RVA or RVHS lead positions.

Phase I and Phase II Oxygen Uptake Kinetics During Atrioventricular Dyssynchrony in Chronotropically Competent Pacemaker Patients

OBJECTIVE

To elucidate the effects of atrioventricular (AV) dyssynchrony on phase I and phase II oxygen uptake (V(O2)) kinetics in chronotropically competent pacemaker patients during exercise of an intensity comparable to activities of daily living.

DESIGN

Blinded patients completed sub-ventilatory threshold (VT) work rate (WR) cycle ergometry exercise in random order during asynchronous AV pacing (AV OFF) and synchronous AV pacing.

SETTING

Tertiary care hospital in a major city.

SUBJECTS

Six chronotropically competent male pacemaker patients (mean [+/- SD] age, 68 +/- 10 years) with high-degree AV block and varying cardiac histories.

RESULTS

The phase I and phase II V(O2) amplitude response and gain (deltaV(O2)/WR ratio) were lower (p < 0.05) and the time course of phase II was slower (p < 0.05) during AV OFF; however, the O2 deficit was similar (p > 0.05) across pacing modes. The stroke volume index (SVI) was consistently lower (p < 0.05) during AV OFF pacing and was significantly correlated with the time course of phase II V(O2). A significant compensatory amplitude response in heart rate (HR) was observed in addition to a higher (p < 0.05) deltaHR/V(O2) ratio during AV OFF. Ventilatory responses were consistent with ventilatory-perfusion mismatching and perceived exertion was higher during asynchronous pacing.

CONCLUSION

This study demonstrated that the contribution of SVI affects V(O2) kinetics and underscores the importance of the atrial contribution to ventricular filling and, consequently, to metabolic and hemodynamic responses. This study supports the theory of an O2 transport limitation and further implicates SV as a potential limiting factor during sub-VT exercise intensities that are comparable to those encountered in activities of daily living.

Prolonged oxygen uptake kinetics during low-intensity exercise are related to poor prognosis in patients with mild-to-moderate congestive heart failure

STUDY OBJECTIVE

To investigate the prognostic value of oxygen uptake (O(2)) kinetics during low-intensity exercise in patients with congestive heart failure.

DESIGN

Prospective cohort study.

SETTING

Tertiary care center.

PATIENTS

One hundred forty-six consecutive patients (128 men) with chronic heart failure, followed up for a mean (+/- SD) duration of 25 +/- 15 months.

MEASUREMENTS

A treadmill exercise test was performed with "breath by breath" gas-exchange monitoring. VO(2) kinetics were defined as the VO(2) deficit (ie, Delta VO(2) x time[rest to steady state] - Sigma VO(2)[rest to steady state]) and mean response time (MRT) [ie, VO(2) deficit/Delta VO(2)]. Cardiac death, urgent cardiac transplantation, and hospitalization due to worsening heart failure were considered as the end points.

RESULTS

Thirty patients (21%) died, 11 patients (8%) underwent urgent transplantation, and 32 patients (22%) were hospitalized. In univariate analysis, MRT was the most powerful predictor of survival, survival free of urgent transplantation, and survival free of hospitalization (hazard ratios [HRs] per 10 s, 1.65, 1.72, and 1.61, respectively; all p < 0.0001). The predictive value of MRT exceeded that of peak VO(2) (HR per mL/kg/min, 0.90; p = 0.02, 0.91; p = 0.007, and 0.95; p = 0.08, respectively). In multivariate analysis, MRT (HR per 10 s, 1.73; p = 0.0002), resting systolic BP (HR per 10 mm Hg, 0.65; p = 0.003), and the slope of the ventilatory response to exercise (HR per 10 U, 1.68; p = 0.02) were independent predictors of survival.

CONCLUSIONS

Our results suggest that VO(2) kinetics are strongly related to outcome in heart failure patients. Since it has several additional advantages over peak exercise testing (eg, less time-consuming, less demanding for the patients, less dependent on motivation, and applicable in patients with limitations other than cardiopulmonary disease), it has the potential to become a prognostic test for the assessment of heart failure patients.

Combining low-intensity and maximal exercise test results improves prognostic prediction in chronic heart failure

OBJECTIVES

This study investigated the combination of maximal and low-intensity exercise testing in predicting prognosis in chronic heart failure (CHF), using one single exercise test (two-step protocol).

BACKGROUND

Risk assessment based on any single factor has limited accuracy and reproducibility.

METHODS

Treadmill exercise testing was performed in 202 consecutive CHF patients (174 male; mean age 52 +/- 11 years) using "breath-by-breath" gas exchange monitoring. Oxygen uptake (VO(2)) kinetics were defined as oxygen deficit (DeltaVO(2) x time [rest to steady state] - Sigma VO(2) [rest to steady state]) and mean response time (MRT = oxygen-deficit/DeltaVO(2)). Peak VO(2) (VO(2)max) was defined as the highest VO(2). Mean follow-up was 873 +/- 628 days. The primary end point was cardiac mortality and the need for urgent heart transplantation.

RESULTS

Forty-four patients (22%) died and 15 (7%) were urgently transplanted. In both univariate and multivariate analyses, MRT >50 s was the most powerful predictor of the primary end point (hazard ratio [HR] 4.44), followed by predicted VO(2)max <50% (HR 3.50) and resting systolic blood pressure <105 mm Hg (HR 2.49, all p < 0.001). A majority (n = 130 [64%]) had one or none of these risk factors, with a one-year event rate of only 3%. Patients with two risk factors (n = 45 [22%]) were at medium risk (one-year event rate of 33%). Twenty-seven patients (13%) had all three risk factors, with a one-year event rate of 59%. The area under the curve, using the number of risk factors, was 0.86 +/- 0.04 for the primary end point at one year. These results were independent of medication, in particular, beta-blockade.

CONCLUSIONS

A combination of low-intensity and maximal exercise test results improves assessment of prognosis in patients with CHF.

Programming of sensor driven pacemakers

The chronotropic response is the most important means by which cardiac output is increased and oxygen delivery is maintained in response to increased oxygen consumption during exercise or stress. When the chronotropic response is suboptimal or absent, exercise intolerance results. This condition, called chronotropic incompetence can be treated effectively with a sensor-driven rate-responsive pacemaker. The effectiveness of this therapy assumes that the pacemaker is programmed appropriately. This article focuses on the programming of sensor-driven pacemakers and provides additional suggestions for follow-up testing to ensure maximal benefit from these devices.

Importance of AV synchronous pacing during low intensity exercise evaluated by oxygen kinetics

It has been shown that dual chamber pacing with preservation of AV synchrony (DDD) is superior to fixed rate ventricular (VVI) or rate responsive ventricular (VVIR) pacing modes, as evaluated by ventilatory response to exercise. Previous studies have focused on the benefits of maintained AV synchrony at maximal exercise. However, there are limited data comparing O2 kinetics in different pacing modes during low intensity exercise, representing the majority of daily activities. This study aimed to provide an evaluation of different pacing modes using O2 kinetics during low intensity exercise. Nineteen patients (age 61 +/- 18 years) with complete AV block underwent low intensity treadmill exercise (35 W) with simultaneous evaluation of symptoms and O2 kinetics in three pacing modes. The first test was performed in DDD mode followed by a second test in VVIR mode with a programmed heart rate corresponding to the sinus rate during the first test. After 6 minutes of each test, the mode was switched from DDD to VVIR and vice versa. The third test was performed in VVI mode at 70 beats/min. O2 kinetics were defined as O2 deficit (time [rest to steady state] x delta VO2-sigma VO2 [rest to steady state]) and mean response time (MRT) of oxygen consumption (O2 deficit/delta VO2). The O2 deficit was 551 +/- 134 mL in DDD pacing, 634 +/- 139 mL in VVIR pacing, and 648 +/- 179 mL in VVI pacing (P = 0.001). MRT was 49 +/- 7.8 seconds in DDD pacing, 54.7 +/- 9.5 seconds in VVIR pacing, and 57.4 +/- 11.0 seconds in VVI pacing (P = 0.002). Ten (53%) patients developed symptoms during switch from DDD to VVIR mode whereas the switch from VVIR to DDD mode was not perceived by any patient (P < 0.001). In conclusion, our study shows an impact of AV synchronous pacing and heart rate adaptation on O2 kinetics during low intensity exercise that correspond to casual daily life activities. Our observations may have clinical implications for the management of patients with complete AV block.

Prognostic significance of oxygen uptake kinetics during low level exercise in patients with heart failure

Oxygen uptake kinetics during low-intensity exercise were investigated in 48 patients with congestive heart failure to assess their prognostic value compared with established predictors of prognosis including neurohumoral stimulation. Mean response time of oxygen uptake during low-intensity exercise, which does not require the patient's maximal effort, appears to be an important predictor of prognosis in these patients.

Hemodynamic and cardiorespiratory function following internal atrial defibrillation for chronic atrial fibrillation

Internal atrial defibrillation (IAD) is able to restore sinus rhythm in patients with chronic atrial fibrillation (AF) and failed external electrical and/or pharmacologic cardioversion. To assess whether cardiorespiratory and hemodynamic function improve after IAD, 35 patients were prospectively investigated during constant workload exercise by spiroergometry and Doppler echocardiography before IAD, and 1 day and 1 month after IAD. Oxygen uptake kinetics, ventilation, left atrial mechanical function, and pulmonary artery pressure were determined simultaneously at rest and during steady state. During the serial follow-up, 20 patients maintained sinus rhythm. The time interval for achieving the steady state (146 +/- 53 vs 132 +/- 42 seconds; p = 0.5) and the oxygen deficit (645 +/- 190 vs 670 +/- 174 ml; p = 0.7) were not different before and 1 day after IAD, but decreased significantly after 1 month (98 +/- 16 seconds, p = 0.01 and 487 +/- 72 ml, p = 0.02). Exercise pulmonary artery systolic pressures were 38 +/- 13 mm Hg before IAD, increased significantly to 46 +/- 11 mm Hg on day 1 (p = 0.03), and decreased below baseline values at 1 month to 31 +/- 12 mm Hg (p = 0.07). Peak A-wave velocities increased from 0.51 +/- 0.1 m/s after 1 day to 0.67 +/- 0.2 m/s after 1 month (p = 0.03). Restoration of sinus rhythm in patients with AF resistant to external electrical and/or pharmacologic cardioversion improves hemodynamic and cardiorespiratory function at daily activity exercise levels.

Oxygen uptake kinetics during low level exercise in patients with heart failure: relation to neurohormones, peak oxygen consumption, and clinical findings

OBJECTIVE

To investigate whether oxygen uptake (VO2) kinetics during low intensity exercise are related to clinical signs, symptoms, and neurohumoral activation independently of peak oxygen consumption in chronic heart failure.

DESIGN

Comparison of VO2 kinetics with peak VO2, neurohormones, and clinical signs of chronic heart failure.

SETTING

Tertiary care centre.

PATIENTS

48 patients with mild to moderate chronic heart failure.

INTERVENTIONS

Treadmill exercise testing with "breath by breath" gas exchange monitoring. Measurement of atrial natriuretic factor (ANF), brain natriuretic peptide (BNP), and noradrenaline. Assessment of clinical findings by questionnaire.

MAIN OUTCOME MEASURES

O2 kinetics were defined as O2 deficit (time [rest to steady state] x DeltaVO2 -sigmaVO2 [rest to steady state]; normalised to body weight) and mean response time of oxygen consumption (MRT; O2 deficit/DeltaVO2).

RESULTS

VO2 kinetics were weakly to moderately correlated to the peak VO2 (O2 deficit, r = -0.37, p < 0.05; MRT, r = -0.49, p < 0.001). Natriuretic peptides were more closely correlated with MRT (ANF, r = 0.58; BNP, r = 0.53, p < 0.001) than with O2 deficit (ANF, r = 0.48, p = 0.001; BNP, r = 0.37, p < 0.01) or peak VO2 (ANF, r = -0.40; BNP, r = -0.31, p < 0.05). Noradrenaline was correlated with MRT (r = 0. 33, p < 0.05) and O2 deficit (r = 0.39, p < 0.01) but not with peak VO2 (r = -0.20, NS). Symptoms of chronic heart failure were correlated with all indices of oxygen consumption (MRT, r = 0.47, p < 0.01; O2 deficit, r = 0.39, p < 0.01; peak VO2, r = -0.48, p < 0. 01). Multivariate analysis showed that the correlation of VO2 kinetics with neurohormones and symptoms of chronic heart failure was independent of peak VO2 and other variables.

CONCLUSIONS

Oxygen kinetics during low intensity exercise may provide additional information over peak VO2 in patients with chronic heart failure, given the better correlation with neurohormones which represent an index of homeostasis of the cardiovascular system.