Limited maximal exercise capacity in patients with chronic heart failure: partitioning the contributors - PubMed (original) (raw)

Limited maximal exercise capacity in patients with chronic heart failure: partitioning the contributors

Fabio Esposito et al. J Am Coll Cardiol. 2010.

Abstract

Objectives: This study aimed to assess the factors limiting maximal exercise capacity in patients with chronic heart failure (CHF).

Background: Maximal exercise capacity, an important index of health in CHF, might be limited by central and/or peripheral factors; however, their contributions remain poorly understood.

Methods: We studied oxygen (O2) transport and metabolism at maximal cycle (centrally taxing) and knee-extensor (KE) (peripherally taxing) exercise in 12 patients with CHF and 8 healthy control subjects in normoxia and hyperoxia (100% O2).

Results: Peak oxygen uptake (VO2) while cycling was 33% lower in CHF patients than in control subjects. By experimental design, peak cardiac output was reduced during KE exercise when compared with cycling (approximately 35%); although muscle mass specific peak leg VO2 was increased equally in both groups (approximately 70%), VO2 in the CHF patients was still 28% lower. Hyperoxia increased O2 carriage in all cases but only facilitated a 7% increase in peak leg VO2 in the CHF patients during cycling, the most likely scenario to benefit from increased O2 delivery. Several relationships, peak leg VO2 (KE + cycle) to capillary-fiber-ratio and capillaries around a fiber to mitochondrial volume, were similar in both groups (r = 0.6-0.7).

Conclusions: Multiple independent observations, including a significant skeletal muscle metabolic reserve, suggest skeletal muscle per se contributes minimally to limiting maximal cycle exercise in CHF or healthy control subjects. However, the consistent attenuation of the convective and diffusive components of O2 transport (25% to 30%) in patients with CHF during both cycle and even KE exercise compared with control subjects reveals an underlying peripheral O2 transport limitation from blood to skeletal muscle in this pathology.

Copyright 2010 American College of Cardiology Foundation. Published by Elsevier Inc. All rights reserved.

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Figures

Figure 1

Figure 1. Exercise Modalities

A schematic representation of the relatively small ratio of cardiac capacity to skeletal muscle recruitment during standard cycle ergometry (A), the much greater ratio during single-leg knee-extension (B), and the subsequently contrasting physiological responses to these exercise paradigms.

Figure 2

Figure 2. Oxygen Transport and Use

A comparison of oxygen transport and use parameters assessed at maximal cycle (A) and knee-extensor (KE) (B) exercise in patients with chronic heart failure (CHF) (n = 12) and control subjects (n = 8). Control data have been normalized to 100% as a point of reference for the data collected in the patients with CHF. CaO2-CvO2 = arterial-venous oxygen content difference; DMO2 = diffusional oxygen conductance; leg VO2 = 1-leg oxygen uptake; QO2 = 1-leg oxygen delivery.

Figure 3

Figure 3. Effect of Hyperoxic Breathing

The effect of hyperoxia (100% oxygen) on peak leg VO2 assessed at maximal cycle (A) and KE exercise (B) in patients with CHF (n = 12) and control subjects (n = 8). Control data have been normalized to 100% as a point of reference for the data collected in the patients with CHF. Abbreviations as in Figure 2.

Figure 4

Figure 4. Capillarity and Peak Oxygen Uptake

The relationship between capillary/fiber ratio and peak VO2 assessed at maximal cycle (A) and KE exercise (B) in patients with CHF (n = 9) and control subjects (n = 6). The decision to combine the correlation for both patients and control subjects was based upon the limited number of the data points and observation that the addition of the patients improved the original relationship displayed by the control subjects. Abbreviations as in Figure 2.

Figure 5

Figure 5. Mitochondrial Volume and Capillarity

The relationship between mitochondrial volume and number of capillaries around a muscle fiber in patients with chronic heart failure (CHF) (n = 9) and control subjects (n = 6). The decision to combine the correlation for both patients and control subjects was based upon the limited number of the data points and observation that the addition of the patients improved the original relationship displayed by the control subjects.

Figure 6

Figure 6. Convective and Diffusive Components of O2 Transport

A schematic illustration of the convective and diffusive components that interact to determine peak oxygen uptake (VO2) in both CHF and control subjects during both cycle (large muscle mass) and KE exercise (small muscle mass). In both scenarios patients with CHF exhibit an attenuated diffusional conductance (slope of the Fick law line). Additionally, it should be noted that the Fick principle lines are not straight, because they are directly reflective of the hemoglobin dissociation curve. Therefore, a left-shifted hemoglobin dissociation curve (greater hemoglobin oxygen affinity), resulting in a lower venous oxygen partial pressure (PO2), will bisect the Fick law line earlier and reduce maximal VO2 and vice versa. This would be the equivalent of anchoring the existing CHF Fick principle line at its origin on the x axis and altering its shape (a left-shifted O2 dissociation curve) to pass through point D (reduced peak VO2) or a right-shifted O2 dissociation curve that would pass through point E (elevated peak VO2). Explanations for point A, B, and C are given in the text. Abbreviations as in Figure 2.

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