Effects of Acute Leg Ischemia during Cycling on Oxygen and Carbon Dioxide Stores

Article excerpt

INTRODUCTION

Progressive physical deconditioning is common in patients with chronic diseases, such as congestive heart failure and chronic obstructive pulmonary disease. One limitation these patients face is an inability to exercise with sufficient intensity to provide adequate training stimuli. However, regional training of muscles without taxing the central circulation can improve whole-body exercise capacity in these patients [1]. An unusual potential tool to facilitate regional muscle rehabilitation is exercise training during reduced limb blood flow [2-3]. Such "ischemic limb training" with limb pressure cuffs has improved limb strength and exercise endurance in physically fit subjects [4-5], diminished postoperative disuse atrophy of knee extensors [6], and induced favorable biochemical and structural changes in muscles [7-8]. Ischemic limb training with low-intensity exercise in patients with congestive heart failure has also reduced exertional dyspnea [9]. We recently demonstrated that leg-extension exercise endurance was enhanced with a 6-week training program of very light leg-extension exercise with ischemia induced by thigh cuff inflation [10].

Superimposing ischemia on exercising limbs provokes the muscle metaboreflex, whereby pulmonary ventilation ([[??].sub.E]) and systemic blood pressure are elevated by a chemoreflex stimulated by buildup of metabolic byproducts in the ischemic limbs; the most likely candidate is hydrogen ion concentration ([H.sup.+]) [11]. The oxygen ([O.sub.2]) stores ([O.sub.2]s) and carbon dioxide ([CO.sub.2]) stores ([CO.sub.2]s) in the region where blood flow is occluded, as well as in the whole body, will be affected during this ischemia and after circulation is restored as a result of ventilatory, blood flow, and biochemical perturbations. The magnitude and time course of these gas store changes will affect regional and whole-body acid-base status, will cause secondary ventilatory and gas exchange fluctuations during and after exercise, and may induce transient hypoxemia and hypercapnia, such as noted following passive changes in posture [12].

Although rapid transient changes in [O.sub.2]s and [CO.sub.2]s during exercise workload transitions have been studied and quantified [13], gas store changes induced by limb ischemia have received little attention. Specifically, the quantitative relationship is not well defined between [O.sub.2] repayment and [CO.sub.2] elimination after exercise requiring energy partially derived from anaerobic sources [14] and these measurements with the anaerobic component artificially superimposed have not been reported. Therefore, this study was an initial attempt to estimate the time course and magnitude of changes in [O.sub.2]s and [CO.sub.2]s during and after acute, temporary ischemia of the legs applied by cuff inflation during steady state exercise on a cycle ergometer.

METHODS

Subjects

Five men and one woman volunteered as subjects. Informed consent was obtained from each person, as approved by the University of New Mexico Human Research Review Committee. All were physically fit and regularly taking part in physical recreation and fitness activities, including jogging and cycling. Their ages ranged from 24 to 62 yr, with a mean body weight and body mass index of 82.5 kg and 25.0 kg/[m.sup.2], respectively. Their maximal [O.sub.2] uptake ([??][O.sub.2]max) averaged 48 [mL.min.sup.-1].[kg.sup.-1] (range: 42-56). The [O.sub.2] uptake ([??][O.sub.2]) during exercise before ischemia (baseline) averaged 35.7 percent (range: 30%-42%, standard error of the mean = 1.7%) of the subjects' [??][0.sub.2max]. This percentage was not related to age (r = -0.22).

Ergometer Exercise and Inflation Cuffs

We placed cuffs on each upper thigh (SC-17, Hokanson Co; Bellevue, Washington) and each lower leg (SC22) using adhesive tape to keep them in position during exercise. …