Monitoring physical activity in ambulatory patients with chronic cardiac failure

Static or dynamic work in patients with chronic cardiac failure elicits a variety of pathophysiologic responses that impair the ability of the cardiopulmonary unit to sustain O2 delivery at a rate that is commensurate with the O2 requirements of working skeletal muscle. Regional abnormalities in the circulation of skeletal muscle may further compromise nutritive blood flow to muscle. As a result, the patient experiences a sense of fatigue during physical activity. Dyspnea, or an abnormal awareness of breathing, may also accompany exercise in these patients. This sense of breathlessness may be due to an abnormally elevated work of breathing secondary to decreased pulmonary compliance, a heightened chemical drive to ventilation, and a possible imbalance in the O2 supply and demand of the respiratory muscles. The ambulatory monitoring of physical activity in patients with chronic cardiac failure has not been systematically examined. Potential monitoring techniques that may provide accurate and reproducible results regarding the exercise response in these patients include the thoracic impedance principle to measure minute ventilation and portable O2 uptake sensors.