Measurement of cardiac output by open-circuit acetylene uptake: a computer model to quantify error caused by ventilation-perfusion inequality.

The ability to assess cardiac output (Q(T)) noninvasively has been the focus of interest for many researchers. While the open-circuit acetylene (C2H2) method seems promising, it is prone to error due to ventilation-perfusion (V/Q) inequality. Measurements during exercise, at high altitude or in patients with chronic obstructive pulmonary disease (COPD) could be unreliable and further validation studies under these circumstances may be needed.

Beta-adrenergic or parasympathetic inhibition, heart rate and cardiac output during normoxic and acute hypoxic exercise in humans.

Acute hypoxia increases heart rate (HR) and cardiac output (Qt) at a given oxygen consumption (VO2) during submaximal exercise. It is widely believed that the underlying mechanism involves increased sympathetic activation and circulating catecholamines acting on cardiac beta receptors. Recent evidence indicating a continued role for parasympathetic modulation of HR during moderate exercise suggests that increased parasympathetic withdrawal plays a part in the increase in HR and Qt during hypoxic exercise.

Human autonomic activity and its response to acute oxygen supplement after high altitude acclimatization.

It is well established that after acclimatization at high altitude, many sympathetic pathways are hyperactive yet heart rate (HR) remains unchanged. In this study, we attempted to determine if this unchanged heart rate is due to compensatory mechanisms such as changes in parasympathetic activity or levels of receptors for autonomic neurotransmitters. We also examined the role played by hypoxia in these autonomic adaptations to high altitude.

Role of the autonomic nervous system in the reduced maximal cardiac output at altitude.

After acclimatization to high altitude, maximal exercise cardiac output (QT) is reduced. Possible contributing factors include 1) blood volume depletion, 2) increased blood viscosity, 3) myocardial hypoxia, 4) altered autonomic nervous system (ANS) function affecting maximal heart rate (HR), and 5) reduced flow demand from reduced muscle work capability. We tested the role of the ANS reduction of HR in this phenomenon in five normal subjects by separately blocking the sympathetic and parasympathetic arms of the ANS during maximal exercise after 2-wk acclimatization at 3,800 m to alter maximal HR.

Validity of pulse oximetry during maximal exercise in normoxia, hypoxia, and hyperoxia.

During exercise, pulse oximetry is problematic due to motion artifact and altered digital perfusion. New pulse oximeter technology addresses these issues and may offer improved performance. We simultaneously compared Nellcor N-395 (Oxismart XLTM) pulse oximeters with an RS-10 forehead sensor (RS-10), a D-25 digit sensor (D-25), and the Ivy 2000 (Masimo SETTM)/LNOP-Adt digit sensor (Ivy) to arterial blood oxygen saturation (Sa(O(2))) by cooximetry.