Proceedings of The Physiological Society
University of Cambridge (2008) Proc Physiol Soc 11, PC46
The response to hypoxia: a comparison of the Human Patient Simulator (HPS) with human data
R. Helyer1, A. Coombs1,2, A. Cousins1,2, H. Dee1,2, E. Kermode1, C. Rogers1,2, E. Lloyd1
1. Department of Physiology & Pharmacology and AIMS Centre For Excellence in Teaching & Learning, University of Bristol, Bristol, United Kingdom. 2. Intercalating MB ChB students, University of Bristol, Bristol, United Kingdom.
The Human Patient Simulator (HPS 337; METI, Sarasota, Florida) has a computer driven mechanical lung and gas exchange mechanism, designed to model the human respiratory system. We have previously demonstrated that the HPS is a useful tool for illustrating physiological principles but requires adjustments to the modeling software in order to improve the fidelity of the quantitative response to perturbations such as simulated hypovolaemia . The aim of this study was to compare the response of the HPS to hypoxia with available human data in order to determine the accuracy of the HPS response and the utility of the HPS for teaching high altitude human physiology. Data can be obtained from the HPS for a range of respiratory variables including breathing rate, tidal volume, simulated alveolar and arterial partial pressures of oxygen and carbon dioxide, and arterial oxygen saturation. The simulator was intubated and baseline measurements were made of the respiratory variables including the partial pressures of oxygen and carbon dioxide in alveolar (PAO2 and PACO2) gas whilst “breathing” atmospheric air at ambient pressure. The responses were then determined to breathing hypoxic gas mixtures (range = 19-5% O2) applied using Douglas bags and a three-way valve system to separate inspired and expired gases. The data were used to construct an Oxygen-Carbon Dioxide diagram for comparison with published human data [2,3,4]. In response to hypoxia the HPS showed a linear relationship between PAO2 and PACO2 over the entire range investigated (PAO2 = 25-110 mmHg; n = 6). Published human data for unacclimatised individuals shows a linear relationship between PAO2 and PACO2 over a PAO2 range of 60-100 mmHg but below this range there is a non-linear relationship [2,3]. This is due to hyperventilation resulting in a respiratory alkalosis when PAO2 decreases below approximately 60mmHg .The HPS failed to exhibit appropriate degrees of respiratory alkalosis in response to PAO2 below 60 mmHg, equivalent to breathing atmospheric air at an altitude of around 2500m (8500ft) and a barometric pressure of 550 mmHg [approximated values derived from 5] We conclude that the HPS is a useful tool for demonstrating trends in the physiological response to changes in the environment, in this case hypoxia, but that the model requires some adjustment in order to more accurately represent human data and demonstrate high altitude physiology.
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