Problems with capnography at high altitude*

Abstract

SummaryThe aim of this study was to investigate why capnographs malfunction at high altitude, in order to suggest ways of improving instrument reliability. Four machines were decompressed to simulated high altitude and measurements taken. The study showed that the reduced air density at altitude has effects upon the pump, causing difficulty in entraining gas, and that high altitude has additional effects upon calibration of the instruments. There is ongoing interest in the effects of carbon dioxide (CO 2 ) on acclimatisation and mountain sickness at high altitude [1][2][3]. Experiments involving the measurement of CO 2 with capnography, however, have frequently been hampered by instrument malfunction. Most capnographs have been designed for use at low altitudes, not high altitude.We recently conducted experiments at three different altitudes in Bolivia and experienced problems with the instruments. We used a Datex-Ohmeda S ⁄ 5 critical care monitor (Datex-Ohmeda, Helsinki, Finland) and a K4b2 portable metabolic monitor (Cosmed, Rome, Italy). The Datex S ⁄ 5 worked well at 3600 m (P atm 63.0 kPa). At 4800 m (P atm 57.8 kPa) there was increased pump noise and an intermittent screen message indicating an air leak and recommending that the water trap be checked. At 5260 m (P atm 54.3 kPa), the machine gave no CO 2 recordings. We were unsure whether the instrument was malfunctioning because of the low barometric pressure, cold or damage but found that it functioned normally on descent to 3600 m. The K4b2 gave consistent readings at each altitude but again there was increased pump noise as the altitude increased.There are published data on the effect of changes in barometric pressure on the calibration of capnographs. These influence the gas flow rate and the infra-red spectroscopy measurements. These effects on the measurements can be corrected by recalibration at the ambient pressure [4,5]. However, in spite of satisfactory calibration at high altitude, the instruments still failed to perform adequately.In order to understand why the capnographs malfunctioned at high altitude we studied four side-stream analysers at several simulated high altitudes in a hypobaric chamber. By understanding the effect of altitude on these monitors, we might be able to identify the most suitable instrument for experimental use in field conditions MethodsFour capnographs were studied, all of which were sidestream analysers:1 Datex-Ohmeda S ⁄ 5 portable critical care monitor (Datex-Ohmeda Division, Instrumentarium Corporation) 2 K4b2 portable metabolic monitor (Cosmed)

Effect of exercise on cerebral perfusion in humans at high altitude

“…The Cosmed K4b 2 was chosen for its portability and performance at high altitude (6), which has been confirmed subsequently by the authors (36). Gases were collected via a tight-fitting Cosmed-modified Hans Rudolph facemask.…”

Exercise intolerance at high altitude (5050m): Critical power and W′

“…correcting for the transport delay between the turbine and gas analyzers and for the analyzer rise time (Beaver et al, 1973). The accuracy of the telemetric system has been previously established (Palange et al, 2000;Pattinson et al, 2004). Heart rate was calculated beat-by-beat, and SpO 2 was monitored non-invasively using finger pulse oximetry (Masimo Rad-5, Masimo Corporation, Irvine, CA, USA).…”

“…Prior to each test, the zero-load setting on the ergometer was checked and a calibration was performed with a 4 kg weight. Ventilatory and pulmonary gas exchange variables were measured breath-by-breath in all tests using a portable system (K4b 2 , COSMED, Italy), which has previously been used at HA (Pattinson et al, 2004). The system comprised a face mask, analyzer unit (containing O 2 and CO 2 gas analyzers), heart-rate monitor and battery.…”

Physiology and pathophysiology at high altitude: considerations for the anesthesiologist

“…Capnograph malfunction is common at HA, possibly due to reduced gas fl ow rates through the sample chamber, the effect of low barometric pressure on the calibration, or the effect of the reduced barometric pressure on the computer software [86]. Furthermore, signifi cant hyperventilation and low P CO 2 levels may make colorimetric P CO 2 detectors unreliable [87].…”

“…Barometric pressure also affects the function of capnographs, which need to be recalibrated at the ambient pressure [86]. Capnograph malfunction is common at HA, possibly due to reduced gas fl ow rates through the sample chamber, the effect of low barometric pressure on the calibration, or the effect of the reduced barometric pressure on the computer software [86].…”