> A pulse oximeter is a device that measures blood oxygen saturation. Sensor > clips to earlobe or finger. > Then you know your saturation level and can adjust oxy flow to keep it good
If anyone is using a pulse oximeter it is important that they have some understanding of the physiology behind it. Pulse oximetry is a useful, non-invasive indirect measure of the partial pressure of arterial oxygen, but it has limitations. It utilizes the difference in infra-red absorption of saturated (oxygenated) and de-saturated (de-oxygenated) arterial and venous blood. The sensor has to pick up a good arterial trace to read accurately. This is very dramatically influenced by movement and cold (and we know it can be cold up there!) Indeed, at times it can be wildly inaccurate. The sensor reads carboxy haemoglobin the same as oxy-haemoglobin, so you smokers out there full of carbon monoxide will artificially read a percent or two higher, while actually carrying less oxygen. Oxygen saturation is only an indirect measure of the all-important factor of arterial oxygen tension. The normal healthy range is 96-99%. Decreases below this represent a very significant reduction in arterial oxygen carriage. One medical definition of respiratory failure is a PaO2 of less than 60 mmHg. This is reached at an altitude of about 10,000'. This usually corresponds to a saturation level of about 90%. 90% sounds pretty good compared to 96%, but actually represents an alarmingly low arterial oxygen level. At around 90 %, small changes in saturation also reflect a very large change in oxygen level. Even one or two percent represents a big difference, and that is well and truly within the error limit of the machine. It is well documented in aviation medicine that night vision is detrimentally affected by hypoxia due to the retinas high oxygen consumption. This occurs even at altitudes of 5000'. I wonder if there is a similar reduction in the ability to pick up day-time traffic. Interestingly, the "oxygen above 10,000' " rule does not take into account density altitude. This is based on an ISA of 10,000', ie 15 deg C at sea level. On a hot summer day of 40 deg at ground level, this is ISA+25, which equates to an extra 3000' of altitude. So at 10,000' you're actually already at a density altitude of 13,000, and starting to get quite hypoxic. Remember it's the number of molecules of oxygen (the density) that is all important. One of the most common symptoms of hypoxia is lack of insight and euphoria. After a particularly good day's flying when it was 40 degrees on the ground with thermals up to 14,000' one pilot without supplemental oxygen remarked "I felt a bit short of breath to start with, but after a while I felt fine!" Here endeth the lesson. Dave Long _______________________________________________ Aus-soaring mailing list [EMAIL PROTECTED] To check or change subscription details, visit: http://lists.internode.on.net/mailman/listinfo/aus-soaring
