Eileen Palayiwa

The effect of dyshemoglobins on pulse oximetry: Part I, theoretical approach and part II, experimental results using an in vitro test system

Pulse oximeters are known to be inaccurate in the presence of elevated concentrations of carboxyhemoglobin and methemoglobin. This paper attempts to alleviate some of the confusion that exists between fractional and functional saturation, and to clarify the comparison of each with SpO2. A series of theoretical relationships between pulse oximeter reading (SpO2) and actual oxygen saturation (both fractional and functional) is derived using simple absorption theory. The theoretical relationships are checked using an experimental in vitro test system. This consists of a blood circuit containing a model finger, capable of simulating the pulsatile transmission signals through a real finger. Theoretical predictions and experimental results are compared and are found to agree well in the presence of carboxyhemoglobin, but less well with methemoglobin. Possible reasons are discussed.Pulse oximeters are known to be inaccurate in the presence of elevated concentrations of carboxyhemoglobin and methemoglobin. This paper attempts to alleviate some of the confusion that exists between fractional and functional saturation, and to clarify the comparison of each with SpO2. A series of theoretical relationships between pulse oximeter reading (SpO2) and actual oxygen saturation (both fractional and functional) is derived using simple absorption theory. The theoretical relationships are checked using an experimental in vitro test system. This consists of a blood circuit containing a model finger, capable of simulating the pulsatile transmission signals through a real finger. Theoretical predictions and experimental results are compared and are found to agree well in the presence of carboxyhemoglobin, but less well with methemoglobin. Possible reasons are discussed.

A Real-Time Algorithm to Improve the Response Time of a Clinical Multigas Analyser

Objective. An algorithm to improve the response time of a clinical respiratory multigas analyser is presented. Methods. The algorithm involves the application of a second order differential equation to the analyser gas output signals in real-time. The adjusted analyser output signals are compared with those of a quadrupole respiratory mass spectrometer sampling and analysing simultaneously. Results. Our results show a close correlation between the adjusted clinical gas analyser and the mass spectrometer signals. Lung volumes derived from a non-invasive sinusoidal inert gas forcing technique, in a model test lung, using the adjusted clinical gas analyser and the mass spectrometer signals demonstrated comparable results. Conclusions. The algorithm provides an improvement on the relatively slow response times of the clinical gas analyser for breath-by-breath time-dependent applications. The same algorithm can also be applied to other instruments which have slow response times.

Nitrous oxide solubility in halothane and its effect on the output of vaporizers

The absorption of nitrous oxide in halothane was studied by bubbling nitrous oxide and nitrous oxide/oxygen gas mixtures through a halothane bottle, using 100% oxygen as a control. The gas volume emerging from the halothane bottle was measured each minute, over a period of up to 15 minutes. When oxygen was used as a control gas, the averaged flow rate dropped slightly over the experimental period, due to the cooling of the halothane. However, in the presence of nitrous oxide, the initial flow rate of the gas emerging from the halothane bottle was greatly diminished, but then accelerated rapidly to reach that obtained with oxygen. The results suggested that nitrous oxide dissolved in large quantities in halothane, and the data are consistent with an Ostwald coefficient in excess of 4.0.

Self-calibrating interference refractometer using a linear photodiode array for anaesthetic gas analysis

The laboratory standard for measurement of gas concentrations in binary mixtures is the manually operated interfence refractometer. We describe an automatic interference refractometer for theatre use incorporating a linear photodiode array and using digital electronics for signal analysis. The support system performs automatic self-calibration, samples gases from an anaesthetic machine, feeds them to the refractometer and presents information on gas concentrations and possible alarm conditions to the anaesthetist. The instrument may be incorporated into a new anaesthetic machine or may be an addition to an existing one. The instrument may also be applied in other fields where concentrations of known gases need to be monitored automatically.