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The photoplethysmorgraphic signal (PPG) from forearm skin and the ECG were recorded simultaneously from healthy subjects. The optical signal was derived with a fibre-optic probe which consisted of 61 fibre pairs. The peak-to-peak averaged a.c.-signal was calculated. The dependency of this signal and the signal-to-noise ratio (SNR) on the light wavelength and on sample volume was studied. The light intensities used at 560 nm were 0.015 and 0.029 mWmm-2 and at 940 nm we used 0.029, 1.37 and 2.77 mWmm-2. A theoretical model for calculation of the sample volumes was also developed. The amplitude and SNR of the a.c.-signal at 560 nm is larger than at 940 nm at the same intensity (0.029 mWmm-2). At 940 nm using 1.37 and 2.77 mWmm-2 the a.c.-signal increased with the number of fibre pairs (volume) while the SNR was little affected. At 560 nm it is suggested that the major contribution to the a.c.-signal is pulsations in the ascending arterioles and at 940 nm from pulsations in the larger vessels deeper in the tissue volume under study.

A new method, which uses a fibre-optic probe to monitor respiratory and heart rates simultaneously, has been developed and evaluated. The results provide evidence that it is possible to monitor respiratory and heart rates using the reflection mode of photoplethysmography (PPG). The fibre-optic probe makes it possible to monitor from different sites on the patient, and the method is convenient to use. In addition, the probe is X-ray transparent, insensitive to electromagnetic interference (e.g. during MRI investigation) and may be made very light and small. Therefore the method is suitable for the observation of both adults and neonates, in hospitals as well as in other environments.

Heart- and respiration rate related signals were measured simultanously using reflection mode of photoplethysmography (PPG) through a fiber optic sensor. The sensor was attached to the ear-lobe. Both heart- and respiration related variations in blood perfusion could be observed after filtering the PE signal. The frequency content of the signal was observed by calculating the power spectral density.

Ten newborn infants, born at 25-41 wk of gestation and nursed in a neonatal intensive care unit or in a neonatal intermediate care unit, underwent monitoring of heart and respiratory rates with a new technique using an optical sensor, 0-60 d postnatally. The aim of this study was to compare the heart and respiratory rates recorded in infants by photoplethysmography with a probe positioned on three monitoring sites, namely the leg, buttock and interscapular region, with the rates recorded by ECG and transthoracic impedance, respectively. The recordings were compared in order to determine which individual heart beats and respiratory cycles were recorded with one or both relevant techniques. A high degree of association (r =0.99) was found between the respiratory rates recorded by photoplethysmography and by transthoracic impedance. Recordings of heart rates by ECG were also highly associated with the photoplethysmographic rates as measured at two (r = 0.99) of the three monitoring sites. It is concluded that heart and respiratory rates in infants can be monitored satisfactorily by the new method based on photoplethysmography. This method offers the advantage of recording several physiological parameters non-invasively with a single probe during the neonatal period.

Visual observation was chosen as the reference method for measuring the respiratory rate in ten healthy volunteers. The new fibre-optic and acoustic sensors were simultaneously compared with capnography and transthoracic impedance plethysmography during normoventilation in the respiratory rate range of 6-24 breaths per minute and at a fixed respiratory rate of 13 breaths per minute. In addition a simulation of central apnoea was performed. All the measurements were recorded on an analogue tape recorder and a strip-chart recorder and analysed off line. The analyses of the recordings were performed by a person who was unable to see the monitoring systems. There was no discrepancy in the results of these methods. Each of the methods responded rapidly to an apnoeic event. The new fibre-optic and acoustic sensors correlate well with more traditional methods such as capnography and transthoracic impedance plethysmography for respiratory rate monitoring.