G. Kwant

A fiberoptic reflection oximeter.

A catheter tip oximeter is described consisting of a cardiac catheter containing optical fibers, an incandescent light source, a light detection unit and a processing unit. Half of the optical fibers guide the light to the blood at the tip of the catheter, the other half the backscattered (reflected) light to the detection unit. The detection unit contains a dichroic mirror, transmitting most of the light with λ<800 nm and reflecting most of the light with λ>900 nm, thus splitting the light into two beams. These pass through interference filters with nominal wavelengths of 640 and 920 nm respectively, and are focused on silicium barrier layer photocells. The photocell signals are amplified and fed into a divider giving the ratio of measuring (R640) and compensating (R920) photocell output. The relationship between logR640/R920 and oxygen saturation is represented by a slightly curved line. The relation may be linearized by subtracting a constant voltage from the divider output before taking the logarithm. The slope of the calibration line is dependent on the total haemoglobin concentration. Nonetheless an average calibration line can be used between 70 and 100% oxygen saturation. For 78 measurements of pig blood samples in this range (haemoglobin concentration between 96 and 161 g ·l−1), the standard deviation of the difference between the fiberoptic oximeter and a Radiometer OSM1 oxygen saturation meter was 1.9% saturation, for 152 samples over the entire saturation range the standard deviation of the difference was 3.1% saturation. The influence of the flow velocity of blood on the light reflection depends on wavelength as well as on oxygen saturation. Therefore, complete compensation for the flow effect is not possible by simple means.

Influence of variations in pH and PCO2 on scalp tissue oxygen tension and carotid arterial oxygen tension in the fetal lamb

A description is given of the effect of hypercapnic acidaemia and hypocapnic alkalaemia on scalp tissue PO2 as measured with a subcutaneous needle-electrode and a transcutaneous electrode in 6 fetal lambs. The experiments were carried out under general anaesthesia with the fetus kept in utero. Hypocapnia was induced by hyperventilating the ewe and hypercapnia was achieved by administering extra CO2 to the ewe. Fetal carotid arterial, subcutaneous and transcutaneous PO2 were continuously recorded, and fetal and maternal arterial pH and arterial PCO2 were determined from blood samples taken at short intervals. In each experiment the H+ Bohr factor of fetal and maternal blood was measured. During hypocapnic alkalaemia, there was a fall in all fetal PO2 levels, whereas a marked rise was observed during hypercapnic acidaemia. The variations in fetal PO2 observed in vivo even exceeded the variations due to the H+ Bohr effect (measured in vitro). This was due to small variations in fetal carotid arterial oxygen saturation, which tended to fall during hypocapnic alkalaemia and to rise during hypercapnic acidaemia. The results of these findings strongly suggest that tissue PO2, as measured with the subcutaneous and transcutaneous electrodes, is dependent on the H+ Bohr effect. This adds to the uncertainty as to the value of subcutaneous and transcutaneous PO2 monitoring during labour as an early warning system for impending fetal asphyxia.

CONTINUOUS INTRA-ARTERIAL PH MEASUREMENT

SummaryA flexible glass electrode catheter with a diameter of 3 mm has been developed for pH measurement in the arterial system of dogs. In combination with a galvanically isolated amplifier, an undisturbed pH signal could be obtained from the aorta. The system was fast enough to truly record pH changes synchronous with respiration and was shown to be insensitive to variations in blood flow velocity. Good agreement was found between pH catheter readings and pH values of simultaneously taken arterial samples as measured with a conventional capillary glass electrode.

Carboxyhemoglobin: Determination and significance in oxygen transport

Apart from being the direct cause of quite a few cases of near-fatal poisoning, carbon monoxide is a very common noxious agent functioning as an additional factor in human disease. The extent of the possible role of carbon monoxide in human pathology is demonstrated by the following data from the Diakonessenhuis Groningen, a 400-bed private hospital1. During a five-month period the carboxyhemoglobin fraction (FHbCO) was measured in the blood of all pre-operative patients. In 64.4% of the 1358 cases FHbCO was 10%. The highest value measured in this series was 15.5%. Thus it appears that in a considerable number of patients HbCO fractions are present that are of possible pathophysiological significance. This shows the practical importance of the pathophysiology of carboxyhemoglobin and the need for easy and reliable methods for the determination of FHbCO in blood.

Continuous Measurement of Scalp Tissue Oxygen Tension and Carotid Arterial Oxygen Tension in the Fetal Lamb

Scalp tissue PO2, carotid arterial PO2 fetal heart rate were continuously measured in the anaesthetized fetal lamb in utero while variations in oxygen supply were brought about. In some experiments the transcutaneously measured fetal scalp PO2 was recorded in addition. Scalp tissue PO2 was measured using specially designed miniature needle-type oxygen electrode, incorporated in an easily applicable spiral scalp electrode as commonly used for fetal heart rate monitoring. The measurements showed that fetal carotid arterial hypoxaemia is always nearly immediately followed by fetal scalp tissue hypoxia, and that the recovery of scalp tissue PO2 after a hypoxic period has a remarkably varying time course. Fetal heart rate usually decreased during hypoxia, but in some instances it did not change or even increased, demonstrating that heart rate is not always a reliable indicator of fetal hypoxia. PO2 values obtained with the transcutaneous method were higher than those with the needle electrode, because of the effect of the heating system of the transcutaneous electrode on tissue blood flow and haemoglobin oxygen affinity. It would seem that during hypoxaemia the decrease in scalp tissue PO2 is poossibly the combined result of the fall in arterial PO2 and a concomitant decrease in blood flow through the skin.

Chronic hypoxemia in the 5-week-old lamb

The mechanisms involved in the adaptation to chronic hypoxemia, so as to maintain normal systemic oxygen delivery (SOD), are not quite clear. Therefore we studied these mechanisms by inducing chronic hypoxemia in 6 newborn lambs. We placed an inflatable balloon around the pulmonary artery after atrial septostomy, and inserted catheters. On days 3-4 after surgery the balloon was gradually inflated. In another 6 lambs only catheters were placed. At 5 weeks of age the lambs were studied. The hypoxemic lambs had a lower aortic and mixed venous oxygen saturation (60±10 (SD) vs 93±2, and 34±7 vs 58±2%, resp., p<0.001), and higher hemoglobin concentration (132±15 vs 101±10 g·1−1, p<0.002). Their P50 as well as their systemic blood flow were not significantly different. The heart rate of the hypoxemic lambs, however, was higher (205±22 vs 153±42, p<0.05). Although SOD in the hypoxemic lambs was lower than in the control lambs, this difference was not statistically significant (17.0±3.4 vs 21.2±5.9 ml.min−1kg−1). The mean weight gain of the hypoxemic lambs was lower (90±39 vs 153±34 g·day−1, p<0.02), while their oxygen consumption was the same as in the control lambs (7.0±2.2 vs 7.2±1.5 ml·min−1kg−1).These data show that to adapt to chronic hypoxemia the lamb increases its hemoglobin concentration to improve arterial oxygen content, and its heart rate to maintain cardiac output. We speculate that the costs of chronic hypoxemia, such as increased cardiac work, are probably effected at the expense of growth.

Determination of interdependent ligand effects on human red cell oxygen affinity.

Human whole blood oxygen affinity was determined as P50, i.e. PO2 for oxygen saturation (SO2) = 0.5, in a new system which allows the measurement of SO2, PO2 and pH continuously and independently, with control of PCO2 and temperature. The influence of pH on P50 expressed as the H+ factor (delta log P50/delta pH)PCO2 was measured under conditions of varying PCO2, temperature and concentration of 2,3-diphosphoglycerate (2,3-DPG), resulting in a set of data expressing second-order inter-ligand interactions. The H+ factor appeared to be only slightly dependent on PCO2. Similarly, the CO2 factor (delta log P50/delta log PCO2)pH shows only a minor dependence on pH. The H+ factor in linearly related to the temperature: at 17 degrees C and 42 degrees C the H+ factor is about -0.53 and -0.36, respectively. Likewise, the temperature factor (delta log P50/delta T)PCO2,pH is linearly related to pH. A pilot study on the effect of varying intra-erythrocytic 2,3-DPG concentrations on the oxygen affinity showed that a very low 2,3-DPH/Hb4 ratio apparently does not influence the H+ factor. A high ratio, however, seems to lower the H+ factor considerably.