B. Oeseburg

The oxygen binding capacity of human haemoglobin

SummaryThe internationally standardized HiCN method for measuring the total haemoglobin concentration of the blood (cHb*) is based on the chemical composition of the haemoglobin molecule. If all haemoglobin measured in the blood by this method were active, i.e., capable of reversibly binding O2, the O2 binding capacity (β) would be 1.39 ml · g−1 The experimental values of β to be found in the literature vary over a wide range, values as low as 1.24 ml · g−1 being no exception. We determined β for 36 blood samples of healthy men, using for the determination of the O2 content 3 independent methods, i.e., a manometric, a polarographic and a chemical one. Before the measurements the blood was equilibrated with humidified pure O2 for 3 h. Appropriate corrections for dissolved O2 were made. The mean β was 1.368 ml · g−1, with a s.d. of 0.017 ml · g−1 (median 1.372 ml · g−1; range 1.322–1.387 ml · g−1). This mean β corresponds to 98.4% active haemoglobin. The inactive haemoglobin consisted of 0.7% HbCO, Hi and SHb (“dyshaemoglobin”) and 0.9% unidentified inactive haemoglobin. In 4 samples a high fraction of unidentified inactive haemoglobin was found (2.8–4.3%). Taking into account that in addition to this fraction a considerable amount of dyshaemoglobin (especially HbCO) may be present in the blood of normal men, it must be concluded that calculating the O2 carrying capacity of the blood by multiplyingcHb* by a constant value of β, may be subject to an appreciable error. If β=1.39 ml · g−1 is used, the O2 carrying capacity of the blood may easily be found 10–15% too high.The internationally standardized HiCN method for measuring the total haemoglobin concentration of the blood (c Hb * ) is based on the chemical composition of the haemoglobin molecule. If all haemoglobin measured in the blood by this method were active, i.e., capable of reversibly binding O2, the O2 binding capacity (β) would be 1.39 ml · g−1 The experimental values of β to be found in the literature vary over a wide range, values as low as 1.24 ml · g−1 being no exception. We determined β for 36 blood samples of healthy men, using for the determination of the O2 content 3 independent methods, i.e., a manometric, a polarographic and a chemical one. Before the measurements the blood was equilibrated with humidified pure O2 for 3 h. Appropriate corrections for dissolved O2 were made. The mean β was 1.368 ml · g−1, with a s.d. of 0.017 ml · g−1 (median 1.372 ml · g−1; range 1.322–1.387 ml · g−1). This mean β corresponds to 98.4% active haemoglobin. The inactive haemoglobin consisted of 0.7% HbCO, Hi and SHb (“dyshaemoglobin”) and 0.9% unidentified inactive haemoglobin. In 4 samples a high fraction of unidentified inactive haemoglobin was found (2.8–4.3%). Taking into account that in addition to this fraction a considerable amount of dyshaemoglobin (especially HbCO) may be present in the blood of normal men, it must be concluded that calculating the O2 carrying capacity of the blood by multiplyingc Hb * by a constant value of β, may be subject to an appreciable error. If β=1.39 ml · g−1 is used, the O2 carrying capacity of the blood may easily be found 10–15% too high.

In-Vivo Experiments with a pH-ISFET Electrode

Thein vivo stability and pH sensitivity of 19 intravascular pH electrodes for continuous monitoring have been investigated. The sensors were mounted in indwelling vascular catheters (7F) which were inserted into the arteries of seven anaesthetised and mechanically ventilated dogs. Variations in arterial pH, ranging from 6·82 to 7·72, were obtained by infusion of sodium bicarbonate and hydrochloric acid and by hyper/ hypoventilation with various volume fractions of carbon dioxide in the inspiratory gas mixture. The pH sensor output potential was compared within vitro pH determinations of arterial blood samples. After an initial stabilisation period following their introduction into the arterial blood, the electrodes showed an average long-term drift of 2·3 mVh−1. When this drift was taken into account, a typical pH sensitivity of 50 mV per pH was found. The relationship between the electrode potentials and thein vitro pH values was linear and in almost all cases the correlation coefficient (r) was above 0·9. The electrodes responded rapidly enough to reflect breath-to-breath oscillations in pH.

Intravascular pH-ISFET, a method of the future

Since blood pH reflects the metabolic state as well as the functioning of the respiratory and circulatory systems, it appears to be one of the vital parameters that should be continuously monitored. The point to point measurement of pH by sampling has limitations, since results are available after some time only. A continuous pH signal is necessary in rapidly changing situations such as often occur during intensive treatment. The application of ion-selective field-effect transistor (ISFET) technology to intravascular pH measurement seemed attractive since these devices offer a short response time, are suitable to miniaturization and fabrication in large quantities, and have a low output impedance. The system was tested in dog experiments. Two to four pH-ISFET catheters were tested simultaneously in each dog. Arterial samples for blood-gas analysis with a Radiometer ABL2 system were taken during periods in which plasma-pH was stable. Clinical trials were performed in patients who were admitted to the intensive care unit after coronary artery bypass grafting. Testing the pH-ISFET system in dogs (N = 12) and comparing it with ABL2 measured samples (n = 132 with a pH range of 6.72-7.86), resulted in a delta pH (ISFET-ABL2) of 0.016 +/- 0.024 pH with a maximum deviation of 0.06. The clinical trials proved that the system is well-suited for monitoring intravascular pH in critically ill patients.

SUBCUTANEOUS OXYGEN TENSION IN THE FETAL SCALP DURING LABOUR CONTINUOUS MONITORING WITH A NEEDLE ELECTRODE

Continuous intrapartum PO2 recordings were made in 25 fetuses, using a needle electrode that measures PO2 in the subcutaneous (sc) tissue of the scalp or the breech. This small electrode is combined with a spiral ECG electrode and can be easily applied in early labour. One hour after application of the electrode in the early first stage, a mean fetal scPO2 of 29.4±7.5 mm Hg was found. In the course of labour, scPO2 gradually fell to a mean value of 22.2±5.6 mm Hg, recorded just before the onset of bearing down contractions, and stabilised in the second stage of labour. ScPO2 values of less than 10 mm Hg were not recorded for a longer period except in one case of severe fetal distress. The relation between scPO2 just before delivery and cord arterial PO2 revealed a coefficient of correlation of 0.85. In general, scPO2 appeared higher than tcPO2 as described in the literature. This is ascribed to the different technique of measurement: the needle electrode measures in the deeper layers of the scalp, whereas the tc electrode is supplied with oxygen from the more superficial capillaries, where blood flow is more liable to be affected by mechanical factors.

Performance of a pH monitoring system in vivo

In vivo tests were performed on a pH monitoring system. For this purpose 12 catheters were introduced into the arterial bloodstream of dogs. In the tip of the catheters (6F) a pH-sensitive ISFET sensor and an Ag/AgCl reference electrode were incorporated. Blood pH was varied by ventilation changes and injection of acidic and basic solutions into the animal’s blood. After stabilisation and in vivo calibration the output of the microprocessor-controlled pH monitor was compared with that of an in vitro pH/blood-gas analyser. The average deviation of the monitor output was 0·016 pH over a measurement period of 4 h. The maximum deviation observed was 0·06 pH. The system’s response is fast enough for monitoring changes in blood pH in clinical conditions.

INTRA-AORTIC DECREASE IN BLOOD-PLASMA PH

SummaryUsing glass electrode catheters, a decrease in blood plasma pH was found in four dogs when the electrode at the tip of the catheter was passed from central to more peripheral sites of the arterial system. This finding can at least in part be explained by assuming that in the lung capillaries erythrocytes and plasma equilibrate separately with alveolar gas.

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.