Blood Gas Monitoring

Blood gas always means the oxygen and carbon dioxide con­tents of the blood. Because most of the oxygen in the blood exists in combination with hemoglobin, the oxygen content of the blood can be expressed in terms of the ratio of the amount of oxyhemoglobin to that of total hemoglobin; this ratio is called the oxygen saturation. A small amount of oxygen, usu­ally less than 1%, remains in the plasma as dissolved oxygen, and its amount is expressed in terms of oxygen partial pres­sure. Although there is a relationship between oxygen satura­tion and oxygen partial pressure, this relationship is nonlin­ear so that saturation increases steeply with increasing oxygen partial pressure when the latter lies in the range 20 to 40 mm Hg (2.7 to 5.3 kPa), but tends to saturate when the oxygen partial pressure reaches above 60 mm Hg (8 kPa). In normal arterial blood, oxygen saturation is above 98%, and oxygen partial pressure is approximately 100 mm Hg (13.3 kPa). The main purpose of monitoring oxygen level is to con­firm the oxygen transport which sustains metabolic demand.

Carbon dioxide is highly soluble in body fluids, and it is also converted, reversibly, to bicarbonate ions. Therefore, blood plasma as well as interstitial fluids have an apparently large storage capacity for carbon dioxide. However, changes in the carbon dioxide content of the body fluids causes a change in the acid-base balance of those body fluids, which is expressed by pH. Thus, it is important to maintain an ade­quate carbon dioxide level in the body fluids. It is therefore monitored by measuring the partial pressure of carbon diox­ide of arterial blood.

Blood gas levels can be measured by taking a blood sample and analyzing it using a blood gas analyzer which provides information about the partial pressures of oxygen and carbon dioxide, and about the pH of the blood. However, in a patient whose respiration is unstable, blood gas values may fluctuate so that frequent measurement is required, and hence continu­ous blood gas monitoring is preferred.

Arterial blood oxygen saturation can be monitored nonin — vasively using a pulse oximeter (15). Due to the difference in the spectral absorption of oxyhemoglobin and reduced hemo­globin, the oxygen saturation of a particular blood sample can be determined by absorption measurements at two wave­lengths, typically in a red band between 600 nm and 750 nm and in an infrared band between 850 nm and 1000 nm. How­ever, the tissue in vivo contains both arterial and venous blood, and hence light absorption occurs by both components. To obtain the arterial component selectively, the pulsatile component is extracted. As shown in Fig. 6(a), light absorp­tion is usually measured in a finger. Two light-emitting di­odes of different wavelengths, for example 660 nm and 910 nm, are operated alternately, and the transmitted light is de­tected by a photocell. The pulsatile components of both wave­lengths are then extracted by a bandpass filter. Arterial oxy­gen saturation is determined from the ratio of these two components.

Although the pulse oximeter is reliable enough and has been used successfully for patient monitoring in most cases, measurement sites of the transmittance measurement are limited, and thus a reflection-type pulse oximeter in which

(a)

Thermistor Heating element Electrolyte

Oxygen permeable membrane

backscattered light is measured has been developed (16). In back-scattered light measurement, a difficulty arises due to the fact that the optical pathlength may vary when absorp­tion is varied, although it is not changed as much in transmis­sion measurement. In principle, this difficulty can be solved by using more than three wavelengths, however, a reflection — type pulse oximeter with comparable performance to the transmission-type oximeter has not yet been developed. In some applications, the reflection-type oximeter is highly ap­preciated. For example, it is applied to fetal monitoring dur­ing labor in which the sensor is applied to the skin of the fetal head (17).

The oxygen content in arterial blood can also be measured continuously and noninvasively with the aid of a transcutane — ous oxygen electrode (18,19). The configuration of the probe is shown in Fig. 6(b). The principle employed is that of polaro — graphic measurement, by which current drains proportionally to the amount of oxygen that reaches the cathode by diffusion through the oxygen permeable membrane. Because the oxy­gen flux is determined by the gradient in oxygen partial pres­sure at the membrane, and the oxygen partial pressure at the electrode surface is reduced to zero by the electrode reaction; the current that results from the oxygen flux depends upon the oxygen partial pressure on the outside of the membrane. When the probe is used for measuring arterial oxygen partial pressure, the electrode body is heated to approximately 42 or 43 °C. At this temperature, arteriovenous shunts in the skin tissue fully open, thus allowing large amounts of blood to flow through the tissue, far more than is required nutritionally, so that the venous blood has almost the same oxygen content as that of the arterial blood. Consequently, the oxygen partial pressure in the tissue reaches almost the same level as that of the arterial blood, and thus the arterial oxygen partial pressure can be measured transcutaneously.

Both the pulse oximeter and the transcutaneous oxygen electrode can be used for monitoring blood oxygenation; how­ever, each method has advantages and disadvantages. The pulse oximeter is safe, easy to use, inexpensive, and sensitive at lower partial pressures. However, for higher oxygen partial pressure where oxygen saturation is almost 100%, a pulse oxi­meter can not detect any change in oxygen partial pressure. Higher oxygen partial pressures may occur during, for exam­ple, oxygen therapy. In such a condition, oxygen partial pres­sure may vary in wider range, and thus a transcutaneous oxy­gen electrode can be a good monitor of gas exchange in the lung.

Drainage eye

Balloon

Urinary

drainage

(a)

Temperature output

Carbon dioxide partial pressure in the blood can also be measured transcutaneously using a heated carbon dioxide electrode, similar to the transcutaneous oxygen electrode. The carbon dioxide electrode consists of a pH electrode covered with a carbon dioxide permeable membrane (20). This type of electrode has been used for neonatal monitoring. The com­bined oxygen and carbon dioxide electrode which consists of a transcutaneous carbon dioxide electrode and a transcutane — ous oxygen electrode is also available (21).

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