Blood partial pressure measurements

The value of the partial pressure measured at the skin surface depends in a complex way on blood partial pressure, constitution of the skin, local perfusion, metabolism in the associated tissue, cardiac output, and application temperature. An increased temperature of 43 °C raises the gas permeability and expands the capillary vessels of skin which are filled with more artial blood. The local hyperemia has the disadvantage of limiting the application time at a certain site. Assuming stable circulation conditions, transcutaneously measured values correlate with arterial partial pressure by a factor of 1.2 (neonates) to 1.0 (small children) [1]. The measured value for adults proved to be very unreliable. In the case of unstable conditions or shock with a reduction of peripheral blood flow, the transcutaneous value drops very early. Inconvenience in routine use is caused by long preparation times of the sensor, the need for periodic membrane changes, the long run-in time of freshly prepared sensors, the necessity for periodic calibrations and the slow response time to changes in partial pressure. 

FIGURE 22.1 Summary of oxygen partial pressure measurements in blood and joints of patients with rheumatoid and osteoarthritis [1,4], conpared to air at standard temperature and pressure (STP). 

Carbon dioxide devices were originally developed by Severinghaus and Bradley (59) to measure the partial pressure of carbon dioxide in blood. This electrode, still in use today (in various automated systems for blood gas analysis), consists of an ordinary glass pH electrode covered by a carbon dioxide membrane, usually silicone, with an electrolyte (sodium bicarbonate-sodium chloride) solution entrapped between them (Figure 6-17). When carbon dioxide from the outer sample diffuses through the semipermeable membrane, it lowers the pH of the inner solution ... 

The quantity P50, a measure of O2 concentration, is the partial pressure of Oj that half-saturates a given hemoglobin. Depending on the organism, P50 can vary widely, but in all instances it will exceed the PO2 of the peripheral tissues. For example, values of P50 for HbA and fetal HbF are 26 and 20 mm Hg, respectively. In the placenta, this difference enables HbF to extract oxygen from the HbA in the mother s blood. However, HbF is suboptimal postpartum since its high affinity for O2 dictates that it can deliver less Oj to the tissues. 

Respiratory alkalosis is associated with low partial pressure of02 (25 to 35 mm Hg) and alkaline pH, but normal bicarbonate. The first two values are measured by arterial blood gas, which also yields partial pressure of carbon dioxide and arterial oxygen saturation. Circulating arterial oxygen saturation can also be measured by an oximeter, which is a noninvasive method that is fairly accurate and useful at the patient s bedside. 

Figure 9.15. In vivo partial pressure of oxygen in arterial blood of dogs over the period of 10 h. The solid lines represent the analyses monitor by the instrument. Two different polymer solutions are shown. The dots represent the batch gas analysis measured with a Nova Biomedical blood gas analyzer. (From Ref. 21 with permission.)...

As arterial C02 tension is practically identical to alveolar C02 partial pressure, it can be used as a surrogate measurement. This is desirable as measuring arterial C02 tension involves only a simple blood gas analysis. The term Paco2, therefore, becomes Paco2 and so the equation is often written as... 

As mentioned above, gases in solution are often measured in partial pressures. Recall that Dalton s Law of Partial Pressures states that the total pressure is the sum of the pressures partially exerted by all of the component, noninteracting gases. For one gaseous component in equilibrium with the same component dissolved in a liquid, the partial pressure of that gas in solution is the pressure that gas would exert in the atmosphere. It is symbolized by either p or P followed by the specific gas (eg, either or pO ). For example, typical blood P02 levels range between 75 and 100 torr (or, 10.0 to 13.3 kPa since the pascal (Pa) is an SI unit). 

Pulmonary absorption of volatile anesthetics across the alveolar-capillary barrier is very rapid because of the relatively high lipid-water partition coefficients and small molecular radii of such agents. The driving force for diffusion is a combination of the blood-air partition coefficient (which is a measure of the capacity of blood to dissolve drug) and the difference in partial pressure between the alveoli and the arterial and venous blood. Agents with high blood-air partition coefficients require more drug to be dissolved in the blood for equilibrium to be reached. 

The complexities of definitions occur primarily because concentration can be expressed in so many different variables. In the above, we have assumed that it is expressed in mass per volume or moles per volume. The concentration can equally be well expressed as a mole fraction, which in the liquid phase is commonly indicated by the symbol X and in a gas phase is written as >t. In gases, one can also express concentrations as partial pressures. In some cases, especially in medicine, the concentration can be expressed in other more arcane units. For example, oxygen tension measures the amount of oxygen present in blood, but it is expressed as the partial pressure that would exist... 

Gas-selective electrodes are a particularly important application of the glass electrode. For example, the carbon dioxide electrode is a self-contained system with a glass electrode and a concentric silver-silver chloride electrode enclosed by a C02 permeable membrane. The latter holds a thin film of bicarbonate solution in contact with the glass membrane, which provides a junction to the silver/silver chloride reference electrode. The electrode, which is illustrated schematically by Figure 2.2, has found extensive application in monitoring C02 levels in blood and probably will find increasing application in other systems that require continuous measurement of C02 partial pressures. The electrode response is based on the reaction... 

Carbon dioxide sensor — An electrochemical device for measuring the -+ partial pressure of carbon dioxide, originally developed for measurements with blood by Sever-inghaus and Bradley [i]. It consists of a glass electrode as used for pH measurements covered with a membrane selectively permeable for CO2, e.g., silicone. Between membrane and glass electrode an electrolyte solution containing NaCl and Na2C03 is entrapped. 

At room temperature -> Nernstian slope (59 mV/decade of concentration change) is usually observed. Partial pressure can be derived by applying Henrys law. A catheter configuration suitable for measurements inside blood vessels has been described [ii]. Using other electrolyte constituents and membranes semipermeable for other types of gas sensors for other analytes were developed (including N02, S02, H2S, HF [iii]). Various CO2 sensors with galvanic solid electrolyte cells have been designed [iv]. 

A pulmonary artery (Swan-Ganz) catheter can be used to determine central venous pressure (CVP) pulmouary artery pressure CO aud pulmonary artery occlusive pressure (PAOP), an approximate measure of the left ventricular eud-diastolic volume aud a major determinaut of left veutricular preload. CO (2.5 to 3 L/min) and mixed venous oxygeu saturatiou (70% to 75%) may be very low in a patient with extensive myocardial damage. Respiratory alkalosis is associated with low partial pressure of O2 (25 to 35 mm Hg) and alkaline pH, but uormal bicarbouate. The first two values are measured by arterial blood gas, which also yields partial pressure of carbon dioxide and arterial oxygen saturation. Circulating arterial oxygen saturation can also be measured by an oximeter, which is a noninvasive method that is fairly accurate and useful at the patient s bedside. 

Measurement of CO in the expired breath is based on the assumption that CO in alveolar air is in equilibrium with the partial pressure of CO in blood which in turn is in equilibrium with CO bound to Hb, that is COHb (Douglas et al, 1912). 

Determination of gas pressures in expired air or blood depends on the application of certain physical principles (Table 27-4). The partial pressure of a gas dissolved in blood is by definition equal to the partial pressure of the gas in an imaginary ideal gas phase in equilibrium with the blood. At equilibrium, the partial pressure (tension) of a gas is the same in erythrocytes and plasma, so that the partial pressure of a gas is the same in whole blood and plasma. The partial pressure of a gas in a gas mixture is defined as the substance fraction of gas (mole fraction) times the total pressure. The tension of a gas in a liquid is, in fact, a measure of the chemical activity of the gas in the liquid. In the physicochemical literature, it is called the fugacity. 

It assumes a standard body temperature of 37 °C and that the measuring device also holds the sample of blood at exactly 37 °C. This assumption requires special concern for thermal stability of the instrument. Just as important, it impHes that in circumstances such as imposed hypothermia, when a patient s temperature is not 37 °C, blood gas values determined at 37 °C might need to be corrected to the actual body temperature to obtain an estimate of blood gas partial pressures in the patient. Temperature corrections are addressed later in this section. 

It recognizes that the partial pressures of measured gases in the blood coexist with a constant and standard saturated vapor pressure (SVP), which is identical for both the calibration conditions of the instrument and measurement conditions of the blood sample. 

In the Henderson-Hasselbalch equation, pK and a are used as constants for a temperature of 37 °C. The temperature-controlled sample chamber of an instrument is specified to be 37 °C 0.1 C, and it is at that temperature that all measurements of pH and partial pressure of gases are made. The body temperature of a febrile patient may be elevated to 40 °C to 41 °C, or a patient may be made hypothermic for cardiopulmonary bypass surgery and have a temperature as low as 23 °C. Most blood gas instruments, on keyboard entry of a patient s actual temperature, can calculate and present... 

Answer D. Saturation of the blood with inhaled anesthetics is more rapid if they have a low blood-gas partition coefficient. This results in the more rapid achievement of a partial pressure of the dissolved anesthetic molecules commensurate with their movement out of the blood into the alveolar spaces of the lung, where they are eliminated. Note that the same physicochemical characteristic is responsible for the rapid onset of the anesthetic action of sevoflurane. Although redistribution of anesthetics between tissues occurs, it is not responsible for rapid recovery. MAC values are a measure, of anesthetic potency. With the exception of halothane (and methoxyflurane), inhaled anesthetics are not metabolized to a significant extent. Naloxone is an opioid receptor antagonist. 

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