Other Methods Of Evaluating Blood Gases

Pulmonary Artery Catheters

A number of pulmonary artery catheters have been developed to continuously monitor mixed venous oxygen saturation (Sv o2). The normal Svo2 is about 70 to 75%. A change in Svo2 could serve as an early warning of inadequacy of perfusion and oxygenation. A rise in Sv o2 can signal an increase in cardiac output and D o2 beyond that required for metabolism, shunting of blood from tissues (sepsis), or an inability of the peripheral tissues to extract and utilize O 2 (cyanide poisoning). It can also be simply a reflection of the location of the catheter tip in a persistently wedged position so that pulmonary capillary (oxygenated) blood is being analyzed. A fall in Svo2 below 50 to 60% is usually due to a significant decrease in cardiac output or lung function and requires urgent investigation. It is important to remember that, while a change in Svo2 may indicate important physiologic change, there can be major changes in the patient's condition without corresponding changes in the Sv o2. A major criticism of the usage of Svo2 to guide therapy is that it is a measure of all of the blood returning to the lungs and can give no information about the adequacy of perfusion of individual organ systems, such as the kidneys, brain, heart, or liver.

Noninvasive Monitoring

PULSE OXIMETRY The use of pulse oximetry for monitoring Sao2 and pulse amplitude in the fingers, nose, or toes can provide early warning of pulmonary or cardiovascular deterioration before it is clinically apparent. This technique employs a microprocessor that continuously measures pulse rate and oxyhemoglobin saturation. The photosensor is not heated and does not require calibration. Oxyhemoglobin is red and reduced hemoglobin is blue, and each has a different absorption of light at their given wavelengths. Because the ratio of transmittance at each of the two wavelengths (660 nm, red, and 940 nm, infrared) varies according to the percentage of oxyhemoglobin, pulse oximeters can be programmed to calculate and display the percentage of oxyhemoglobin saturation at each pulse.

There is a predictable correlation between noninvasive Sa o2 monitoring and measured arterial oxygen saturation from arterial blood gas over a wide range of values. Pulse oximetry has only a minimal error, of 1 to 2 percent, above 60% saturation. However, a number of factors can limit the effectiveness and accuracy of pulse oximetry, including impaired local perfusion (e.g., in patients who are hypothermic or on vasopressors); ambient light, particularly flourescent (easily eliminated by placing a towel over the pulse oximeter); nail polish (particularly blue, which absorbs near 660 nm); abnormal hemoglobin; and very high P o2. Carboxyhemoglobin falsely raises oxyhemoglobin saturation readings because bedside pulse oximeters read carboxyhemoglobin as oxyhemoglobin, while methemoglobin lowers them (at high levels of methemoglobin the pulseoximeter will read 85% regardless of the actual blood oxygenation, as may be seen after the treatment of cyanide poisoning). Fetal hemoglobin has nearly the same absorption spectrum as hemoglobin A and thus has little effect on the readings.

It is important to remember that a pulse oximeter does not provide a good measure of the adequacy of ventilation of patients on supplemental oxygen. A quick calculation using the alveolar gas equation shows that a person on 50% oxygen would be able to increase the Pa co2 to greater than 230 mm Hg before that saturation would drop below 90 [Pao2 = 0.5(713) - 230/0.8 = 356 - 287 = 69].

CAPNOGRAPHY By providing a real-time estimate of Paco2, capnography is a useful and accurate means of assessing ventilation, respiratory gas exchange, and carbon dioxide production, and it can give some indication of cardiovascular status (primarily cardiac output). Although the measurement of end-tidal carbon dioxide partial pressure (PETco2) usually underestimates Paco2 by about 3 mmHg, it may be greater if the patient has a high dead space. However, the difference between PETco2 and Paco2 is constant for a given patient, provided that the V ds/Vt ratio and airway resistance are not changing.

Mainstream and side-stream infrared capnometers are commercially available. A mainstream capnometer connects directly to the endotracheal tube, thus providing real-time breath-by-breath analysis. The major disadvantages of this system are its size and bulk and the fact that it cannot be used in nonintubated patients. Side-stream capnometers aspirate gas at the sample site. The principal advantages of this system are that it reduces mechanical dead space and can be used in nonintubated patients; however, many mechanical factors related to gas sampling require much expert attention and time, and can affect the results.

Because carbon dioxide production is directly dependent on metabolic rate, there are a large number of conditions that can lower PET co2. However, sudden decreases in PETco2 suggest mechanical problems in the airway, hypoventilation, or increased dead space. A gradual decrease in the PET co2 is usually due to changes in the lung itself. Increases in the PETco2 are generally due to hypermetabolic states or unrecognized inadequate ventilation. If a simultaneous Pa co2 value is available, one can estimate the P(A - a)co2 (using PETco2 as a proxy for PAco2). Normally, this is less than 3 mmHg, and if it suddenly increases, one should suspect a pulmonary embolus or drastic reduction in cardiac output.

The most frequent use of PETco2 is to evaluate the adequacy of ventilation. Inadvertent esophageal intubation, tracheal extubation, and endotracheal tube obstruction can be readily detected. These monitors can reduce the number of arterial blood gas determinations obtained and can be very useful in weaning patients from mechanical ventilatory support. They can also be useful in determining the adequacy of circulation during cardiopulmonary resuscitation.

Pulse oximetry and, to some extent, capnography have become standard monitors in most locations that provide for the acute care of unstable patients. While understanding that they have limitations, the clinician can use them for second-to-second indications of the adequacy of ventilation and oxygenation.

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