Blood Gas and Pulmonary Function Monitoring
James M. Adams
I. GENERAL PRINCIPLES.
Blood gas monitoring in neonatal critical care units allows (i) assessment of pulmonary gas exchange; (ii) determination of hemoglobin oxygen saturation and arterial oxygen content; and (iii) evaluation, although limited, of adequacy of tissue oxygen delivery. Both invasive and noninvasive techniques are used in the clinical setting.
II. OXYGEN USE AND MONITORING.
In emergency situations, sufficient oxygen to abolish cyanosis should be administered. Oxygen monitoring with pulse oximetry should be initiated as soon as possible, and the concentration of oxygen should be adjusted to maintain saturation values within a targeted range. An oxygen blender and pulse oximeter should be used whenever supplemental oxygen is administered. Monitoring of oxygen use is necessary to reduce both hypoxic injury to tissues and to minimize oxidative injury to the lungs or the immature retina of the premature infant.
Arterial blood gas measurements. Arterial PO2 and PCO2 are direct indicators of efficiency of pulmonary gas exchange in babies with acute lung disease. Arterial oxygen tension (PaO2), measured under steady state conditions from an indwelling catheter, is the “gold standard” for oxygen monitoring.
Usual values. Most sources consider 50 to 80 mm Hg to be an acceptable target range for newborn PaO2. Premature infants who require respiratory support may exhibit wide swings in PaO2 values. In such circumstances, a single blood gas value may not accurately reflect the overall trend of oxygenation.
Sampling. To minimize sampling and dilutional artifacts, arterial blood gas samples should be collected in dry heparin syringes that are commercially available for this purpose. Most blood gas analyzers allow determination of blood gas values, as well as other whole blood parameters, on 0.2 to 0.3 mL samples. Samples should be analyzed within 15 minutes or preserved on ice if sent to a remote laboratory site. Blood gas sampling by percutaneous puncture is utilized when the need for measurement is infrequent or an indwelling catheter is not available. However, the discomfort of the puncture may result in agitation and a fall in PaO2, such that the value obtained underestimates the true steady state value.
Capillary blood gas determination. This technique requires extensive warming of the extremity, free-flowing puncture, and strictly anaerobic collection. Under these conditions, capillary sampling may be useful for determination of pH and
PCO2. Proper collection techniques are often difficult to guarantee in the clinical setting however, and capillary sampling should not be used for determination of PaO2.
Continuous blood gas analysis via an indwelling catheter has been advocated to provide rapid, real-time data and reduce the volume of blood required for repeated blood gas measurements. However, because of technical limitations, a role for these devices in neonatal intensive care has not been established.
Noninvasive oxygen monitoring provides real-time trend data that are particularly useful in babies exhibiting frequent swings in PaO2 and oxygen saturation. Noninvasive devices also may reduce the frequency of blood gas sampling in some patients.
Pulse oximetry is the primary tool for noninvasive oxygen monitoring in neonates. Pulse oximeters provide continuous measurement of hemoglobin oxygen saturation (SpO2) with a high level of accuracy (±3%) when compared to control values measured by co-oximetry, at least down to the range of 70%.
General characteristics. Oximeters depend upon different absorption characteristics of oxygenated versus reduced hemoglobin for various wavelengths of light. Differences in transmission of two (usually red and near infrared) or more wavelengths through tissues with pulsatile blood flow are measured. Using the measured values, the proportion of oxygenated and reduced hemoglobin is calculated and displayed as percent saturation.
Disadvantages. Pulse oximetry does not measure the PaO2 and, thus, is insensitive in detecting hyperoxemia. Due to the shape of the oxyhemoglobin dissociation curve, if SpO2 is >95%, PaO2 is unpredictable. Under such conditions, PaO2
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