A computed tomography (CT) scan of the chest is the imaging test of choice for evaluating pleural masses, bronchiectasis, and mediastinal lesions as well as delineating pleural from parenchymal lesions. CT scans with intravenous contrast provide excellent information about the pulmonary vasculature and great vessels and can detect pulmonary embolism. High-resolution CT scans are used to assess lung parenchyma (congenital pulmonary malformations, interstitial lung disease) and the airways (bronchiectasis). The speed of current
CT scanners makes it possible to scan most children without sedating them. However, sedation may be required in infants and toddlers to decrease motion artifact. Magnetic resonance imaging (MRI), useful in visualizing cardiac and great vessel anatomy, is less useful for evaluation of pulmonary parenchymal lesions.
Ultrasonography can be used to delineate some intra-thoracic masses and is the imaging procedure of choice for assessing parapneumonic effusion/empyema. It is useful for assessing diaphragmatic motion in small children.
Measures of Respiratory Gas Exchange
A properly performed arterial blood gas analysis provides information about the effectiveness of both oxygenation and ventilation. However, arterial samples are more difficult to obtain, so capillary and venous blood samples are more commonly used. The Pco2 from a capillary sample is similar to that from arterial blood. The Pco2 in venous samples is approximately 6 mm Hg higher than arterial Pco2. The ratio of the serum bicarbonate concentration to Pco2 determines the pH. Capillary or venous samples should not be used to assess oxygenation.
There are both respiratory and metabolic causes of acidosis (see Chapter 37). In the presence of an alkalosis or acidosis, respiratory compensation (altering Pco2 to maintain a normal pH) can occur within minutes, but renal compensation (altering the serum bicarbonate level) may not be complete for several days. Recall that both the respiratory and metabolic compensation are incomplete, so pH will remain on the side of the primary insult (whether acidosis or alkalosis).
Pulse oximetry measures the O2 saturation of hemoglobin by measuring the blood absorption of two or more wavelengths of light. It is noninvasive, easy to use, and reliable. Because of the shape of the oxyhemoglobin dissociation curve, O2 saturation does not decrease much until the Po2 reaches approximately 60 mm Hg. Pulse oximetry may not accurately reflect true O2 saturation when abnormal hemoglobin is present (carboxyhemoglobin, methemoglobin), when perfusion is poor, or if no light passes through to the photodetector (nail polish).
The measurement of Pco2 is accomplished most reliably by blood gas analysis. However, there are noninvasive monitors that record exhaled Pco2 (end-tidal CO2), which is representative of alveolar Pco2. End-tidal Pco2 measurements are most commonly used in intubated and mechanically ventilated patients, but some devices can measure Pco2 at the nares. Transcutaneous electrodes can be used to monitor Pco2 and Po2 at the skin surface, but are less accurate. Noninvasive techniques of CO2 measurement are best suited for detecting trends rather than for providing absolute values.
Measurement of lung volumes and airflow rates using spirometry are important in assessing pulmonary disease. The patient inhales to TLC and then forcibly exhales until no more air can be expelled. During the forced expiratory maneuver, forced vital capacity (FVC), forced expired volume in the first second (FEV1), and forced expiratory flow (FEF) rates are measured. These are compared to predicted values based on patient age, gender, and race, but rely mostly on height. Most children above 6 years of age can perform spirometry. Infant pulmonary function testing is possible, using sedation and sophisticated equipment.