Mild desaturation that is not clinically apparent may be the onlyearly finding in complex congenital heart defects. Comparingpulse oximetry between the right arm and a lower extremity mayallow diagnosis of a ductal dependent lesion in which desaturatedblood flows right to left across a PDA to perfuse the lower body.This is now becoming a routine screening test in newborn infantsto rule out congenital heart disease.
The 12-lead electrocardiogram (ECG) provides information about the rate, rhythm, depolarization, and repolarizationof the cardiac cells and the size and wall thickness of the chambers. It should be assessed for rate, rhythm, axis (P wave, QRS, and T wave), intervals (PR, QRS, QTc) (Fig. 139-2), and voltages (left atrial, right atrial, left ventricular, right ventricular) adjusted for the child’s age.
The P wave represents atrial depolarization. A criterion for right atrial enlargement is an increase of the amplitude of the P wave, reflected best in lead II. The diagnosis of left atrial enlargement is made by prolongation of the second portion of the P wave, exhibited best in the chest leads.
The PR interval, measured from the beginning of the P wave to the beginning of the QRS complex, increases with age. Conduction time is shortened when the conduction velocity is increased (glycogen storage disease) or when the atrioventricular node is bypassed (Wolff-Parkinson-White syndrome). A prolonged PR interval usually indicates slow conduction through the atrioventricular node. Diseases in the atrial myocardium, bundle of His, or Purkinje system may also contribute to prolonged PR intervals.
The QRS complex represents ventricular depolarization. A greater ventricular volume or mass causes a greater magnitude of the complex. The proximity of the right ventricle to the chest surface accentuates that ventricle’s contribution to the complex. Changes in the normal ECG occur with age. Normative data for each age group must be known to make a diagnosis from the ECG.
The QT interval is measured from the beginning of the QRS complex to the end of the T wave. The corrected QT interval (corrected for rate) should be less than 0.45 second (QTc = QT/ precedingR to R interval). The interval may be prolonged in children with hypocalcemia or severe hypokalemia. It is also prolonged in a group of children at risk for severe ventricular arrhythmias and sudden death (prolonged QT syndrome). Drugs such as quinidine and erythromycin may prolong the QT interval.
A systematic approach to reading a chest radiograph includes assessment of extracardiac structures, the shape and size of the heart, and the size and position of the pulmonary artery and aorta (Fig. 139-3). Assessment of the location and size of the heart and cardiac silhouette may suggest a cardiac defect. On a good inspiratory film, the cardiothoracic ratio should be
Figure 139-2 Nomenclature of electrocardiogram (ECG) waves and intervals.