Besides noninvasive imaging tools, there are other noninvasive investigational tools that are frequently used in the evaluation of cardiac patients. They include exercise stress testing, long-term ECG monitoring, and ambulatory blood pressure (BP) monitoring.
I. Exercise Stress Testing
Exercise stress testing plays an important role in the evaluation of cardiac symptoms by quantifying the severity of the cardiac abnormality and assessing the effectiveness of management. Although some exercise laboratories have developed bicycle ergometer protocols, the treadmill protocols, such as the Bruce protocol, are well standardized and widely used because most hospitals have treadmills.
A. Monitoring During Exercise Stress Testing
During exercise stress testing, the patient is continually monitored for symptoms such as chest pain or faintness, ischemic changes or arrhythmias on the ECG, oxygen saturation, and responses in heart rate and BP.
Heart rate . Heart rate is measured from the electrocardiographic (ECG) signal.
The maximal heart rate ranges between 188 and 210 beats/min. The mean maximal heart rates reported are virtually identical for boys and girls: 198±11 for boys and 200±9 for girls.
Heart rate declines abruptly during the first minute of recovery to between 140 and 150 beats/min.
Inadequate increments in heart rate may be seen with sinus node dysfunction, in congenital heart block, and after cardiac surgery.
An extremely high heart rate at low levels of work may indicate physical decondition or marginal circulatory compensation.
Blood pressure . BP is measured in the arm with the auscultatory method or an oscillometric device. Accuracy of BP measurement, especially systolic BP, is doubtful during exercise.
Systolic pressure increases linearly with progressive exercise. Systolic pressure usually rises to as high as 180 mm Hg with little change in diastolic pressure. Maximal systolic pressure in children rarely exceeds 200 mm Hg. During recovery it returns to baseline in about 10 minutes.
The diastolic pressure ranges between 51 and 76 mm Hg at maximum systolic BP. Diastolic pressure also returns to the resting level by 8 to 10 minutes of recovery.
High systolic pressure in the arm, to the level of what is considered hypertensive emergency, raises a concern in both children and adults, but it probably does not reflect the central aortic pressure. The major portion of the rise in arm systolic pressure during treadmill exercise probably reflects peripheral amplification due to vasoconstriction in the nonexercising arms (associated with increased blood flow to vasodilated exercising legs); central aortic pressure would probably be much lower than the systolic pressure in the arm in most cases. Fig. 5.1 is a dramatic illustration of a relationship between the central and peripheral arterial pressures measured directly with arterial cannulas inserted in the ascending aorta and radial artery during upright exercise in young adults. Note that when the radial artery systolic pressure is over 230 mm Hg, the aortic pressure is only 160 mm Hg and there is very little increase in diastolic pressure ( Fig. 5.1 ). This phenomenon is known as peripheral amplification of systolic pressure, discussed in Chapter 1 (see Fig 1.3 ). Therefore the usefulness of arm BP in assessing CV function during upright exercise is questionable, except in the case of failure to rise.
Failure of BP to rise to the expected level may be much more significant than the level of the rise in arm BP. The failure reflects an inadequate increase in cardiac output. This is commonly seen with cardiomyopathy, LVOT obstruction, coronary artery diseases, or the onset of ventricular or atrial arrhythmias.
ECG monitoring . The major reasons for ECG monitoring are to detect exercise-induced arrhythmias and ischemic changes.
Exercise -induced arrhythmias: Arrhythmias that increase in frequency or begin with exercise are usually significant. Type and frequency before and after the exercise and occurrence of new or more advanced arrhythmias should be noted. Occurrence of serious ventricular arrhythmias may be an indication to terminate the test.
ST -segment depression is the most common manifestation of exercise-induced myocardial ischemia.
For children, down-sloping or sustained horizontal depression of the ST segment of 2 mm or greater when measured at 80 msec after the J point is considered abnormal (see Fig. 2.20 ).
Most guidelines for adult exercise testing consider ST-segment depression of 1 mm or greater as an abnormal response. If the ST segment is depressed at rest, an additional depression of 1 mm or greater should be present to be significant.
Specificity of the exercise ECG is poor in the presence of ST-T abnormalities on a resting ECG or with digoxin use.
When there is an abnormal depolarization (such as BBB, ventricular pacemaker, or WPW preexcitation), interpretation of ST-segment displacement is impossible.
Oximetry . Normal children maintain oxygen saturation greater than 90% during maximal exercise when monitored by pulse oximetry. Desaturation (<90%) is considered an abnormal response and may reflect pulmonary, cardiac, or circulatory compromise. Children who received lateral tunnel Fontan operations with fenestration may desaturate during exercise due to R-L shunt through the fenestration.
B. Endurance Time
There is a high correlation between the maximum oxygen consumption (Vo 2 max) and endurance time. Thus endurance time is the best predictor of exercise capacity in children. The endurance data reported by Cummings et al. in 1978 have served as the reference for several decades. Two reports from the U.S. (Chatrath et al., 2002; Ahmed et al., 2001) indicate that the endurance time has been reduced significantly since the 1970s. It is concerning that endurance times reported from two other countries (Italy in 1994; Turkey in 1998) are similar to those published by Cummings et al. and are significantly longer than those reported in the two U.S. reports. This may be an indication that U.S. youth are less physically fit than the youth from other countries, which may lead to increased risk of CAD and stroke in the U.S. population. A new set of endurance data from a recent U.S. study is presented in Table 5.1 .
|AGE GROUP (YR)||PERCENTILES||MEAN±SD|