Besides imaging, there are other noninvasive investigative tools that are frequently used in the evaluation of cardiac patients. In this chapter, the following will be discussed.
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Stress testing
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Long-term electrocardiography (ECG) recording
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Ambulatory blood pressure monitoring
Stress Testing
Stress testing plays an important role in the evaluation of cardiac symptoms by quantifying the severity of the cardiac abnormality, providing important indications of the need for new or further intervention, and assessing the effectiveness of management. The cardiovascular system can be stressed either by exercise or by pharmacologic agents.
The maximum oxygen uptake (Vo 2 max) that can be achieved during exercise is probably the best index of describing fitness or exercise capacity (also called “maximum aerobic power”). Vo 2 max is defined by the plateau of oxygen uptake (Vo 2 ) that occurs despite continued work. Beyond this level of Vo 2 max, the work can be performed using anaerobic mechanism of energy production, but the amount of work that can be performed using anaerobic means is quite limited. There is a linear relationship between the heart rate and progressive workload or Vo 2 max.
Cardiovascular Response in Normal Subjects
During upright dynamic exercise in normal subjects, the heart rate, cardiac index, and mean arterial pressure increase. In addition, the systemic vascular resistance (SVR) drops, and blood flow to the exercising leg muscles greatly increases. Heart rate increase is the major determinant of increased cardiac output seen during exercise. The heart rate reaches a maximum plateau just before the level of total exhaustion. For subjects between 5 and 20 years of age, the maximal heart rate is about 195 to 215 beats/min. For subjects older than 20 years, the maximal heart rate is 210 to 0.65 × Age.
Blood pressure (BP) response varies depending on the type of exercise. During dynamic exercise , systolic BP increases, but diastolic and mean arterial pressures remain nearly identical, varying within a few mm Hg from their levels at rest. BP response to isometric exercise is quite different from the response to dynamic exercise. With isometric exercise, both systolic and diastolic pressures increase.
Although similar changes occur in pulmonary circulation as those seen in the systemic circulation, the increase in the mean pulmonary artery (PA) pressure is more than twice that of systemic mean arterial pressure (e.g., 100% increase), and the drop in pulmonary vascular resistance (PVR) is much less than that in SVR (e.g., 17% decrease in PVR vs. 49% decrease in SVR). Because of this, children with pulmonary hypertension or those with RV dysfunction (after Fontan operation or surgery for tetralogy of Fallot [TOF]) may respond abnormally to exercise and demonstrate a decreased exercise capacity.
Cardiovascular Response in Cardiac Patients
1
Congenital Heart Defects
- a.
Patients with minor congenital heart defects (CHDs) (e.g., small left-to-right shunt lesions or mild obstructive lesions) have little or no effect on exercise capacity.
- b.
Large left-to-right shunt lesions decrease exercise capacity because a ventricle that has a much increased stroke volume at rest has a limited ability to increase the stroke volume further.
- c.
In patients with severe obstructive lesions, the ventricle may not be able to maintain an adequate cardiac output, so that during exercise, the systemic BP may not increase appropriately, and decreased blood flow to exercising muscles may lead to premature fatigue.
- d.
In cyanotic lesions, the arterial hypoxemia tends to increased cardiac output and decreased mixed venous oxygen saturation, thereby limiting the usual increment in stroke volume and oxygen extraction that occurs with exercise. Furthermore, these patients have an increased minute ventilation at rest and during exercise. In this way, ventilatory as well as cardiac mechanisms may limit exercise capacity.
2
Postsurgical Patients
- a.
For many patients with CHDs, normal or near-normal exercise tolerance is expected after surgery unless there are significant residual lesions or myocardial damage.
- b.
After a successful Fontan operation for functional single ventricle, exercise capacity improves, but it remains significantly less compared with normal. This results from both subnormal heart rate response to exercise and abnormal stroke volume (resulting from reduced systemic ventricular function). Cardiac arrhythmias also are common in patients both before and after the Fontan operation and may contribute to the decreased exercise capacity.
- c.
After arterial switch operation for D-TGA, more than 95% of the children have normal exercise capacity. However, up to 30% of patients have chronotropic impairment with a peak heart rate of less than 180 beats/min. Up to 10% of the patients develop significant ST-segment depression with exercise.
Exercise Stress Testing
Some exercise laboratories have developed bicycle ergometer protocols, but the equipment is not widely used. The treadmill protocols are well standardized and widely used because most hospitals have treadmills. In this chapter, exercise tests, in particular, that using the Bruce protocol will be presented. In the Bruce protocol, the level of exercise increases by increasing the speed and grade of the treadmill for each 3-minute stage.
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. In the Bruce protocol, children are not allowed to hold onto the guardrails except to maintain their balance at change of stage because this can decrease the metabolic cost of work and therefore increase the exercise time.
Monitoring During Exercise Stress Testing
1
Endurance Time
Oxygen uptake is difficult to measure in children. However, there is a high correlation between maximal Vo 2 and endurance time, and thus endurance time is the best predictor of exercise capacity in children.
The endurance data reported by Cumming et al in 1978 have served as the reference for several decades. Recently, however, two reports from the United States ( Ahmed et al, 2001 ; Chatrath et al, 2002 ) 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 Cumming 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 coronary artery disease and stroke in the U.S. population. A new set of endurance data from a recent U.S. study is presented in Table 6-1 . The endurance times for boys and girls are close until early adolescence, at which time the endurance time of girls diminishes and that of boys increases.
| Age Group (yr) | Percentiles | Mean ± SD | ||||
|---|---|---|---|---|---|---|
| 10 | 25 | 50 | 75 | 90 | ||
| Boys | ||||||
| 4–5 | 6.8 | 7.0 | 8.2 | 10.0 | 12.7 | 8.9 ± 2.4 |
| 6–7 | 6.6 | 7.7 | 9.6 | 10.4 | 13.1 | 9.6 ± 2.3 |
| 8–9 | 7.0 | 9.1 | 9.9 | 11.1 | 15.0 | 10.2 ± 2.5 |
| 10–12 | 8.1 | 9.2 | 10.7 | 12.3 | 13.2 | 10.7 ± 2.1 |
| 13–15 | 9.6 | 10.3 | 12.0 | 13.5 | 15.0 | 12.0 ± 2.0 |
| 16–18 | 9.6 | 11.1 | 12.5 | 13.5 | 14.6 | 12.2 ± 2.2 |
| Girls | ||||||
| 4–5 | 6.8 | 7.2 | 7.4 | 9.1 | 10.0 | 8.0 ± 1.1 |
| 6–7 | 6.5 | 7.3 | 9.0 | 9.2 | 12.4 | 8.7 ± 2.0 |
| 8–9 | 8.0 | 9.2 | 9.8 | 10.6 | 10.8 | 9.8 ± 1.6 |
| 10–12 | 7.3 | 9.3 | 10.4 | 10.8 | 12.7 | 10.2 ± 1.9 |
| 13–15 | 6.9 | 8.1 | 9.6 | 10.6 | 12.4 | 9.6 ± 2.1 |
| 16–18 | 7.4 | 8.5 | 9.5 | 10.1 | 12.0 | 9.5 ± 2.0 |
2
Heart Rate
Heart rate is measured from the ECG signal. A heart rate of 180 to 200 beats/min correlates with maximal oxygen consumption in both boys and girls. Therefore, an effort is made to encourage all children to exercise to attain this heart rate. The mean maximal heart rates for all subjects were virtually identical, 198 ± 11 beats/min for boys and 200 ± 9 beats/min for girls. Heart rate declined abruptly during the first minute of recovery to 146 ± 19 beats/min for boys and 157 ± 19 beats/min for girls.
Inadequate increments in heart rate may be seen with sinus node dysfunction, in congenital heart block, and after cardiac surgery. Sinus node dysfunction is common after surgery involving extensive atrial suture lines, such as the Senning operation or Fontan operation. It is also common after repair of TOF. Marked chronotropic impairment significantly decreases aerobic capacity. Trained athletes tend to have lower heart rates at each exercise level. An extremely high heart rate at low levels of work may indicate physical deconditioning or marginal circulatory compensation.
3
Blood Pressure
Blood pressure can be measured with a cuff, a sphygmomanometer, and a stethoscope. Numerous commercially available electronic units are also available to measure BP during exercise. However, one must be concerned with the accuracy of these devices. Accurate measurement of BP, especially diastolic pressure, is probably not possible during exercise.
Systolic BP increases linearly with progressive exercise. Systolic BP usually rises to as high as 180 mm Hg ( Table 6-2 ) with little change in diastolic BP. Maximal systolic pressure in children rarely exceeds 200 mm Hg. During recovery, it returns to baseline in about 10 minutes. The diastolic BP ranges between 51 and 76 mm Hg at maximum systolic BP. Diastolic BP also returns to the resting level by 8 to 10 minutes of recovery.
| Age Group (yr) | Rest | Maximal | Recovery (min) | ||
|---|---|---|---|---|---|
| 6 | 8 | 10 | |||
| Boys | |||||
| 5–7 | 105 ± 10 | 141 ± 13 | 111 ± 14 | 108 ± 9 | 106 ± 12 |
| 8–9 | 107 ± 10 | 149 ± 15 | 111 ± 10 | 107 ± 9 | 105 ± 6 |
| 10–11 | 108 ± 7 | 153 ± 13 | 112 ± 8 | 107 ± 9 | 106 ± 8 |
| 12–13 | 111 ± 12 | 165 ± 19 | 118 ± 12 | 113 ± 15 | 110 ± 9 |
| 14–15 | 120 ± 12 | 179 ± 23 | 124 ± 15 | 118 ± 16 | 115 ± 12 |
| 16–18 | 122 ± 14 | 182 ± 17 | 136 ± 16 | 125 ± 13 | 125 ± 14 |
| Girls | |||||
| 5–7 | 106 ± 9 | 143 ± 15 | 103 ± 4 | 104 ± 8 | 98 ± 6 |
| 8–9 | 108 ± 9 | 149 ± 11 | 114 ± 14 | 108 ± 11 | 108 ± 11 |
| 10–11 | 106 ± 11 | 145 ± 12 | 106 ± 10 | 104 ± 8 | 102 ± 7 |
| 12–13 | 112 ± 12 | 163 ± 16 | 120 ± 14 | 113 ± 10 | 108 ± 6 |
| 14–15 | 111 ± 10 | 166 ± 16 | 117 ± 13 | 112 ± 12 | 111 ± 10 |
| 16–18 | 118 ± 14 | 170 ± 17 | 125 ± 14 | 119 ± 13 | 117 ± 14 |
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