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.
• Stress testing
• Long-term electrocardiography (ECG) recording
• Ambulatory blood pressure monitoring
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 (Vo2 max) that can be achieved during exercise is probably the best index of describing fitness or exercise capacity (also called “maximum aerobic power”). Vo2 max is defined by the plateau of oxygen uptake (Vo2) that occurs despite continued work. Beyond this level of Vo2 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 Vo2 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 Vo2 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.
PERCENTILES OF ENDURANCE TIME (MIN) BY BRUCE TREADMILL PROTOCOL
SD, standard deviation.
From Chatrath R, Shenoy R, Serratto M, Thoele DG: Physical fitness of urban American children. Pediatric Cardiol 23:608-612, 2002.
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.
High systolic BP in the arm, to the level of what is considered hypertensive emergency, probably does not reflect the central aortic pressure, and the usefulness of arm BP in assessing cardiovascular function during upright exercise is questionable except in the case of failure to rise. The major portion of the rise in arm systolic BP during treadmill exercise probably reflects peripheral amplification caused by 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 BP in the arm in most cases. Figure 6-1 is a dramatic illustration of a relationship between the central and peripheral arterial pressures measured directly with arterial cannulas inserted in to the ascending aorta and radial artery during upright exercise in young adults. Note that when the radial artery systolic BP is over 230 mm Hg, the aortic pressure is only 160 mm Hg and that there is a very little increase in diastolic BP.
FIGURE 6-1 Simultaneous recording of aortic and radial arterial pressure waves in a young adult during rest (A) and 28.2% (B), 47.2% (C), 70.2%, and (D) of maximal oxygen uptake during treadmill exercise. AA, ascending aorta; RA, radial artery. (From Rowell LB, Brengelmann GL, Blackmon JR, Bruce RA, Murray JA: Disparities between aortic and peripheral pulse pressure induced by upright exercise and vasomotor changes in man. Circulation 37:954-964, 1968.)
SYSTOLIC BLOOD PRESSURE RESPONSE TO BRUCE TREADMILL PROTOCOL
From Ahmad F, Kavey R-E, Kveselis DA, Gaum WE: Response of non-obese white children to treadmill exercise, J Pediatr 139:284-290, 2001.
An excessive rise in the peripheral BP has been reported in patients who have had surgical repair of coarctation of the aorta, patients with hypertension and those with the potential to develop hypertension, hypercholesterolemic patients, and patients with aortic regurgitation, but information on the central aortic pressure is lacking in these reports.
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, left ventricular outflow tract obstruction, coronary artery diseases, or the onset of ventricular or atrial arrhythmias.
4. ECG Monitoring
The major reasons for monitoring ECG during exercise testing are to detect exercise-induced arrhythmias and ischemic changes. A complete ECG should be recorded at rest, at least once during each workload, and for several intervals after exercise.
a. Exercise-induced arrhythmias
Arrhythmias that increase in frequency or begin with exercise are usually significant and require thorough evaluation. The type and frequency before and after the exercise and occurrence of new or more advanced arrhythmias should be noted. The occurrence of serious ventricular arrhythmias may be an indication to terminate the test. Changes in the QTc duration, including the recovery period, should be evaluated.
b. Changes suggestive of myocardial ischemia
ST-segment depression is the most common manifestation of exercise-induced myocardial ischemia. For children, downsloping of the ST segment 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. 3-23). Most guidelines for adult exercise testing, however, recommend the ST-segment depression of 1 mm or greater as an abnormal response. With progressive exercise, the depth of ST-segment depression may increase, involving more ECG leads, and the patient may develop anginal pain. Five to 10 minutes after the termination of the exercise, the ST changes (and T-wave inversion) may return to the baseline. Occasionally, the ischemic ST-segment response may appear only in the recovery phase.
The following lists some points in the evaluation of ST-segment shift in certain situations.
1) Specificity of the exercise ECG is poor in the presence of ST-T abnormalities on a resting ECG or with digoxin use
2) If the ST segment is depressed at rest (which occurs occasionally), the J point and ST segment measured at 60 to 80 msec should be depressed an additional 1 mm or greater to be considered abnormal.
3) When there is an abnormal depolarization, such as bundle branch block, ventricular pacemaker, or Wolff-Parkinson-White preexcitation, interpretation of ST-segment displacement is impossible.
4) In patients with early repolarization and resting ST-segment elevation, return to the PQ junction is normal. In such cases, ST depression should be determined from the PQ point, not from the elevated J point.
5) There is a poor correlation between ST-segment changes and nuclear perfusion imaging in such conditions as anomalous origin of the coronary artery from the PA, Kawasaki’s disease, and postoperative arterial switch operation.
Pulse oximetry measurement of blood oxygen saturation is useful during exercise testing of children who have CHD. Normal children maintain oxygen saturation greater than 90% during maximal exercise when monitored by pulse oximetry. Desaturation (less than 90%) during exercise is considered an abnormal response and may reflect pulmonary, cardiac, or circulatory compromise. Children who received lateral tunnel Fontan operation with fenestration may desaturate during exercise because of right-to-left shunt through the fenestration.
Safety of Exercise Testing
A properly supervised exercise study is safe. Exercise testing should be performed under the supervision of a physician who has been trained to conduct the test, with patient safety in mind. The examiner should pay close attention to the subject during the treadmill exercise testing and be alert to stopping the treadmill when the patient can no longer exercise or appears to be in jeopardy. At these times, an observer should be positioned to assist the subject. A well-stocked crash cart should be available in the laboratory. A defibrillator should be present. Additional equipment should include a delivery system for oxygen as well as ventilation and suction apparatus.
Indications of stress testing vary with institutions and cardiologists. However, some of the more common indications for exercise testing in children are as follows:
1. Evaluate specific signs or symptoms that are induced or aggravated by exercise.
2. Assess or identify abnormal responses to exercise in children with cardiac, pulmonary, or other organ disorders, including the presence of myocardial ischemia and arrhythmias.
3. Assess the efficacy of specific medical or surgical treatments.
4. Assess the functional capacity for recreational, athletic, and vocational activities.
5. Evaluate a prognosis, including both baseline and serial testing measurements.
6. Establish baseline data for institution of cardiac, pulmonary, or musculoskeletal rehabilitation.
Good clinical judgment should be foremost in deciding contraindications for exercise testing. Absolute contraindications include patients with acute myocardial or pericardial inflammatory diseases or patients with severe obstructive lesions in whom surgical intervention is clearly indicated (American Heart Association (AHA), 2006, Clinical Stress Testing in Pediatric Age Group).
The patients with following diagnoses are considered a high-risk group.
1. Acute myocarditis or pericarditis
2. Severe aortic or pulmonary stenosis
3. Pulmonary hypertension
4. Documented long QT syndrome
5. Uncontrolled resting hypertension
6. Unstable arrhythmias
7. Routine testing on Marfan’s syndrome
8. Routine testing after heart transplantation
Termination of Exercise Testing
Three general indications to terminate an exercise test are (1) when diagnostic findings have been established and further testing would not yield any additional information, (2) when monitoring equipment fails, and (3) when signs or symptoms indicate that further testing may compromise the patient’s well-being. The following indications for termination of exercise testing have been recommended by AHA 2006 Clinical Stress Testing in Pediatric Age Group.
1. Failure of heart rate to increase or a decrease in ventricular rate with increasing workload associated with symptoms (e.g., extreme fatigue, dizziness)
2. Progressive fall in systolic pressure with increasing workload
3. Severe hypertension, above 250 mm Hg systolic or 125 mm Hg diastolic, or BP higher than can be measured by the laboratory equipment
4. Dyspnea that the patient finds intolerable
5. Symptomatic tachycardia that the patient finds intolerable
6. Progressive fall in oxygen saturation to less than 90% or a 10-point drop from resting saturation in a patient who is symptomatic
7. Presence of 3 mm or greater flat or downward-sloping ST-segment depression
8. Increasing ventricular ectopy with increasing workload
9. Patient requests termination of the study
Alternative Stress Testing Protocols
Besides treadmill exercise, there are other types of stress testing that can be performed, including the 6-minute walk test, pharmacologic stress tests, and exercise-induced bronchospasm (EIB) provocation tests.
Six-Minute Walk Test
This test may be more appropriate for assessing exercise tolerance in children with moderate to severe exercise limitation for traditional exercise testing.
The patient is encouraged to try to cover as much distance or as many laps on a measured course (often 30 m) as possible in 6 minutes. Patients using supplemental oxygen should perform the test with oxygen. Portable oximeters may be used if available to the patient. If monitoring equipment is not available, oxygen saturation and heart rate are monitored before and after the test. The total distance walked is the primary outcome. At least two practice tests performed on a separate day are advisable. At this time, reference values for healthy children and adolescents are not available. However, the test is useful for following disease progression and measuring the response to medical interventions.
Pharmacologic Stress Protocols
This protocol is used when conventional exercise testing is unsuitable or impractical, such as for patients who are too young and those who are unable to perform exercise test. After pharmacologic stimulations, either echocardiography or nuclear imaging is performed. Two types of pharmacologic agents are used:
• Agents that increase myocardial oxygen consumption (dobutamine, isoproterenol), which simulate the effects of exercise.
• Agents that cause coronary dilatation (adenosine, dipyridamole). Adenosine cause dilatation of normal coronary artery segments, resulting in a shunting of myocardial blood flow away from diseased segments. Dipyridamole inhibits adenosine reuptake, resulting in the same physiology.
The following are the dosages of the pharmacologic agents recommended in a statement from the AHA (Paridon et al, 2006). Dobutamine is administered in gradually increasing doses from a starting dose of 10 mcg/kg/min to a maximal dose of 50 mcg/kg/min in 3- to 5-minute stages to achieve the target heart rate. Atropine (0.01 mg/kg up to 0.25 mg aliquots every 1–2 minutes up to a maximum of 1 mg) can be administered to augment the heart rate, usually given at 50 mcg/kg/min of dobutamine. Esmolol (10 mg/mL dilution at a dose of 0.5 mg/kg) should be available to rapidly reverse the effects of dobutamine in the event of adverse reaction or development of ischemia. If echocardiography is used, the imaging should be performed at rest and at each dosing stage. Radioisotope for nuclear myocardial perfusion scan should be injected 1 minute before the infusion of dobutamine at maximal dosage is stopped.
Adenosine is infused at 140 mcg/kg/min for 6 minutes. If echocardiography imaging is used, it should be continuous throughout the infusion. Nuclear isotope is given at 3 minutes into the infusion. Dipyridamole is infused over the same time period at a dose of 0.6 mg/kg/min. Radioisotope delivery and echocardiography imaging should be performed at the peak physiologic effect of the dipyridamole, usually 3 to 4 minute after completion of the infusion. Administration of aminophylline is routinely used in many centers after termination of the dipyridamole infusion.
Exercise-Induced Bronchospasm Provocation
Bronchial reactivity is measured while the subject exercises for 5 to 8 minutes on a treadmill at an intensity of 80% of maximum capacity. The exercise room should be as cool (temperature, 20° to 25°C) and dry as possible.
The exercise protocol should be to quickly increase the intensity to 80% maximum capacity within 2 minutes (using predicted heart rate maximum as a surrogate). If the intensity is not reached quickly, the patient may develop refractoriness to bronchospasm. An incremental work used in many exercise tests, such as the Bruce protocol, is less likely to be effective in evaluating EIB because of too short a duration of high ventilation; therefore, it should not be used in the evaluation of EIB.
Exercise is preceded by baseline spirometry. Spirometry is repeated immediately after exercise and again at minutes 5, 10, and 15 of recovery. Most pulmonary function test nadirs occur within 5 to 10 minutes after exercise. Accepted criteria for a significant decline in forced expiratory volume in 1 second (FEV1) after exercise are variable. Declines of 12% to 15% in FEV1 are typically diagnostic.
Long-Term Electrocardiography Recording
Long-term ECG recording is the most useful method to document and quantitate the frequency of arrhythmias, correlate the arrhythmia with the patient’s symptoms, and evaluate the effect of antiarrhythmic therapy. There are several different types of long-term ECG recorders, which detect arrhythmias for varying lengths of time. The Holter monitor is used to record events occurring in 24 (or up to 72) hours, event recorders record arrhythmic episodes for up to 30 days, and implantable loop recorders record rhythm up to 14 months.
The Holter monitor, invented by Dr. Norman Holter, is a device that records the heart rhythm continuously for 24 (to 72) hours using ECG electrodes attached on the chest. The heart rhythm is recorded onto a cassette tape or flash card technology and then processed at a heart center. Two simultaneous channels are usually recorded. This helps distinguish artifacts from arrhythmias. This recorder is useful when a child has symptoms almost daily. This type of monitoring is not helpful in the detection of episodes that occur infrequently (e.g., once a week or once a month). Patients are given a diary so they can record symptoms and activities. The monitor has a built-in timer that is used with the patient’s diary to allow subsequent correlation of symptoms and activities with arrhythmias. The importance of keeping an accurate and complete diary must be impressed on patients and parents. Events of interest can be picked out and printed for review. A wide variety of information can be obtained from the recording, including heart rates, abnormal heartbeats, and recording of rhythm during any symptoms.
The Holter recordings should reveal the frequency, duration, and types of arrhythmias, as well as their precipitating or terminating events. Significant arrhythmias rarely cause symptoms such as palpitation, chest pain, and syncope (<10% of cases). Marked bradycardia (<50 beats/min in infants, <40 beats/min in older children), supraventricular tachycardia with a rate faster than 200 beats/min, and ventricular tachycardia are potentially life threatening. These arrhythmias do occur and may worsen during sleep.
Ambulatory ECG monitoring is obtained for the following reasons:
1. To determine whether symptoms such as chest pain, palpitations, and syncope are caused by cardiac arrhythmias
2. To evaluate the adequacy of medical therapy for an arrhythmia
3. To screen high-risk cardiac patients such as those with hypertrophic cardiomyopathy or those who have had operations known to predispose to arrhythmias (e.g., Mustard, Senning, Fontan-type operation)
4. To evaluate possible intermittent pacemaker failure in patients who have an implanted pacemaker
5. To determine the effect of sleep on potentially life-threatening arrhythmias
Interpretation of the results usually includes the following:
1. A description of the basic rhythm and the range of the heart rate
2. If there is bradycardia, description of its rate, rhythm, duration (or number of beats) and the presence of escape beats, and so on
3. For extreme tachycardia, description of the rhythm, mode of initiation and termination, and its duration
4. Description of any abnormalities in AV conduction
5. Description of any arrhythmias, including their characteristics, duration, and frequency
6. Correlation of the arrhythmias with the patient’s activities and symptoms
7. If the patient complained of anginal pain, correlation of ST-segment changes with activities
Holter Findings in Normal Children
Meaningful interpretation of the Holter recordings performed in patients with organic heart disease or significant systemic illness requires knowledge of the range of heart rate and rhythm variations in normal subjects of comparable age. Holter ECG recordings of healthy pediatric populations have demonstrated that variations in rate and rhythm, which were previously thought to be abnormal, occur quite frequently.
Premature or Low-Birth-Weight Infants
The minimum heart rate of premature or low-birth-weight infants can be as low as 73 beats/min; the maximum heart rate can be as high as 211 beats/min. Junctional rhythm may be observed in 18% to 70%, premature atrial contractions (PACs) in 2% to 33%, and premature ventricular contractions (PVCs) in 6% to 17%. First-degree atrioventricular (AV) block or Wenckebach’s second-degree AV block occurs in 4% to 6%. Sudden sinus bradycardia and sinus pause occur especially frequently.
In full-term neonates, the heart rate can be as low as 75 beats/min and as high as 230 beats/min. Junctional rhythm may be present in 28%, PACs in 10% to 35%, and PVCs in 1% to 13%. First-degree AV block or Wenckebach’s (Mobitz type 1) second-degree AV block may be recorded in 25% of neonates. Sinus pause is quite frequent.
Southall et al (1981) reported the following findings in healthy 7- to 11-year-old children.
• The mean highest heart rate was 164 ± 17 beats/min.
• The mean lowest heart rate was 49 ± 6 over 3 beats’ and 56 ± 6 over 9 beats’ duration.
• At their lowest rate, 45% had junction escape rhythm lasting up to 25 minutes.
• AV conduction:
• PR interval ≥0.20 sec in 9%
• Mobitz type I second-degree AV block in 3%
• Isolated supraventricular or ventricular premature beats in 19% (less than 1 per hour)
• Sinus pause seen in 65%. The maximum duration of sinus pause on each child was 1.36 ± 0.23 seconds
Dickinson and Scott (1984) reported the following in healthy 14- to 16-year-old boys.
• Sinus arrhythmia in all cases with heart rate ranging from 45 to 200 beats/min during the day.
• During sleep, the heart rate can become as low as 23 beats/min.
• At their lowest rate, escape rhythm was present in 26%.
• AV conduction:
• Sudden variation in PR interval was noted in 41%
• Sinus arrest or sinoatrial block in 15%
• First-degree AV block in 12%
• Wenckebach’s (Mobitz type 1) second-degree AV block in 11%
• PVCs (occurring in 26% to 57%), which include multiform PVCs; PACs (occurring in 13% to 20%) were also observed.
• Short episodes of ventricular tachycardia were present in 3%.
Event monitors are devices that are used by patients over a longer period (weeks to months, typically 1 month). The monitor is used when symptoms suggestive of an arrhythmia occur infrequently. A drawback of this device is that the patient must be able to press the event button to begin recording. The information collected by the event monitor can be sent over the phone to a doctor’s office, clinic, or hospital. Two general types of cardiac event monitors are available.
1. Looping Memory (Presymptom) Event Monitor
The term “looping” refers to the memory of the monitor, which means that when activated, the monitor can save the previous 30 to 60 seconds of data stored in the memory loop. Two electrodes are attached on the chest. The monitor is always on but only stores the patient’s rhythm when the patient or caregiver pushes the button. Most monitors will save the rhythm for 30 to 60 seconds before the device is activated. The patient can send the stored ECGs by telephone directly to the event monitoring center. This feature is especially useful for people who pass out when their heart problems occur and can press the button only after they wake up.
2. Nonlooping Memory (Postsymptom) Event Monitor
This monitor is capable of real-time recording of the cardiac rhythm without having electrodes attached to the chest. This device is used to record symptoms that last longer than 45 to 60 seconds. It is a small device that has small metal discs that function as the electrodes. When symptoms occur, the device is pressed against the chest to start the recording. The device records and stores the events in solid-state memory. It can store up to four to six such events before it is necessary to transmit the information.
Implantable Loop Recorder
For patients with very infrequent symptoms, such as once every 6 months, neither Holter recorders nor 30-day event recorders may yield diagnostic information. In such patients, implantable loop recorders, about the size of a pack of chewing gum, are implanted beneath the skin in the upper left chest. The patient uses a hand-held activator to record and permanently store the cardiac rhythm when symptoms occur. The device can be “interrogated” through the skin to determine what the heart was doing when the symptoms occurred. This device was shown to be instrumental in establishing the diagnosis in patients with infrequent syncope on whom other recording devices failed to document the cause of syncope.
Ambulatory Blood Pressure Monitoring
Blood pressure is not a static variable; rather, it changes not only from daytime to nighttime but also from minute to minute. Casual BP measurement provides only a snapshot of the daytime BP pattern, which is higher than nighttime readings. In some patients, there is a transient elevation of systolic, diastolic, or mean BP when BP is measured in a health care facility (i.e., “white-coat hypertension”). This could lead to an overdiagnosis of hypertension and to unnecessarily aggressive and costly diagnostic studies and treatment. Ambulatory BP monitoring (ABPM) has emerged as a technology that addresses some of the limitations of casual BP measurements because it permits the observation of BP throughout day and night in a nonmedical environment. Many antihypertensive organizations recommend BP measurement outside the clinic setting, including self-measurement of BP at home or any other place. Although self-measurement of BP was better than clinic BP measurement, ABPM was even better than self-measured BP in the diagnosis of hypertension. Some researchers advocate the use of ABPM in all patients with casual BP elevation.
Multiple BP measurements are obtained with a preapplied BP cuff, usually using the oscillometric method (with or without the ECG’s R-wave gating) for a 24-hour period while children participate in their normal daily activities. Typically, recording frequency varies from every 15 to 30 minutes for daytime or waking measures and every 20 to 60 minutes for nighttime or sleep measures. Sleep periods can be identified by either diary or actigraphy. The most common complaint is sleep disruption. The use of ABPM is usually limited to children 5 to 6 years of age or older. Successful recording has been reported in more than 70% of children younger than 6 years of age. Activity influences the BP readings. Both ambulatory systolic and diastolic BPs varied by 10 mm Hg from lowest to highest levels of activity. Most hypertension specialists recommend that children continue their normal activities but refrain from contact sports and vigorous exercises.
There are three basic calculations of ABPM.
1. Mean BP value. The mean BP value can be determined for the entire 24-hour period or for day and night periods separately. Wühl and colleagues (2002) have published mean normative ambulatory BP levels for gender and height from mid-European white children (see Table B-8 for boys and Table B-9 for girls in Appendix B).
2. BP load. BP load is defined as the percentage of valid BP measures above a set threshold such as the 95th percentile of BP for age, gender, and height. This may be a better measure of the hemodynamic stress placed on end organs susceptible to hypertensive injury. BP load in excess of 25% to 30% are typically considered elevated. Loads in excess of 50% may be predictive of left ventricular hypertrophy. Most experts use a combination of mean BP and BP load to categorize ABPM results as normal or abnormal (Table 6-3).
3. Nocturnal dipping. The nocturnal dipping is calculated by subtracting the mean sleep BP from the mean awake BP and dividing this value by the mean awake BP. Normal nocturnal dipping is generally defined as a 10% or greater decline in mean systolic and diastolic ambulatory BP levels from day to night ([Mean daytime ABPM – Mean nighttime ABPM]/ Mean daytime ABPM × 100]). Nondipping (defined as a decline of <10%) has been associated with hypertensive end-organ injury, end-stage renal disease, renal transplantation, or insulin-dependent diabetes mellitus. Black children have higher sleep BP levels and less significant decreases in BP during sleep compared with age-matched white counterparts.
ABPM is useful in evaluation of hypertension in the following ways.
a. White-coat hypertension. ABPM is critical in distinguishing white-coat hypertension from true hypertension. Office measurement of BP often fails to account for the transient, stress-induced elevation of BP.
b. Secondary hypertension in adolescents. ABPM may be useful in differentiating primary from secondary hypertension. Adolescents with secondary hypertension have been shown to have greater nocturnal systolic BP loads and greater daytime and nocturnal diastolic BP loads than children with primary hypertension. Decreased nocturnal dipping may be another sign favoring secondary hypertension.
c. Abnormality in ABPM, such as nighttime systolic BP and BP load, is more strongly associated with the end-organ damage, such as left ventricular mass, than with a casual BP measurement in children and adults.
d. ABPM may help quantify the level of BP elevation as shown in Table 6-3. Different levels of hypertension can be defined by using the mean ambulatory systolic BP and systolic BP load.
e. ABPM also allows evaluation of the need for and the effectiveness of pharmacologic therapy for hypertension.
Contraindications to ABPM may include atrial fibrillation, coagulation disorders, and for some brands of equipment, latex allergy.
Although the advantages of ABPM are clear, there are still some technical difficulties and problems with normative ambulatory BP levels in children. With the auscultatory method, it is difficult to keep a microphone under the BP cuff over the brachial artery, and ambient noise levels interfere with accurate detection of Korotkoff sounds. Thus, oscillometric devices are more popular. When one uses an auscultatory method, one should not use the normative data of Wühl et al (2002) because oscillometric BP readings have been shown to be higher than the auscultatory BP levels (see an earlier section of Blood Pressure Measurement in Chapter 2). Another problem is that this technology is not always available, and the cost of the study may be prohibitive.
SUGGESTED SCHEME FOR STAGING OF AMBULATORY BLOOD PRESSURE LEVELS IN CHILDREN
BP, blood pressure.
From Urbina E, Alpert B, Flynn J, et al: Ambulatory blood pressure monitoring in children and adolescents: Recommendations for standard assessment. Hypertension 52:433-451, 2008.