Measurement of Systemic Blood Pressure

Measurement of blood pressure in children is usually first performed at the time of their first presentation to a pediatric department, or as part of their first school physical examination. Unfortunately, it is often omitted. Common reasons cited by clinicians include the practical difficulties in making measurements in infants and young children, changing normative limits throughout childhood, and the belief that hypertension is primarily an adult disease. There are several aspects to measurement that need consideration. These include the type of measuring machine, the type of cuff and bladder, the technique of measurement, and variables relating to the patient, the observer, and the environment in which the measurements are made.

Cuff and Bladder

The cuff is the inelastic covering that encases an inflatable rubber bladder. The cuff is usually made of cloth, using semisynthetic or any other material that will reliably and evenly transmit pressure over the artery once the bladder is inflated. A combination that allows removal of the bladder from the cuff so that the cuff can be washed at regular intervals is probably more desirable.

The inflatable bladder, rather than the cuff, needs to be of the correct size. Failure to select the correct size of bladder remains the most common error in the measurement of blood pressure. “Miscuffing” refers to the inaccuracy introduced as a result of using a bladder that is too small and narrow, or too long relative to the circumference of the upper arm. It is advisable to choose a size of cuff in relation to the circumference of the arm as opposed to its length ( Table 60.1 ). Although debated, most researchers in this area agree that the width for children should be about 40% of the circumference of the upper arm. This is because measurements made using circumference rather than length as the yardstick correlate best with intra-arterial readings. In both adults and children, the use of cuffs that are too small in length, or too narrow in width, will overestimate the pressures measured by errors ranging from 3 over 2 to 12 over 8 mm Hg, with even greater ranges of error in obese individuals. This is sometimes referred to as cuff hypertension. Too large a cuff is also recognized as a cause of underestimation of pressure in both adults and children. Length of the bladder is much more important in children than adults, and also leads to overestimation of the measured pressure.

The suggested solution to the problem of the size of the cuff is to have a range of sizes of bladder available. In the United Kingdom, cuffs and bladders measuring 4 cm in width and 13 cm in length, and then 8 by 18 cm, 12 by 18 cm, 12 by 26 cm, and 12 by 40 cm, are available. These usually suffice for arms of all sizes across the pediatric age range, including lean and obese children. Other national societies recommend different sizes. The bladder should be centered over the brachial artery and should encircle from 80% to all of the upper arm. In practice, the widest cuff should be used to permit auscultation of the antecubital fossa. Clinically, to reduce errors related to issues related to miscuffing, the authors suggest introducing recording of cuff size in patient case records so that sequential measurements can be compared in the same patient.

Instruments

The mercury sphygmomanometer combined with an inflated cuff and auscultation remains the gold standard for the measurement of blood pressure in children. Widely accepted percentiles for normal pressures through childhood have been developed using such a standard mercury sphygmomanometer instrument. Following concerns regarding the safety of mercury for users in the clinical environment, for technicians who have to service the instrument, and for the environment itself, these instruments have now been phased out of clinical practice.

An issue with any method using auscultation is the introduction of the phenomenon of terminal digit preference, and bias of the observer because of knowledge of previous measurements. Any instrument that eliminates or reduces these two is to be welcomed. This initially led to the development of the random-zero sphygmomanometer, which was developed and shown to minimize or eliminate both these items. Unfortunately, these devices were shown subsequently significantly to underestimate blood pressure, and were therefore abandoned.

Automated oscillometric devices have similarly been developed. These improve on the shortcomings of the auscultatory method by eliminating both terminal digit preference and the bias of the observer. They work by the detection of pressure pulses in the cuff. These are generated as a result of the volume pulses of the artery. When blood starts flowing through the artery at the point of systolic pressure, a pressure pulse is generated. As pressure within the deflating cuff is reduced in a stepwise manner, a series of pressure pulses generate the pulse oscillogram. The pulse amplitudes of this oscillogram provide an envelope curve, with the maximal value on this curve equating to the mean arterial pressure. Systolic and diastolic pressures are calculated from preset algorithms in the instrument microchip, and are a function of the mean. The algorithms are specific to the instrument, and are not declared by the manufacturers, although one would presume these algorithms have been improved over the years by the manufacturers. Although these automated devices are increasingly being used in primary care and pediatric departments, there are some particular concerns that need to be highlighted. The Dinamap devices are the most commonly studied automated oscillometric devices reported in pediatric series. An earlier version of this device, model 1846 SX, had been shown to have superior correlation with intra-arterial measurements. Other studies, however, reported higher mean systolic measurements using models 1846, 8100, and 845 when compared with the random-zero sphygmomanometer and mercury sphygmomanometer. Diastolic measurements had been reported to have better agreement. A more recent report using the Dinamap 8100 monitor highlighted the discrepancy between the two methods, with measurements using the device higher by a mean of 10 mm Hg for systolic and 5 mm Hg for diastolic blood pressure. Therefore, although normative limits have been proposed using the Dinamap 8100 instrument from the United Kingdom, caution needs to be proposed before applying these limits in clinical practice. Another particular practical observation with oscillometric devices is the phenomenon of measurements being higher by about 3 to 5 mm Hg on first measurement, despite control of factors involving the patient, the observer, and the environment. The second reading has been reported to be more accurate.

Automatically inflated cuffs have recently been introduced. These instruments measure both systolic and diastolic pressures, recording the results at preset intervals by detecting oscillations in the pressure from the cuff. They are especially useful in the care of the critically ill child, saving nursing time and reducing disturbance to the patient. The calibration needs to be checked frequently if the result is to be regarded as accurate, but they are useful in detecting changes. Indeed, they will alarm automatically if preset parameters are exceeded.

Aneroid sphygmomanometers have also gained popularity in clinical practice because of their portability and their reliance on techniques similar to the standard mercury sphygmomanometer. Because of this, however, they have no influence on the biases existing with the mercury sphygmomanometer. The devices have proven their accuracy when regular 6-month maintenance is in place to service the instruments.

The majority of devices in clinical use, nonetheless, have not been evaluated independently for accuracy using the two most widely accepted protocols for validation. These protocols have been proposed by the British Hypertension Society and the Association for the Advancement of Medical Instrumentation. Several updates of validation have been published, but the best method of finding up-to-date information is on the nonprofit website http://www.dableducational.com .

Technique of Measurement

The child should be relaxed and quiet when blood pressure is recorded. Should this not be possible, but readings are made, then the conditions should be recorded. A simple description of the process of measurement of a child at a level appropriate for age will usually lead to a cooperative patient, thus allowing accurate recordings. Measuring pressures in infants who are crying is not useful. The standard position is the sitting position when children are older than 3 years, with the fully exposed arm supported or resting on a table at the level of the heart. An arm higher than the heart will underestimate the pressure, while a lower position will produce overestimations. In younger children and infants, pressure should be routinely measured when the patient is in the supine position. The sphygmomanometer should be placed at the level of the eye of the observer to eliminate error from parallax. The cuff should be inflated about 30 mm Hg above the point at which the radial pulse disappears. In some patients, there will be a silent gap between the systolic and diastolic pressures. Simply inflating the cuff until the sound disappears in such individuals may produce a serious underestimation of systolic pressure. Once inflated, the cuff should be deflated at a rate of 2 to 3 mm Hg/s while auscultating with the stethoscope. The sudden distension of the collapsed artery at the systolic pressure is associated with a clear tapping sound, defined as phase 1 of the Korotkoff sounds. The murmur of turbulent blood flowing through the partially occluded artery is phase 2. Phase 3 is a high-pitched sound produced when the artery, closed during diastole, opens in systole. When the artery no longer closes during diastole, the tapping sounds are low pitched and muffled and quieter. This is phase 4. Phase 5 is when the sounds disappear. This is variable and may not occur in some children. The fourth phase, however, tends to overestimate the diastolic pressure, while the fifth phase underestimates it. Although the fifth sound is widely accepted as the optimal measurement of diastolic pressure among adolescents and adults, there has been considerable debate in the literature concerning its value in children younger than 13 years. For these younger children, the fourth sound had generally been preferred, and was recommended by the Task Force Reports on Blood Pressure Control in Children. An international committee, nonetheless, recommended the fifth sound, and the update of the Task Force Recommendations also advocates using this sound. It is preferred because of its easier identification by observers and the comparability it provides for measuring diastolic pressures across age groups. The two sounds are not equal in most children, and may vary considerably, by up to 10 mm Hg.

The measurement of blood pressure in infants, and in some older children, is sometimes difficult because the sounds are inaudible. The old flush technique is unreliable. It has now been superseded by the use of Doppler ultrasound, using an ultrasonic beam to detect motion of the arterial wall when the cuff is deflated. This technique is remarkably reliable for measurement of the systolic pressure, and corresponds to the intra-arterial pressure better than any other method, including auscultation. It is less reliable in the measurement of diastolic pressure, even when machines specifically designed for diastolic pressure are used. All hospital pediatric departments, especially those specializing in pediatric cardiology and nephrology, should have an ultrasonic machine for measurement of blood pressure. It is particularly useful in determining pressure in the legs in children with suspected aortic coarctation. Recording of the systolic blood pressure by palpation of the radial artery underestimates the pressure by as much 11 mm Hg. The blood pressure measurement should be recorded immediately, without rounding off the measured value to the nearest preferred whole value.

The number of measurements to be obtained for each visit in order for the observer to be assured that the measured pressure is accurate is also important. In practical terms, an observer should attempt to take at least two but preferably three measurements at each visit. The second and third measurements are a truer reflection of the pressure, the first being least accurate, especially when measured with an automated oscillometric instrument. Measurements over three visits should be considered to provide a better reflection of a given blood pressure.

In 2016, the European Society of Hypertension published revised guidelines regarding the definitions of hypertension, with those 16 years and older now being categorized as hypertensive per adult definitions ( Tables 60.2 and 60.3 ).

In contrast, the American Academy of Pediatrics (AAP) has categorized adolescents greater than or equal to 13 years of age as hypertensive based on absolute cutoff BP values. The AAP guidelines have also now included new normative pediatric blood pressure tables based on normal-weight children only. As the European Guidelines still refer to the data from the Fourth Report, the authors have referred to the Fourth Report and recent guidelines from the European Society of Hypertension throughout this chapter (see Tables 60.2 and 60.3 ). It is important to highlight that these recent guidelines from the AAP and ESH, although they have key differences in cutoff levels for abnormal BP both by age and by level of BP and differences in terminology related to abnormal BP, continue to have similarities in methods for screening, confirmation of diagnosis, and method for assessment for hypertension.

Ambulatory Measurement of Blood Pressure

Ambulatory monitoring using a small, portable, programmable monitor, and performed over a continuous 24-hour period, is the method of choice for the diagnosis of hypertension and assessment of treatment in adults. There is now a large body of literature highlighting its superior role in the diagnosis and evaluation of blood pressure in children. The monitors typically use an oscillometric technique. They can be programmed to record the blood pressure frequently, for example every 30 minutes during the day and hourly at night. The readings are stored for later downloading, display, and analysis by personal computer. The monitor can be worn on the belt or in the pocket, and the recording is made as the children carry on their normal daytime and nighttime activities.

There are some inherent disadvantages of recording a single measurement of blood pressure, since pressure itself is a continuous variable. In addition, there is the phenomenon of white coat hypertension (WCH), especially in children (discussed in more detail later). Measures made in the clinic also provide no information of pressures when the patient is asleep. Monitoring over a 24-hour period has several advantages over solitary measurements. It is a more accurate representation, being more physiologic in nature and detecting the circadian variation in pressure. Multiple measurements are made in the regular environment, including measurements when the patient is both awake and asleep. Measurements are automatically stored, removing observer error and bias. The technique has been shown to have higher reproducibility than measurements made in the clinic in adults. This has also been shown in children, although reproducing the nocturnal decline in pressure has not been observed as consistently. With improving technology, the instruments themselves have become smaller, and this has resulted in their improved tolerability by children, even in infants and toddlers as young as 2 months of age, although there are no normative limits for this population. As normative standards for ambulatory measurements exist only for children older than 5 years, it is not recommended to use ambulatory blood pressure monitoring (ABPM) over a 24-hour period in younger children.

Increasing age, and higher mean arterial pressures, have previously been described as factors that improve the rate of success of ambulatory measurements made over 24 hours. Monitoring is well tolerated, with more successful measurements achieved when patients are asleep than when awake. This is related to the normal unrestricted daytime activity of the child causing errors in measurement primarily due to movement.

Accurate documentation of the waking and sleeping periods is important for correct interpretation of any study made over 24 hours. A number of studies in children have considered measurements only during a fixed waking period, between 08:00 and 20:00, or sleeping, from 24:00 to 06:00, in effect ignoring measurements made over 6 hours during the period of 24 hours. This method has been preferred by some, as it improves accuracy in correctly classifying measurements made when the patient is awake and asleep. It is important to identify awake and sleep periods accurately, as inaccuracies in this will result in the misclassification of 10% of children as being hypertensive. No study has thus far evaluated if ignoring one-quarter of recording time, as described earlier, impacts on the association of adverse target organ findings with abnormalities on 24-hour ABPM.

Normative data for ambulatory monitoring are published but includes a relatively small cohort of children. Modified normative data from this initial report are recommended and have been developed using the least mean square method of analysis to account for the non-Gaussian distribution of the original cohort providing 90th and 95th percentiles for daytime and nighttime values stratified according to sex and height. The criteria for defining hypertension in children are different when using data derived from office measurements as opposed to ambulatory monitoring, with higher values for daytime levels using 24-hour monitoring of normotensive children compared with measurements made in the clinic. Findings in children with hypertension, however, are inconsistent; although similar to adults with increasing blood pressure values, ambulatory BP values are usually lower than measured ambulatory levels. Ambulatory devices allow for the assessment of systolic and diastolic pressures, expressed as mean and standard deviation of values during the 24-hour period, including daytime and nighttime. The pressure load and dipping state, described later, can also be assessed.

The pressure load as assessed over 24 hours expresses the percentage of measurements above the 95th percentile but provides no information regarding the amplitude and duration of the pressures above the 95th percentile. In adults, this load has been shown to be more accurate than mean pressures in predicting damage to end organs. A recent review suggested a load of less than one-quarter to be normal, and greater than half to be more closely related to damage to the end organs. Attempts have also been made to determine the agreement between mean pressures greater than the 95th percentile and a load of 30% when diagnosing hypertension on the basis of ambulatory monitoring. These data, along with data from others, suggest that mean pressures above the 95th percentile correlate better with a load in excess of 50%. In the authors’ opinion, an isolated abnormality of BP load on ABPM is unlikely to be of major clinical relevance, and the definition of abnormal BP levels should be based on actual measured BP values, as significantly elevated BP levels will be associated with additional abnormalities on 24-hour ABPM.

The physiologic nocturnal dip in pressure and the rise with waking, or the morning surge, have been well described in adults. The absence of any dip in adults is related to cardiovascular and cerebrovascular morbidity and mortality. This relationship has not been described in children. A state of nondipping, however, has been observed in several groups of children, including those with chronic kidney disease, diabetes mellitus, obesity, and those with autonomic dysfunction. Untreated children with secondary hypertension have been reported to develop nocturnal hypertension and the nondipping state, as opposed to patients with primary hypertension who may maintain their dipping status. The morning surge as described in adults, however, is not usually reported in children.

Normal Blood Pressure in Childhood

A large number of studies have been undertaken to establish the range of blood pressure found in normal children, with the results for boys summarized in Fig. 60.1 . As a general rule, a steady increase has been observed with age, with no significant difference between the sexes during childhood. Most studies, unfortunately, differ in some respect, for example in the sizes of cuff used, with measurements taken with the patient sitting or supine, using different end points for diastolic pressure, and so on.

The 95th centiles for systolic and diastolic blood pressure in boys taken from various sources. Data from the Rochester (Minnesota) study, including data from the Muscatine study ; a Scandinavian study ; the Bogalusa study ; the Zurich study ; and the Londe study. The conditions and measurement parameters varied. Children were seated, except in the study of Londe, where they were supine. The widths of the cuffs used were 9 and 13 cm (Scandinavian), two-thirds of upper arm (Londe), at least two-thirds of upper arm (Rochester), mostly 10 and 12.5 cm (Bogalusa), and the largest possible (Zurich). Standard mercury sphygmomanometers were used except in the Zurich study, where random-zero manometers were used. There is an overlap between use of phase 4, muffling of sounds, and phase 5, cessation of sounds.

(From Leuman EP. Blood pressure and hypertension in childhood and adolescence. In: Frick P, von Harneck GA, Martini GA, et al., eds. Advances in Internal Medicine and Pediatrics . Berlin: Springer; 1979:109–183.)

These differences in technique make it difficult to interpret apparent differences between ethnic groups. Adult black individuals have an increased prevalence of hypertension and damage to target organs when compared with whites. Nigerian children were found to have higher pressures than black American children. Higher values for black than white children might be expected in view of the higher prevalence of hypertension in black adults. These were reported by the Bogalusa study, but similar, lower, and no difference in values have also been reported. A significant increase in the proportion of hypertensive 18-year-old black adolescents has also been reported, albeit that another study found no difference in the renin-angiotensin-aldosterone system when comparing black and white children and adolescents. It has been suggested that the evaluation of blood pressure in an individual should be based on height and weight rather than age. It is not clear whether sexual maturation has an effect, independent of body mass, on blood pressure.

Obesity poses special problems because of the essential requirement for an adequate cuff in making measurements. Where adequate sizes have been used, obese adolescents have been found to have a higher incidence of hypertension. Corrections have been made according to the circumference of the arm and the size of the cuff used in the measurement (see Table 60.1 ). The deleterious influence of salt on blood pressure in children has become clearer, and is discussed briefly later in this chapter. No clinically significant difference seems apparent in blood pressure between bottle- and breast-fed infants.

Normal Values for Blood Pressure Measured in the Clinic

Pressures during childhood vary physiologically with gender, age, and height. The practicing clinician requires standards with which to compare the blood pressure of individual patients (see Tables 60.2 and 60.3 ). In a recent report of the European Society of Hypertension, data from the Fourth Report from the working group of the National High Blood Pressure Education Program in the United States have been used to define hypertension, high-normal (previously called “prehypertension”), and normotension. In contrast to previous definitions, adolescents 16 years or older would be considered hypertensive using absolute cutoff of BP values like adults, defining high-normal (130–139 mm Hg systolic and 85–89 mm Hg diastolic) and hypertension (≥140/90 mm Hg). Updated guidelines from the American Academy of Pediatrics, as discussed previously, show similar although not identical categorization.

The Fourth Report from the working group of the National High Blood Pressure Education Program in the United States provides 50th, 90th, 95th, and 99th percentiles for pressures obtained by auscultation in formats specific for gender, age, and height. It provides data over and above those from the previous reports of the Task Force on Blood Pressure Control in Children, giving comprehensive values for seven different centiles for height at each age from 1 to 17 years, as shown in Table 60.2 for boys and Table 60.3 for girls. In addition, the new data describe stages of hypertension, defining normotension as below the 90th percentile, high-normal as between the 90th and 95th percentiles, the first stage of hypertension as between the 95th and 99th percentiles plus 5 mm Hg, and the second stage of hypertension as greater than the 99th percentile plus 5 mm Hg, similar to the seventh report of the joint national committee. It provides guidance on the speed at which evaluation, treatment, and referral should be made in a child with elevated blood pressure. The age- and gender-specific percentiles for blood pressure in girls and boys from birth to 12 months of age are shown in Figs. 60.2 and 60.3 , respectively.

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