This test assesses heart rate variability, a parasympathetic nervous system function. The test is performed by instructing the patient to breathe deeply and regularly at a rate of 6 breaths per minute for 1 min. This is repeated after a minute of rest. Values for this parameter are age dependent, and a reduction in heart rate variability is considered abnormal. The authors utilize the data published by Ingall et al. [1] as age-based norms in their laboratory. The presumed purpose of the reflex is to provide adequate blood volume to absorb incoming oxygen during deep inspiration. When an individual inhales deeply, both air and vascular spaces expand and require increased lung blood volume. This need is met through an increase in heart rate during inspiration, triggered by vagal parasympathetic inhibition. When the individual exhales, the heart rate decreases, due to parasympathetic excitation [2]. In teenage years, this heart variability may become very large, probably due to high vagal tone. The nucleus tractus solitarius orchestrates this response to pulmonary stretch receptor afferents (J-receptors) [3] also accounting for baroreflex responses to blood pressure changes and intrinsic central respiratory rhythms.

The Valsalva maneuver (VM) (Fig. 16.1) evaluates cardiac parasympathetic, cardiac sympathetic, and vasomotor sympathetic functions in response to low-pressure baroreceptor afferents from the right atrium and the great veins. The patient generates a continuous expiratory pressure of 40 mmHg by blowing against a fixed resistance and then suddenly releases the pressure after 15 s. This sudden high pressure in the chest cavity impedes venous return to the heart and reduces ventricular filling and stroke volume. Phase I and III are mechanical phases unrelated to autonomic physiology. During phase I, blood pressure rises for a few seconds as the held pressure is transmitted directly as a pressure wave through the vascular system. Phase II is a sympathetic nervous system-mediated response to the decline in cardiac output, resulting in vasoconstriction and tachycardia to restore blood pressure. The lost cardiac output is reflected in a drop in systolic pressure, while vasoconstriction causes a rise in diastolic pressure, resulting in a marked reduction in pulse pressure. When the subject releases pressure, blood pressure drops transiently during the mechanical phase III . The dominant effect occurs when blood fills the heart again, reaching higher levels than baseline, due to thoracic pressure normalization in the face of continued vasoconstriction. The baroreflex triggers a relative bradycardia through sympathetic withdrawal and parasympathetic excitation. Since vasodilation is slow, the blood pressure overshoots temporarily before returning to baseline. The result is usually read as a ratio of the fastest heart rate during phase II and the slowest heart rate during phase IV. If the ratio is below the age-based normal value, one must determine if this is due to an inadequate bradycardia during phase IV or inadequate tachycardia during phase II. In most centers, results of this study are repeated three times, with the two largest responses included in the dataset [2]. The values vary with age, and we currently utilize the pediatric values published by Ingall et al. [1].

This test evaluates sympathetic vasomotor responses . The patient must remain supine for a minimum of 10 min to obtain reliable baseline values and then passively tilted to 70°. The length of time of the tilt varies greatly across centers, being 10 min in many neurologic autonomic centers and up to 45 min when performed by cardiologists. Currently, in our institution, we tilt children without history of syncope for 30 min, and if there is a history of recurrent fainting, the tilt is extended to 40 min. In our clinical experience, many subjects would be diagnosed as normal had the tilt-table test been stopped at 10 min or may be erroneously diagnosed with POTS due to a cardioacceleration in the first 10 min, though this is not sustained in the ensuing time upright. The clinical significance of such findings is still unknown. A study performed by Carew et al. [4] in adolescents to adult age group (14–60 years) showed that 75 % of the subjects with complaints of orthostatic intolerance develop a sustained increase in heart rate to fulfill the heart rate criteria for postural tachycardia syndrome (POTS) within the first 3 min of head-up tilt and by 7 min had developed the diagnostic criteria for POTS. None of the subjects in the control group (no orthostatic intolerance symptoms) had sustained tachycardia. Thirty six percent of the subjects with POTS developed reflex syncope between 7.4 and 32 min into the head-up tilt [4]. This frequency of syncope in POTS is remarkably similar to that found by Ojha et al. of 38 % [5]. Based on these various data sources, perhaps children should be tilted for a minimum of 30 min, or less if they experience a pre-syncopal or syncopal event. Not every pediatric autonomic center agrees with this recommendation, and some tilt for 5–10 min. During the test, all symptoms should be documented (and rated on a numeric rating scale) so they can later be correlated with vital sign changes. It is of particular importance if children replicate their gastrointestinal complaints during the upright portion of the tilt test, as they will often benefit from treatment aimed at orthostatic intolerance [6].

The tilt-table test may demonstrate four patterns (Fig. 16.2): (a) normal response, (b) postural tachycardia syndrome (POTS), (c) orthostatic hypotension (OH), and (d) reflex syncope. In our clinical experience, children seldom demonstrate true orthostatic hypotension, while POTS and POTS associated with reflex syncope is the more common finding.