Model-Based Assessment of Cardiovascular Autonomic Control in Children with Obstructive Sleep Apnea

Autor: Thomas G. Keens, Maida Lynn Chen, Michael C. K. Khoo, Zheng Lin, Sally L. Davidson Ward, Jarree Chaicharn
Rok vydání: 2009
Předmět:
Zdroj: Sleep. 32:927-938
ISSN: 1550-9109
0161-8105
Popis: ALTHOUGH OBSTRUCTIVE SLEEP APNEA SYNDROME (OSAS) OCCURS QUITE FREQUENTLY IN THE PEDIATRIC POPULATION, WITH A PREVALENCE RATE OF 1 TO 3% in preschool-aged children,1 the cardiovascular consequences of OSAS in children have been less extensively studied, compared with the adult form of sleep-disordered breathing. Most studies have suggested a causal link between OSAS and cardiovascular disease in adults,2–4 primarily in the form of systemic hypertension, myocardial infarction, and stroke. Cardiovascular disease has also been reported to occur in children with severe OSAS, but the more common manifestations are pulmonary hypertension; compromised right ventricular function, including cor pulmonale; and congestive heart failure.5,6 The cumulative evidence in adults suggests that autonomic dysfunction, in the form of reduced parasympathetic activity and elevated sympathetic tone, plays an important role in mediating the link between OSAS and cardiovascular disease.2–4 In contrast, the chronic effects of OSAS on autonomic function in children have been little studied.6 In this study, we hypothesize that the autonomic nervous system is also adversely affected in pediatric OSAS but that the relative impact on the parasympathetic and sympathetic branches differs from what occurs in adults. In recent years, it has become increasingly popular to employ spectral analysis of heart rate variability (HRV) as a simple and cost-effective tool for noninvasive assessment of autonomic function.7 The power of the HRV spectrum in the frequency range of 0.15 to 0.4 Hz, referred to as high-frequency power (HFP), is frequently taken to quantify vagal tone. On the other hand, HRV power from 0.04 to 0.15 Hz, referred to as low-frequency power (LFP), has been shown to reflect both sympathetic and parasympathetic activity.8 The ratio (LHR) between LFP and HFP is therefore known as representing an index of sympathovagal balance,8 with a higher LHR implying a shift toward sympathetic dominance, a decrease in vagal tone, or both.9 The underpinnings of HRV spectral analysis are derived largely from the 1975 study of Katona and Jih,10 which demonstrated, in an animal preparation, a linear relationship between respiratory-related fluctuations in R-R intervals (RRI) and vagal firing rates. Studies using HRV for autonomic-function assessment often overlook the fact that this key observation and the other validation findings that followed11 were obtained under conditions in which respiration was relatively well controlled. However, it has been shown that changes or differences in breathing frequency, tidal volume, or ventilatory pattern can significantly confound the interpretation of autonomic activity that one derives from HRV spectral analysis.12,13 Some interventions that increase sympathetic drive also lead to increases in LFP of blood-pressure variability (BPV).8 Thus, the power of low-frequency BPV oscillations has been proposed by some to represent a quantitative index of sympathetic modulation of the peripheral vasculature. At the same time, however, there are other observations that do not support this view.9 To circumvent the limitations associated with spectral analysis of HRV or BPV, we have developed an alternative approach for noninvasive assessment of autonomic function. This approach employs a closed-loop model that relates HRV to respiration and BPV and relates BPV to changes in heart rate and respiration. The model has been validated in a number of studies on adult subjects with OSAS and normal control subjects under a variety of conditions.14–17 For instance, our group has shown that continuous positive airway pressure therapy in subjects with OSAS leads to improved autonomic function, as reflected in cardiovascular variability.14 In another study, we showed that autonomic control is impaired in subjects with OSAS during both wakefulness and sleep.15,16 We recently extended the model so that temporal changes in the parameters can be estimated when data are collected under time-varying conditions, such as during arousals from sleep.17,18 In this study, we applied both the original and time-varying versions of the closed-loop model to assess cardiovascular autonomic control in pediatric OSAS under conditions of altered orthostatic stress and cold face stimulation (CFS). Changing posture from supine to standing is known to increase sympathetic drive and decrease vagal tone. The CFS test activates the diving reflex, which produces an increase in systemic vascular resistance via an elevation of peripheral sympathetic activity, along with a concomitant bradycardia as a consequence of increased vagal drive.19,20 The combination of the 2 autonomic tests thus allowed us to determine how the model parameters would be affected by conditions in which vagal and sympathetic activity are altered in opposite directions (orthostatic stress), as well as in the same direction (CFS).
Databáze: OpenAIRE
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