Rett syndrome is a neurodevelopmental disorder with immature brainstem and breathing dysrhythmia that includes IBH (Julu et al. 1999). We investigated the CVT response to IBH when there was no significant baroreceptor unloading in healthy control and RS subjects.
Autonomic monitoring of four RS girls aged 6-18 years (mean ± S.E.M., 12 ± 2.75 years) and four healthy female controls aged 6-14 years (10 ± 1.85 years) was carried out after the approval of the local ethics committee and informed consent was given by subjects. Real-time, non-invasive and continuous measurement of mean arterial pressure (MAP), CVT in units of a linear vagal scale (LVS) and cardiac sensitivity to baroreflex (CSB) (Julu et al. 1999) were performed using a combination of NeuroScopeTM (MediFit Instruments LTD, London, UK) and FinapresTM (Ohmeda, Eagleswood, USA). Breathing movements (BM) were monitored continuously using resistant plethythsmograph (PneumoTrace¿, Stevenage, UK). Breathholds lasting more than 10 s in both groups, with depths more than the mean amplitude of normal BM were analysed.
Baseline values in RS girls were: CVT, 4.4 ± 0.85 units of LVS; CSB, 2.7 ± 0.9 ms mmHg-1. In controls, CVT was 9.5 ± 0.55 units in LVS; CSB, 8.3 ± 1.9 ms mmHg-1. During IBH, in RS CVT dropped slowly from 5.9 ± 0.95 to 2.1 ± 0.6 units within 10.9 ± 1.45 s (P < 0.005) (Student’s paired t test, P < 0.05 is significant). In control subjects, the CVT dropped slowly from 13.1 ± 1.65 to 2.7 ± 0.55 units within 13.6 ± 0.45 s (P < 0.005). There was no significant difference in the duration of sustained CVT withdrawal between the two groups (P: n.s.). The baseline MAP in RS was 71.6 ± 13.55 mmHg, and 72.2 ± 8.1 mmHg during IBH, showing no significant change (P: n.s.). In control subjects, baseline MAP was 106.7 ± 13.4 mmHg, and 105.7 ± 14.25 mmHg during IBH (P: n.s.). The close relationship between CVT and CSB previously described during baroreceptor unloading (Al-Rawas et al. 2001) was inconsistent in both groups. Durations of IBH were between 12 and 15.6 s (15.4 ± 1.2 s) in girls with RS and 16.9 and 26.2 s (20.8 ± 1.95 s) in controls.
The results suggest that CVT withdrawal during IBH is not due to baroreceptor unloading and suggest evidence of decoupling of nucleus of tractus solitarii from nucleus ambiguus (NA) during IBH in both RS and controls. We propose that CVT withdrawal is due to the inhibitory effect of lung inflation on NA mediated through pulmonary stretch receptors (PSR) (Daly, 1995). The PSR adapts slowly if lung inflation is sustained (Widdicombe, 1982) and this could explain the protracted duration of CVT withdrawal in our subjects. Our results suggest that inhibition of NA by lung inflation is intact in RS.
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