Dynamic loop gain increases upon adopting the supine body position during sleep in patients with obstructive sleep apnoea

BACKGROUND AND OBJECTIVE

Obstructive sleep apnoea (OSA) is typically worse in the supine versus lateral sleeping position. One potential factor driving this observation is a decrease in lung volume in the supine position which is expected by theory to increase a key OSA pathogenic factor: dynamic ventilatory control instability (i.e. loop gain). We aimed to quantify dynamic loop gain in OSA patients in the lateral and supine positions, and to explore the relationship between change in dynamic loop gain and change in lung volume with position.

METHODS

Data from 20 patients enrolled in previous studies on the effect of body position on OSA pathogenesis were retrospectively analysed. Dynamic loop gain was calculated from routinely collected polysomnographic signals using a previously validated mathematical model. Lung volumes were measured in the awake state with a nitrogen washout technique.

RESULTS

Dynamic loop gain was significantly higher in the supine than in the lateral position (0.77 ± 0.15 vs 0.68 ± 0.14, P = 0.012). Supine functional residual capacity (FRC) was significantly lower than lateral FRC (81.0 ± 15.4% vs 87.3 ± 18.4% of the seated FRC, P = 0.021). The reduced FRC we observed on moving to the supine position was predicted by theory to increase loop gain by 10.2 (0.6, 17.1)%, a value similar to the observed increase of 8.4 (-1.5, 31.0)%.

CONCLUSION

Dynamic loop gain increased by a small but statistically significant amount when moving from the lateral to supine position and this may, in part, contribute to the worsening of OSA in the supine sleeping position.

A randomised crossover trial of clinical algorithm for oxygen saturation targeting in preterm infants with frequent desaturation episodes

OBJECTIVES

Strategies for oxygen therapy for preterm infants, such as the Vermont-Oxford's 'Breathsavers' guidelines, seek to strike a balance between the potential risks of the extremes of hyperoxia and hypoxia in preterm infants. Using an algorithm based on those guidelines, we aimed to compare the proportion of time spent within the SpO2 target range during algorithm-based management of oxygen delivery compared with routine nursing care.

STUDY DESIGN

In a randomised crossover trial, maintenance of SpO2 over a 4-hour period during routine care was compared with algorithm-based control (administered by a dedicated research nurse). SpO2 target (88-92%) and alarm limits (86-94%) were identical in both arms. Infants <32 weeks' gestation were eligible if having >8 desaturations episodes to <85%/4 h while receiving continuous positive airway pressure/synchronised intermittent mandatory ventilation. Data was recorded via the Powerlab system from Masimo oximeters and Babylog 8000+ ventilators.

RESULTS

16 infants with a gestation of 26.7 ± 1.3 weeks (mean ± SD) and birth weight 901 ± 193 g were studied at a postmenstrual age of 30.5 ± 2.4 weeks. The percentage of time spent within target range was 34.6 ± 28.5% during routine care versus 38.3 ± 29.3% during algorithm-based care (p = 0.23). Compliance with alarm limits was 58.4 ± 21.8% during routine versus 64.7 ± 22.1% for algorithm-based care (p = 0.091). The frequency of desaturations, episode severity or number of FiO2 adjustments did not differ between the two care strategies.

CONCLUSIONS

The observation that algorithm-based control did not improve time spent within the SpO2 target and alarm limits suggests nursing care has reached an optimum in the unit studied. Our finding indicates that significant modifications to the algorithm are likely to be necessary to improve target range compliance.