Nasal pressure recording in the diagnosis of sleep apnoea hypopnoea syndrome

Background-Nasal pressure tracing is now being used to measure breathing in ambulatory screening devices for sleep apnoea but it has not been compared with other methods of assessment. Methods-Sleep induced breathing disorders were scored by three different me thods of analysis (thermistry; inductive plethysmography, and nasal pressur e tracing) in 193 consecutive patients referred to our sleep laboratory. Wi th the conventional thermistry method an apnoea was defined as the absence of oronasal flow on the thermistor signal for greater than or equal to 10 s and a hypopnoea as a 50% decrease in the sum signal of inductive plethysmo graphy tracing for greater than or equal to 10 s associated with an arousal and/or a 2% decrease in Sao,. Nasal pressure was measured via nasal prongs connected to a pressure transducer. Using the thermistor signal alone, a h ypopnoea was defined as a 50% decrease in the signal for greater than or eq ual to 10 s associated with an arousal and/or a 2% decrease in Sao,. A simi lar definition of apnoea and hypopnoea was used for nasal pressure, the fal l in pressure being substituted for the thermistor reading. Results-Impaired nasal ventilation prevented adequate measurements of nasal pressure in 9% of subjects. According to the conventional method of interp retation 107 subjects were identified as having the sleep apnoea hypopnoea syndrome (SAHS). The apnoea + hypopnoea index (AHI) was significantly lower using the thermistry method than with conventional analysis (mean differen ce -4.3/h, 95% CI -5.3 to -3.2, p<10(-4)); 39% of conventional hypopnoeic e vents were scored as apnoeas using nasal pressure scoring. Apnoeic and hypo pnoeic events could also be observed without any change in thermistor and s um Respitrace signals that resumed with the occurrence of arousals or awake nings. The AHI was significantly higher with nasal pressure scoring than wi th the conventional method (mean difference 4.5, 95% CI 3.4 to 5.6, p<10(-4 )). The mean difference in apnoea index between conventional and nasal pres sure scoring was -7.5/h (95% CP -8.9 to -6.1). In the 78 patients who did n ot have SAHS according to the conventional method of analysis there was a s ignificant positive relationship between the arousal index and AHI measured by nasal pressure tracing (R = 0.51,p<10(-4)). Seventeen of the 78 patient s had an AHI of >15/h by the nasal pressure method of analysis. Conclusions-Nasal pressure recording provides a simple and reliable measure ment of nocturnal breathing abnormalities and may identify breathing abnorm alities associated with arousals that are missed by other diagnostic method s.