Abstract
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 methods of analysis (thermistry, inductive plethysmography, and nasal pressure tracing) in 193 consecutive patients referred to our sleep laboratory. With the conventional thermistry method an apnoea was defined as the absence of oronasal flow on the thermistor signal for >/=10 s and a hypopnoea as a 50% decrease in the sum signal of inductive plethysmography tracing for >/=10 s associated with an arousal and/or a 2% decrease in SaO2. Nasal pressure was measured via nasal prongs connected to a pressure transducer. Using the thermistor signal alone, a hypopnoea was defined as a 50% decrease in the signal for >/=10 s associated with an arousal and/or a 2% decrease in SaO2. A similar definition of apnoea and hypopnoea was used for nasal pressure, the fall 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 interpretation 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 difference -4.3/h, 95% CI -5.3 to -3.2, p<10(-4)); 39% of conventional hypopnoeic events were scored as apnoeas using nasal pressure scoring. Apnoeic and hypopnoeic events could also be observed without any change in thermistor and sum Respitrace signals that resumed with the occurrence of arousals or awakenings. The AHI was significantly higher with nasal pressure scoring than with 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 pressure scoring was -7.5/h (95% CI -8.9 to -6.1). In the 78 patients who did not have SAHS according to the conventional method of analysis there was a significant positive relationship between the arousal index and AHI measured by nasal pressure tracing (R = 0.51, p<10(-4)). Seventeen of the 78 patients had an AHI of >15/h by the nasal pressure method of analysis. CONCLUSIONS: Nasal pressure recording provides a simple and reliable measurement of nocturnal breathing abnormalities and may identify breathing abnormalities associated with arousals that are missed by other diagnostic methods.