Sleep, 20(12):1175-1J84 © 1997 American Sleep Disorders Association and Sleep Research Society Detection of Respiratory Events During NPSG: Nasal CannulaJPressure Sensor Versus Thermistor Robert G. Norman, Muhammed M. Ahmed, Joyce A. Walsleben and David M. Rapoport Division of Pulmonary and Critical Care Medicine, New York University Medical Center, New York, New York, U.S.A Downloaded from https://academic.oup.com/sleep/article/20/12/1175/2750010 by guest on 01 October 2021 Summary: Recording of respiratory airflow is an integral part of polysomnography (NPSG). It is conventionally monitored with a thermistor that measures temperature as a surrogate of flow. The subjectivity of interpreting hypopnea from this signal has prompted us to measure nasal airflow directly with a simple pneumotachograph consisting of a standard nasal cannula connected to a 2-cm H20 pressure transducer. We manually analyzed respi­ ratory events using simultaneous thermistor and nasal cannula in 11 patients with obstructive sleep apnea syndrome (OSAS) and 9 with upper airway resistance syndrome (UARS). Definite events were scored separately for each signal when amplitude was <50% for> 10 seconds. Events were also scored on the nasal cannula signal when the flattened shape of the signal suggested flow limitation. and these were tabulated separately. Definite events in one signal were tabulated by whether the other signal showed a definite event or not. In addition, nasal cannula events were compared to a more liberal thermistor criterion (any change in the signal for 2:2 breaths). Visually, events were more easily recognized on the nasal cannula signal than on the thermistor signal. In OSAS, 1,873 definite thermistor events were detected. Of these, 99.1 % were detected by nasal cannula, and 0.9% were missed. Of 3,541 definite nasal cannula events, 51.9% were detected by definite thermistor criteria; 75.0% were detected by liberal thermistor criteria; 25.0% were missed. In UARS, 123 definite thermistor events were detected. Of these, 89.4% were detected by nasal cannula and 10.6% were missed. Nine hundred and three nasal cannula events were. detected. However, only 17.2% of these were detected by definite thermistor criteria; 38.6% were detected by liberal thermistor criteria; 61.4% were completely undetected by thermistor. When events identified on the nasal cannula hy flow limitation alone were excluded, the thermistor detected 30.1 % of events by definite criteria and 78.6% by liberal criteria, still leaving 21.4% completely undetected by the thermistor. We conclude that the nasal cannula reliably detects respiratory events seen by thermistor. Additional events (including some characterized only by flow limi­ tation) that help define the UARS, were recognized by nasal cannula but often completely missed by thermistor. We propose that respiratory monitoring during NPSG with nasal cannula significantly improves event detection and classification over that with thermistor. Key Words: Respiratory monitoring-Airflow-Flow limitation-UARS­ OSAS-Hypopnea-Apnea. In the past decade there has been explosive growth couples for this purpose. These devices are based on in the diagnosis of sleep-disordered breathing (SDB)­ measuring relative temperature during expiration and ranging from frank obstructive sleep apnea syndrome inspiration and on the heat content of the air (temper­ (OSAS) to the recently defined upper airway resistance ature and volume) passing over the device. Both syndrome (UARS). In part this has been driven by an thermistors and thermocouples thus only measure flow increased appreciation of the morbidity of excessive indirectly. The use of temperature as a surrogate for daytime sleepiness and the biologically plausible hy­ direct measurement of airflow works well for indicat­ pothesis that significant but mild SOB may lead to ing apnea (absent airflow). However, because the sig­ important cardio- and cerebrovascular consequences nal amplitude bears only a rough correlation to mag­ such as hypertension, myocardial infarction, and nitude of actual airflow [and varies greatly with the stroke. type of thermistor and its position (1)], temperature Polysomnographic diagnosis and assessment of se­ may not be as well related to volume when used to verity of SOB rely on measurement of respiratory air­ assess hypopnea (reduced airflow). Because thermis­ flow. Most laboratories use thermistors and thermo- tors and thermocouples have similar properties we will refer only to thermistors in this paper. Accepted for publication September 1997. Despite the limitations of thermistors it has become Address correspondence and reprint requests to David M. Rapo­ port, M.D., Department of Medicine, New York University Medical conventional to define hypopnea based on reduction in Center, 550 First Avenue, New York, NY 10016, U.S.A. amplitude of the thermistor signal as compared to the 1175 1176 R. G. NORMAN ET AL. local baseline amplitude (2). The exact criteria for hy­ encephalographic (EEG) arousals, AHI < 15 by popnea vary widely, ranging from a 30% to 70% re­ thermistor, and no other sleep disorder diagnosis). Re­ duction to any reduction in amplitude (3,4). In addi­ cordings of central and occipital EEG, electrooculo­ tion, most laboratory use supporting criteria (e.g. pres­ gram (EOG), and submental electromyogram (EMG) ence of 2-4% oxygen de saturation or arousal) to con­ were used to monitor sleep. Leg movements were firm an equivocal event identified on the thermistor monitored with an anterior tibialis EMG. A unipolar channel. While this is useful for preventing excessive electrocardiogram (ECG) was used for cardiac moni­ sensitivity of the thermistor (improving specificity), it toring. Oxygen saturation was monitored with a pulse does not address the issue of identifying events that oximeter. Chest wall and abdominal movement were are entirely undetected by the thermistor. Further com­ monitored with piezoelectric strain gauges. Respira­ plicating the situation is the recent description of tory airflow was simultaneously monitored using a na­ UARS (5). In this syndrome sleep disruption and sal/oral thermistor and a nasal cannula connected to a Downloaded from https://academic.oup.com/sleep/article/20/12/1175/2750010 by guest on 01 October 2021 symptoms similar to OSAS result from repetitive in­ 2-cm H20 pressure transducer (see Appendix for de­ creases in airway resistance (identified by increased tails of cannula setup). Figure 1 shows a picture of the intrathoracic pressure swings) without detectable setup used to collect simultaneous nasal cannula and changes in the thermistor signal. thermistor data. We (6) and others (7) have shown that the identifi­ Respiratory events were manually analyzed from cation of a plateau on the inspiratory flow waveform the thermistor and nasal cannula signals wherever both correlates with an elevated upper airway resistance signals were present and of acceptable quality. Con­ during continuous positive airway pressure (CPAP) ti­ tration, and we have proposed that this may be true ventional scoring of apneas and hypopneas was per­ during spontaneous breathing (8). A recent study formed independently from thermistor and nasal can­ shows that there is an excellent correlation between nula signals (with blinding as to the information from the shape of the flow/time curve and the cross-sec­ the other signals). A definite respiratory event (apnea tional area of the airway as well (9). However, a con­ or hypopnea) was scored for one signal when the am­ ventional pneumotachograph signal requires a tight­ plitude of that signal fell below 50% of the local base­ fitting face mask that may be excessively intrusive for line amplitude for more than 10 seconds. The nasal routine sleep monitoring. To circumvent this limita­ cannula signal was also inspected to identify flow lim­ tion, we have been using a simple alternative to the itation events, defined by a flattening of the contour of pneumotachograph that gives a quantitative flow signal successive inspiratory waveforms for at least 10 sec­ without a face mask. This consists of a standard oxy­ onds (but <2 minutes) at a time when the flow am­ gen nasal cannula placed in the nares and attached to plitude did not meet criteria for hypopnea (>50% am­ a sensitive pressure transducer that detects the pressure plitude). These were tabulated separatedly from nasal fluctuations caused by inspiration and expiration. cannula events defined by amplitude criteria alone. Fi­ Monserrat has shown that the signal obtained from nally the thermistor signal was reexamined during such a device is comparable in both shape and ampli­ each nasal cannula event for any visually detectable tude to that of a conventional pneumotachograph (10). change lasting two or more breaths and suggesting hy­ The characteristics of this simple, inexpensive, and popnea. These were classified as liberal thermistor nonobtrusive device make it ideal for quantitative events. monitoring of respiration during sleep. Because neither the thermistor nor the nasal cannula The present study was designed to compare the na­ signal was the a priori gold standard for detection of sal cannula/pressure transducer combination to a con­ events, analysis of their relative sensitivity was per­ ventional thermistor with respect to their ability to de­ formed symmetrically. Events defined by the therm­ tect respiratory events. In particular, we investigated istor were classified as to whether they were simulta­ the ability of each device to (1) detect apnea and hy­ popnea in OSAS, and (2) provide an index of subtle neously detected or missed by the nasal cannula. Con­ airway obstruction in UARS. versely, events defined by the nasal cannula were clas­ sified as to whether they were detected or missed by the thermistor. This analysis was performed separately METHODS for each individual patient. Data were also then pooled A full night of polysomnography was performed on for all OSAS patients and separately for UARS pa­ 11 patients with obstructive sleep apnea [apnea-hy­ tients. Percentage agreement and 95% confidence lim­ popnea index (AHI) > 15 by thermistor) and 9 patients its for this percentage were calculated.