ACCURACY OF NASAL CANNULA PRESSURE RECORDINGS FOR ASSESSMENT OF VENTILATION DURING SLEEP

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1 Online Supplement for: ACCURACY OF NASAL CANNULA PRESSURE RECORDINGS FOR ASSESSMENT OF VENTILATION DURING SLEEP METHODS Evaluation of Impaired Nasal Ventilation Impaired nasal breathing, as perceived subjectively, was assessed by a visual analog scale. The patient had to set a mark onto a straight line, 12 cm in length, printed on paper. The relative location of the mark with respect to the two ends of the line, labeled not impaired (corresponding to 0%) and completely obstructed (corresponding to 100%) represented the degree of nasal obstruction. In 10 patients, inspiratory nasal resistance was measured at 150 Pa with active anterior rhinomanometry (Rhinotest MP 500; EVG Electronic Vertriebe GmbH, Ludwigshafen, Germany) (E1). Subjectively perceived nasal breathing of these 10 patients was not statistically different from that of the other 10 of the 20 patients. Sleep Studies The polysomnographic montage included EEG (C3A2, C4A1), EOG (LEA2, REA1), submental EMG, ECG, pulseoximetry, respiratory inductive plethysmography, and body position by an accelerometer. Nasal cannulas as used for nasal oxygen administration (nasal cannula Salter Style; Salter Labs, Arvin, CA) were inserted into the nostrils and securely taped to the face. The other end of the tubing was connected to a differential pressure transducer (Validyne MP45) referenced to the pressure inside the face mask (see below). A full face mask (ResMed, Sidney, Australia) was strapped onto the face on top of the nasal cannula tubing. The leak valves (holes) of the mask (designed for CPAP therapy) were occluded with E1

2 silicone glue. An airflow meter (Spiroson; ultrasound transit time flow meter, Isler Engineering, Duernten, Switzerland) (E2), was calibrated by a 1-L syringe, and attached to the main air inlet of the face mask. The flowmeter/mask system was tested airtight to 20 cm H 2 O. Respiratory signals were converted to digital format at 50 Hz with 12-bit resolution, and recorded in a digital polygraph (Respitrace PT16; Noninvasive Monitoring Systems, Miami Beach, FL), and in the polysomnography system (Alice 3; Respironics, Murrisville, PA). The patients were also audiovisually monitored by means of an infrared light source and a low light video camera. Protocol Patients reported to the sleep laboratory at 8 PM. After assessment of nasal breathing on the visual analog scale and by rhinomanometry, the polysomnographic montage was installed. The respiratory inductive plethysmograph was calibrated in supine position by the qualitative diagnostic calibration (QDC) method that sets relative gains of rib cage and abdominal signals so that their sum is proportional to lung volume changes (E3, E4). Lights were turned off at approximately 10 PM. The patient was carefully observed during the night by means of the video camera. The position of the face mask was adjusted, if necessary, to assure air tightness. The recordings were stopped in the morning at approximately 6 AM, after a minimum of 6 h of monitoring. Rhinomanometry was repeated in the morning. Data Analysis Sleep and apnea/hypopnea scoring. Sleep stages were scored by review of 30 s screens according to standard criteria (E5). Apneas/hypopneas were defined as a clear amplitude E2

3 reduction of a measure of breathing to < 50% of baseline for > 10 s (according to the American Academy of Sleep Medicine Task Force) (E6). Baseline was defined as mean amplitude of stable breathing and oxygenation over the previous 2 min or, if breathing pattern was unstable, the mean of the 3 largest breaths during the previous 2 min. Specialized software (EDP V4.3; Noninvasive Monitoring Systems, Miami Beach, FL) and a customized LabView application (Alea Solutions GmbH, Zürich, Switzerland) was used to display and analyze respiratory signals by review of 2.7-min time series on a computer video screen that displayed grids with measurement units. Cursor assisted measurement of amplitude and time was also provided. Successive screens overlapped by approximately 15 s. The following measures of breathing were scored by separate page-by-page display: nasal pressure (P), square root transformed nasal pressure (V[dot]) (E7) (see details below), summed rib cage + abdominal volume from calibrated inductive plethysmography (VolRIP), time derivative of the latter (V[dot]RIP [i.e., RIP-derived flow ]) (E8), and airflow from flowmeter (V[dot]FM). Signals of the inductive plethysmograph (rib cage, abdomen, sum), and nasal pressure were also scored together, with priority on apnea/hypopnea criteria by inductive plethysmography in case of discrepancies. Assuming V[dot]FM square root transformed P (E7), overdetection of hypopnea by P was expected if the same criterion for amplitude reduction as that for V[dot]FM (< 0.5 times baseline) was applied. To account for this, P was also scored with an amplitude reduction criterion of < (i.e., < 25% of baseline). Furthermore, apneas/hypopneas were scored according to Peppard and colleagues (E9) by combined analysis of P, inductive plethysmography, and pulseoximetry. Apnea/hypopnea was defined as absence of any deflection of P > 10 s, or as any discernible reduction in VolRIP > 10 s associated with > 4% oxygen desaturation (E9). E3

4 Recordings were scored independently by 2 observers. Means of corresponding individual apnea/hypopnea indices (AHI) were compared among methods. Square root transformation of nasal pressure. From P sampled at 50 Hz in arbitrary computer units, a surrogate of airflow (V[dot]) was derived by square root transformation as reported by Montserrat and colleagues (E7). To this end, the value of P corresponding to zero flow (P0), was determined during transient disconnection of the nasal cannula from the differential pressure transducer. This value was subtracted from all values of the P raw signal. The square root of the absolute number of the resulting values were computed and multiplied by +1 and 1, respectively, depending on whether the corresponding P raw signal was greater (inspiration) or smaller (expiration) than P0. Thus, for inspiration: V ' = K + 1 P P 0 (equation 1), and I for expiration: V ' = K 1 P P 0 (equation 2). E KI and KE are proportionality coefficients relating P P 0 to respiratory airflow or to V FM according to: K I, E V ' FM =. P P 0 The coefficients were determined by linear regression analysis of P 0 versus V[dot]FM during inspiration and expiration, respectively. P Estimation of ventilation by nasal pressure monitoring. In 5 patients, the stability of the correlation between V[dot] and V[dot]FM over a relatively short period (i.e., 10 successive breaths recorded during stable respiration in NREM sleep in supine position) was E4

5 evaluated. KI and KE were computed for each successive breath. Mean values of KI and KE for the second to the 10 th breaths were expressed as multiples of KI and KE during the first breath. This allowed assessment of the relative deviation of calibration of V[dot] over these 10 breaths. Coefficients of determination (r 2 ) for the regression of V[dot]FM versus V[dot] were computed over the 10 breaths as a measure of the variation in V[dot] related to ventilation. Stability of the correlation among V[dot] and V[dot]FM over the course of the entire night was also assessed. In the evening, immediately after turning off the lights, baseline values for KI and KE, averaged over the inspirations and expirations of a 2-min period of stable, exclusively nasal breathing in supine position, were obtained. Subsequently, mean KI and KE from artifact-free 2-min periods recorded at the beginning of the second, third, and fourth quarter of the night were also analyzed for each patient. Corresponding values for KI and KE were expressed as multiples of the individual KI and KE at baseline. Estimation of ventilation by respiratory inductive plethysmography. Stability of the correlation among the surrogate of airflow derived by differentiating the calibrated sum signal of the respiratory inductive plethysmograph (E8), V[dot]RIP, and V[dot]FM over the course of the night was assessed in a similar way as explained above for V[dot]. Statistics Statistical analysis was performed with Statistica V5.1 (StatSoft, Tulsa, AZ) and Corel QuattroPro V8.0 (Corel, Ottawa, Canada). Normally distributed data were summarized as means ± SE, nonnormally distributed data as medians and quartiles. Agreement in the AHI by two methods was assessed by analysis of bias (mean difference between values derived by two E5

6 methods), limits of agreement (bias ± 2 SD), and mean discrepancy (mean difference among corresponding values from two methods, irrespective of the algebraic sign) (E10). The correlation among epoch by epoch apnea/hypopnea scores derived by different methods was estimated by Cohen s kappa statistics (intraclass correlation). Correlation among visual analog scores of subjectively perceived nasal obstruction with differences among AHI from nasal pressure and flowmeter quantified by the Pearson product moment correlation coefficient. Mean KI and KE at different times were compared by analysis of variance. Statistical significance was assumed at a probability of p < SUPPLEMENT TO RESULTS In Table E1, AHI are compared by various methods with those obtained according to the definition by Peppard and colleaues (E9). The latter provided systematically lower AHI than all other methods. Mean deviations (irrespective of algebraic sign) of AHI by P exceeded corresponding values from V[dot], V[dot]RIP, and V[dot]FM, suggesting a greater precision of the latter three methods in prediction of the AHI according to Peppard and colleagues (E9). In five patients, comparisons of the square root transformed nasal pressure signal with that from the flowmeter over short time periods (i.e., 10 consecutive breaths) revealed close correlation with a mean value ± SE of the coefficient of determination between the two signals of r 2 = 0.94 ± 0.03 (range, 0.93 to 0.96) during inspiration, and r 2 = 0.93 ± 0.01 (range, 0.88 to 0.96) during expiration. There were only minor breath by breath variations of inspiratory and expiratory proportionality coefficients (KI, KE) (Table E2). E6

7 Mean proportionality coefficients (KI and KE) between V[dot]RIP and V[dot]FM remained fairly stable over the duration of the night (Table E3). However, individual values were variable. References 1. Panagou P, Loukides S, Tsipra S, Syrigou K, Anastasakis C, Kalogeropoulos N. Evaluation of nasal patency: comparison of patient and clinician assessments with rhinomanometry. Acta Otolaryngol 1998;18: Buess C, Pietsch P, Guggenbuhl W, Koller EA. A pulsed diagonal-beam ultrasonic airflow meter. J Appl Physiol 1986;61: Sackner M A, Watson H, Belsito AS, Feinerman D, Suarez M, Gonzalez G, Bizousky F, Krieger B. Calibration of respiratory inductive plethysmograph during natural breathing. J Appl Physiol 1989;66: Bloch KE, Li Y, Sackner MA, Russi EW. Breathing patterns during sleep disruptive snoring. Eur Respir J 1997;10: Rechtschaffen A, Kales A. A manual of standardized terminology, techniques and scoring system for sleep stages of human subjects. Public Health Service. Washington DC: U.S. Government Printing Office; Publication No American Academy of Sleep Medicine Task Force. Sleep-related breathing disorders in adults: Recommendations for syndrome definition and measurement techniques in clinical research. Sleep 1999;22: Montserrat JM, Farré R, Ballester E, Felez MA, Pastó M, Navajas D. Evaluation of nasal prongs for estimating nasal airflow. Am J Respir Crit Care Med 1997;155: E7

8 8. Kaplan V, Zhang JN, Russi EW, Bloch KE. Detection of inspiratory flow limitation during sleep by computer assisted respiratory inductive plethysmography. Eur Respir J 2000;15: Peppard PE, Young T, Palta M, Skatrud J. Prospective study of the association between sleep-disordered breathing and hypertension. N Engl J Med 2000;342: Bland M J, Altman DG. Statistical methods for assessing agreement between two methods of clinical measurement. Lancet 1986;i: E8

9 TABLE E1. AGREEMENT OF APNEA/HYPOPNEA SCORES BY VARIOUS MEASUREMENT TECHNIQUES WITH COMBINED NASAL PRESSURE, INDUCTIVE PLETHYSMOGRAPH, AND PULSE OXIMETER ANALYSIS* Evaluated Methods for AHI Estimation Bias (h -1 ) Apnea/Hypopnea Indices Limits of Agreement Bias ± 2 SD (h -1 ) Mean Deviation ± SD (h -1 ) Coefficients of Intra-Class Correlation (κ) among Epoch by Epoch Apnea/Hypopnea Scores by Different Methods (Means ± SE) P to ± ± 0.03 V[dot] to ± ± 0.02 VolRIP to ± ± 0.03 V[dot]RIP to ± ± 0.03 P-VolRIP-RCRIP-ABRIP to ± ± 0.02 V[dot]FM to ± ± 0.03 * The analysis was based on the average of the apnea/hypopnea scores obtained independently by two observers for each of the seven methods in the sleep studies of 20 patients. The reference method was the combined analysis of nasal pressure, inductive plethysmograph, and pulse oximetry according to Peppard and colleagues (E9). The evaluated methods were: P, V[dot]: nasal pressure raw signal, square root transformed nasal pressure; VolRIP, V[dot]RIP: inductive plethysmographic sum volume signal and its time derivative; P-VolRIP-RCRIP-ABRIP: nasal pressure and inductive plethysmographic rib cage, abdominal and sum volume signals; V[dot]FM: flowmeter. Bias: mean difference in apnea/hypopnea index by evaluated minus reference method; mean deviation: mean difference in apnea/hypopnea index by evaluated minus reference method, irrespective of algebraic sign. Cohen kappa intraclass correlation coefficients (κ) were computed for a total of 1,890 epochs of 2.7-min duration from the 20 sleep studies. p < for comparisons of bias versus reference method. p < for comparisons of bias and mean deviation versus P, VolRIP, and P-VolRIP-RCRIP-ABRIP. p < 0.05 for comparisons of mean deviation versus P. E9

10 TABLE E2. SHORT TIME COMPARISONS AMONG SQUARE ROOT TRANSFORMED NASAL PRESSURE AND AIRFLOW BY FLOWMETER* Proportionality Coefficients among V[dot] and V[dot]FM over 10 Successive Breaths Inspiration: KI (Breath 1) = 100% Expiration: KE (Breath 1) = 100% Patient KI (Breaths 2 to 10) Deviation of KI (Breaths 2 to 10) KE (Breaths 2 to 10) Deviation of KE (Breaths 2 to 10) No. in % KI (Breath 1) from KI (Breath 1) in % KI (Breath 1) in % KE (Breath 1) from KE (Breath 1) in % KE (Breaths 1) ± 3 8 ± ± 1 3 ± ± 6 9 ± 6 99 ± 1 3 ± ± 1 3 ± ± 3 6 ± ± 2 4 ± 2 88 ± 1 12 ± ± 1 1 ± ± 2 8 ± 2 All patients 104 ± 2 5 ± 1 99 ± 1 6 ± 5 * Values are means ± SE. Proportionality coefficients among 50 Hz time series of square root transformed nasal pressure (V[dot]) and airflow by flowmeter (V[dot]FM) were calculated for 10 successive inspirations (KI) and expirations (KE). Values for KI and KI for breaths 2 to 10 were expressed in percent of corresponding values for breath 1. The deviations of KI and KE (breaths 2 to 10) from KI and KE (breath 1) were calculated as absolute differences, irrespective of algebraic sign, and expressed in percent of the values of corresponding KI and KE (breath 1). p < 0.05 versus corresponding value for inspiration. E10

11 TABLE E3. OVERNIGHT COMPARISON OF RESPIRATORY INDUCTIVE PLETHYSMOGRAPHY AND AIRFLOW BY FLOWMETER* Proportionality Coefficients among V[dot]RIP and V[dot]FM Inspiration: KI (Epoch 1, after Lights Off) = 100% Expiration: KE (Epoch 1, after Lights Off) = 100% KI (Epoch 2 to 4) Deviation of KI (Epoch 2 to 4) KE (Epoch 2 to 4) Deviation of KE (Epoch 2 to 4) Epoch in % KI (Epoch 1) Medians (Quartile Ranges) From KI (Epoch 1) in % KI (Epoch 1) Medians in % KE (Epoch 1) Medians (Quartile Ranges) from KE (Epoch 1) in % KE (Epoch 1) Medians 2 nd quarter of night 103 (96 to 109) (65 to 116) 35 3 rd quarter of night 108 (64 to 166) (61 to 124) 31 4 th quarter of night 101 (57 to 156) (97 to 115) 37 Epochs 2 to (71 to 149) (65 to 117) 35 * N = 20 patients. As data were not normally distributed, values are summarized by medians and quartiles. Inspiratiory (KI) and expiratory (KE) proportionality coefficients among 50 Hz time series of time derivative of calibrated sum volume signal from respiratory inductive plethysmography (V[dot]RIP) and airflow by flowmeter (V[dot]FM) were calculated for four epochs of 2-min duration. Epoch 1 was immediately after lights off, epochs 2, 3, and 4 at the beginning of the 2 nd, 3 rd, and 4 th quarter of the night. Values for KI and KE for epochs 2 to 4 are expressed in percent of the corresponding value for epoch 1. Deviations correspond to absolute differences, irrespective of algebraic sign, of KI and KE epochs 2 to 4 from values of epoch 1, expressed in percent of values for epoch 1. P = NS for all comparisons among medians of KI and KE at corresponding times. p < 0.05 versus median deviation of KI during 2 nd quarter by analysis of variance. E11

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13 Figure E1. Apnea/hypopnea indices derived from nasal pressure raw (P) and square root transformed signals (V[dot]) were closely correlated. If a hypopnea threshold of < 25% (P [25%]) of the raw signal is taken, AHI are almost identical as that from V[dot]. The dotted and dashed lines represent identity and the prediction equation for the AHI by V[dot] and P (25%), respectively, from that by P (r = 0.99, p < for both regression equations).