Sleep, 15:S5-58 . © 1992 American Sleep Disorders Association and Sleep Research Society
Respiratory Monitoring in Sleep Apnea Syndrome *tP. Levy, *J. L. Pepin, tB. Wuyam and
*D. Veale
*Sleep and Respiration
This review provides a critical analysis of current respiratory monitoring tec~ni.qu~s in diagnosis of sleep apnea syndrome. The correct analysis ofpolysomnography require~ knowledg~ of the h~ll1tatIons of the m~ans of recording used. These limitations, for invasive and noninvasive techmques, are dls.cus~ed III terms.of c.a1culatlOn, differentiation and scoring of respiratory events. Aims and means are stated for momtonng and sconng III research as well as in clinical practice. Key Words: Respiratory monitoring-Sleep apnea syndrome-Polysomnography.
Summary:
~.
::a..
~.
,.
There are a number of different techniques for assessing airflow and respiratory effort in patients with suspected sleep related breathing problems, including the sleep apnea syndrome (SAS). Although these techniques are less complex in terms of recording and analysis than analysis of the sleep electroencephalogram (EEG), they have not been standardized like the EEG with the international 10120 electrode system, which governs the placement of electroencephalographic (EEG) electrodes (l), and the Rechtshaffen and Kales criteria for EEG scores and staging sleep (2). Several different techniques for measurement of respiratory airflow and respiratory effort are in current use. There are insufficient data available, however, to permit recommendation of any single method for standard use (3). A number of requirements must be fulfilled in order to achieve satisfactory ventilatory monitoring in SAS. Techniques must be reliable with regard to measurement, recording and analysis and must also be capable of reliable calibration (4). Naturally the sensors must be sufficiently comfortable when placed so as not to disturb the patient while asleep (5). The primary objectives of recording respiratory parameters during sleep are first to record the number of respiratory events and second to define their nature, i.e. whether events are obstructive, central or mixed. Finally, we wish to evaluate the relationship of respiratory events to oxygen saturation. In this review, we aim to describe the methods of
monitoring respiration in clinical practice and in research. CALCULATION OF RESPIRATORY EVENTS
An apnea is defined as a cessation of airflow for more than 10 seconds (6). The quantitative recording of airflow can be by direct measurement of tidal volume (Vt) or by integration of airflow measured by a pne~ motachograph. Airflow measurement can also be semIquantitative or qualitative. The most reliable method for quantitative measurement of airflow is by means of a pneumotachograph. The major limitations concern patient tolerance rather than validity of the data recorded. There can, however, be leakages through the face mask, leading to underestimation of ventilatory volume. This technique is of limited use in sleep studies because the face mask leads to sleep disturbance, particularly in moderately severe SAS patients with mild hypersomnia (7). In addition, the tight fitting face mask may have reflex effects on ventilatory patterns (8). Krieger and Kurtz, however, studied 20 young subjects using this technique and did not find significant changes in sleep pattern or in ventilatory characteristics (9). Modifications of ventilation have not been studied in apneic subjects. Calibrated inductance plethysmography records variations in thoracic and abdominal movements. This technique is based on the principle that the respitatory system has two degrees of freedom, i.e. chest and abAccepted for publication July 1992. dominal motion. Under specific conditions of meaAddress correspondence and reprint requests to P. A. Levy, Department of Respiratory Disease, CHRU de Grenoble, BP 217 X surement and calibration this technique can give a semiquantitative estimation of ventilation and tidal 38043 Grenoble Cedex 9, France. S5
Downloaded from https://academic.oup.com/sleep/article-abstract/15/suppl_6/S5/2749348 by guest on 28 April 2019
Unit, Department of Respiratory Disease, (Pr Brambilla, Pr Paramelle), and Sleep Laboratory, CHRU Grenoble, 38043 Grenoble, France tPreta Laboratory, Department of Physiology, J. Fourier University, Grenoble, France +.Freeman Hospital, Newcastle upon Tyne, England
S6
P. LEVY ET AL.
DIFFERENTIAnON OF RESPIRATORY EVENTS
Cessation of breathing during sleep can be of two major types: a) central, with complete cessation of diaphragmatic activity and airflow and b) obstructive, in which diaphragmatic activity continues but airflow Sleep, Vol. 15, No.6, 1992
ceases. A combination of these two types of apneas is called mixed apnea. The characterization of the type of apnea can be made directly by measurement of muscular activity of the diaphragm or the intercostals with electromyography. By contrast, indirect estimation can be performed by assessing intrathoracic pressure changes associated with respiratory efforts by using an esophageal balloon to measure pleural pressure changes. Another indirect method of analyzing respiratory effort is the detection of thoracic and abdominal movements by induction plethysmography or magnetometry. Measurement of pleural pressure changes by esophageal balloons is a method of reference (22,23). This method is limited by patient tolerance of the esophageal balloon. Furthermore, the presence of a balloon catheter in the upper airway may modify dynamic airway collapse. This was the case when local anesthetic was applied to the throat in normals (24). Diaphragmatic muscle activity can be recorded by means of esophageal electrodes, but the invasive character of this method has led to a preference for surface electrode recording during sleep. In this situation there is interference with the signal by activity in other muscles besides the diaphragm, namely abdominal muscles. Furthermore, changes in pulmonary volume modify electric activity, particularly when there are large pulmonary volume changes at the time of apneas. Purely qualitative plethysmography can be used to record whether there is movement of the thorax or abdomen. Magnetometry can be used similarly (25). During changes in position, however, these two methods can become unreliable. Induction plethysmography is the more reliable of these types of qualitative recording. The obstructive nature of respiratory events can be confirmed in the majority of cases, but recordings can be inadequate in some very obese patients or in those in whom paradoxical movement ofthorax and abdomen is absent or in whom respiratory efforts are weak. This is the case in 15% of patients with sleep apnea syndrome (26). If absence of chest wall motion suggests that all apneas are central, then direct measurement of respiratory effort is necessary in order to exclude obstructive events (27). An alternative to the esophageal balloon technique is measurement of movement in the suprasternal notch by induction plethysmography at the surface (Psip). This gives an estimate of variations in pleural pressure with respiration (5,28). These data can be obtained by means of two small plethysmography bands around the neck, but these are often not well tolerated by the patient. The static charge sensitive bed (SCSB) (29) consists of a thin mattress placed under the bedclothes and can be used on an ordinary bed. Movement on the surface
Downloaded from https://academic.oup.com/sleep/article-abstract/15/suppl_6/S5/2749348 by guest on 28 April 2019
volume (10). Precision of measurement requires, however, that the coils not move, which is difficult to achieve during sleep, particularly in apneic subjects. Approaches to this problem include the use of a body jacket to which the coils are sewn or glued (12). Analysis requires multiple regression mathematics or use of the least squares method (10,11). The reference method for calibration of inductance plethysmography is the isovolume maneuver, but this is difficult to perform correctlly in a large number of patients. Sackner (5) has proposed an automatic calibration procedure, which utilizes data required during normal respiration. This procedure must be performed over a number of breathing cycles with minimal variability between cycles. Thermistors and thermocouples at the nose and mouth can achieve semiquantitative or qualitative measurement of airflow, but there may be variations in airflow between one nostril and another or between the nose and mouth (13,14). The amplitude of the recording signal is very dependent on the positioning of the recording electrodes in the airflow current. This is best achieved by mounting three thermocouples on nasal spectacles. The distinction of hypopnea from normal respiration is very difficult in the absence of reliable quantitative data on airflow (15). Furthermore, with thermistors one can have false positive recording of respiration with variations in temperature at the mouth without airflow. Impedance measurement of respiration relies on the principle that a linear relationship exists between thoracic electrical impedance and thoracic volume (16). This method of measurement is limited by interference from cardiac oscillations and body movements (5). If one uses four electrodes rather than two, however, one can record apneic events. These limitations to the use of impedance measurement can be overcome by using numeric filtering to smooth the curves, and peripheral movements can be detected by use of an actigraph (17). Tracheal or laryngeal sounds have been used to (:stimate airflow (18-20). More involved spectral analysis allows characterization of snoring and information at the site of airflow obstruction (21). Finally, measurement of expired CO 2 by a rapid analyzer has been found useful for semiquantitative estimation of airflow.
..
S7
RESPIRATORY MONITORING IN SAS
.'
EVALUATION OF THE CONSEQUENCES OF RESPIRATORY EVENTS Noninvasive measurement of oxygen saturation (Sa0 2) is now performed by pulse oximeters, which give a good correlation with intra-arterial measurements down to saturations around 65-75%. Nevertheless, there are differences among instruments with regard to dynamic characteristics. There are important errors associated with repeated oscillations in oxygen saturation in the sleep apnea syndrome if the sampling rate for Sa0 2 measurement is not high enough (30). Furthermore, when the sensor is applied to the finger there is a delay in response in the instrument associated with heart rate (31). The measurement of accurate oxygen saturation is particularly important in evaluating the severity of the sleep apnea syndrome.
SCORING OF RESPIRATORY EVENTS
.'
~.
In the absence of standardized methods of measurement, the description of respiratory events remains an incompletely resolved question. Although there is general agreement on the definition of apneas (cessation of airflow for more than 10 seconds), there have been numerous definitions as to what constitutes an hypopnea. These definitions vary according to the transducers used, as they may provide either quantitative or qualitative analysis. There is also disagreement about the criteria for defining a reduction in ventilation. These vary from a 50% to a 213 reduction compared to the proceeding respirations. There is also question as to whether one associates hypopnea with a desaturation or not. Thus, certain authors consider only apnea index, whereas others take into account hypopneas to define a respiratory disturbance index (RDI), which is the number of apneas and hypopneas per hour of sleep. It has now been established, however, that the consequences ofhypopneas are comparable to those of apneas, and this justifies the use of the RDI in scoring respiratory events in OSAS. The risk offalse
positive recording of central apneas is not inconsiderable with the use of nasal thermistors and strain gauges (32).
MONITORING AND SCORING IN CLINICAL PRACTICE In clinical practice, nasal and bucal thermistors ranged in series and recording of respiratory efforts by magnetometry or plethysmography are the most commonly used techniques. In theory, however, hypopneas and central apneas cannot be definitively established using these techniques. If there are very few such events throughout the whole recording, then taking them into account may not change the overall result substantially. By contrast, if central apneas dominate the recording, then a further monitoring of sleep may be required, taking into account measurement of respiratory efforts by esophageal recording in order to confirm this initial impression. Likewise, the diagnosis by qualitative methods of the hypopnea-type syndrome in sleep ought to be verified by measurement of ventilation using a pneumotachograph or calibrated inductance plethysmography. Finally, as body position may influence the results of qualitative or semiquantitative measurement methods, this may need to be systematically monitored (33). Such monitoring is important, as Shephard and Thawley have shown that position may influence the site of collapse in the upper airway (34). In patients with neuromuscular pathology, in whom inspiratory effort may be minimal, the signals may be particularly difficult to appreciate (35). Therefore, in some patients, who are unable to move because of muscle disease, the absence of positional change makes calibrated inductance plethysmography an excellent tool for studying ventilation during sleep (36).
MONITORING AND SCORING IN RESEARCH PRACTICE Many studies in the literature examining survival or the variability of respiratory events from one night to another or studying epidemiological aspects of OSAS have not precisely described the techniques for examining respiration or have used thermistors or strain gauges (37-39). This has contributed to a difficulty in establishing the level of respiratory events required as a treatment cutoff point. Thus, only studies trying to establish physiopathological mechanisms in respiratory events have systematically used quantitative recordings (15,40). Sleep. Vol. 15. No.6. 1992
Downloaded from https://academic.oup.com/sleep/article-abstract/15/suppl_6/S5/2749348 by guest on 28 April 2019
of this mattress induces a static charge distribution that creates a potential difference which may then be recorded. Thus, the SCSB can be used to estimate the amplitude of respiratory movement. This technique is usually used for screening purposes but can give some indication of breathing efforts during apneas. In practical clinical studies mercury type gauges are frequently used (3). There have been no studies, however, evaluating the usefulness of these types of gauges in sleep apnea, although they would seem to be very sensitive to changes in body position in bed.
S8
P. LEVY ET AL.
CONCLUSION
REFERENCES I. Jasper HH. The ten twenty electrode system of the International Federation. Electroencephalogr Clin Neurophysiol 1985; 10: 371-5. 2. Rechtshaffen A, Kales A. A manual of standardized terminology, technique and scoring system for sleep stages of human sleep. Brain information service, Brain Information Institute, University of California, Los Angeles, 1968. 3. Medical section of the American Lung Association. Indications and standards for cardiopulmonary sleep studies. Am Rev Respir Dis 1989; 139:559-68. 4. Boysen PG, Block AJ, Wynne JW. Sleep monitoring; an increase in demand and a decrease in invasiveness. Chest 1980;77:349-· 50. 5. Sackner MA. Noninvasive cardiorespiratory monitoring during sleep. Eur Resp J 1990;11:85s-105s. 6. Guilleminault C, Van Den Hoed J, Mitler M. Clinical overview of the sleep apnea syndromes. In: Guilleminault C, Dement W, eds. Sleep apnea syndromes. New York: Alan R Liss, 1978:1-· 12. 7. Guilleminault C, Stoohs R, Duncan S. Snoring: daytime sleep· iness in regular heavy snorers. Chest 1991 ;99:40-8. 8. Askanasi J, Silverberg PA, Foster RJ, Hyman AI, Milic Emili J, Kinney JM. Effects of respiratory apparatus on breathing pattern. J Appl Physiol 1980;48:577-80. 9. Krieger J, Kurtz D. Effects ofpneumotachographic recording of breathing on sleep and respiration during sleep. Bull Europ Phy· siopath Resp 1983;19:641-4. 10. Konno K, Mead J. Measurement of the separate volume changes of ribcage and abdomen during breathing. J Appl Physiol1967: 22:407-22. 11. Chadha TS, Watson H, Birch S, Jenouri GA, Schneider AW. Cohn MA, Sackner MA. Validation of respiratory inductive plethysmography using different calibration procedures. Am Rev Respir Dis 1982;125:644-9. 12. Spier S, England S. The respiratory inductive plethysmography: bands versus jerkins. Am Rev Respir Dis 1983;127:784-5. 13. Rao S, Potdar A. Nasal airflow with body in various positions. J Appl PhysioI1970;28: 162-65. 14. Hasagawa M, Kern EB. The human nasal cycle. Mayo Clinic Proc 1977;52:28-34. 15. Gould GA, Whyte KF, Rhind GB, Airlie MAA, Catterall JR, Shapiro CM, Douglas NJ. The sleep hypopnea syndrome. Am Rev Respir Dis 1988;137:895-8. 16. Goldensohn RA, Zablow L. An electrical spirometer. J Appl Physio/ 1959; 14:463-4.
Sleep, Vol. 15, No.6, 1992
Downloaded from https://academic.oup.com/sleep/article-abstract/15/suppl_6/S5/2749348 by guest on 28 April 2019
This review of the different monitoring systems used in the evaluation of respiration during sleep shows the need to know the limits of the materials in use. The correct interpretation of polysomnography requires knowledge of the limitations of the methods used. Conclusions need to take into account the function of the material used for monitoring ventilation. In research or pathophysiological studies, we need to use quantitative measurements of ventilation and respiratory effort. Monitoring of respiration in SAS will require increasing standardization in the future. This is particularly important with the evolution of automatic analysis systems.
17. Gumery P-Y. Non invasive monitoring in respiratory insufficiency. Conception and characterisation of sensors. Ph. D. thesis. Joseph Fourier University, Grenoble, France, 1992. 160 pp. 18. Krumpe PE, Cummiskey JM. Use oflaryngeal sound recordings to monitor apnea. Am Rev Respir Dis 1980;122:797-801. 19. Cummiskey JM, Williams TC, Krumpe PE, Guilleminault e. The detection and quantification of sleep apnea by tracheal sound recordings. Am Rev Respir Dis 1982;126:221-4. 20. Peirick J, Shepard JW Jr. Automated apnea detection by computer: analysis of tracheal breath sounds. Med BioI Eng Comput 1983;21 :632-5. 21. Dalmasso F, Benedetto G, Pogolotti R, Righini G, Spagnolo R. Digital processing of snoring sounds. Eur Respir J 1990; II :445505. 22. Milic-Emili J, Mead J, Turner JM, Glauser EM. Improved technique for estimating pleural pressure from esophageal balloons. J Appl PhysioI1964;19:207-11. 23. Baydur A, Berhakis PK, Zin WA, Jaeger M, Milic-Emili J. A simple method for assessing the validity of the esophageal balloon technique. Am Rev Respir Dis 1982;126:788-91. 24. McNicholas WT, Coffey M, McDonnell T, O'Regan R, Fitzgerald MX. Upper airway obstruction in normal subjects after selective topical oropharyngeal anesthesia. Am Rev Respir Dis 1987; 135: 1316-9. 25. Sharp JT, Druz WS, Foster JR, Wicks MS, Chokroverty S. Use of the respiratory magnetometer in diagnosis and classification of sleep apnea. Chest 1980;77:350--3. 26. Tobin MJ, Cohn MA, Sackner MA. Breathing abnormalities during sleep. Arch Intern Med 1983;143:1221-8. 27. Staats BA, Bonekat HW, Harris CD, Offord KP. Chest waH motion in sleep apnea. Am Rev Respir Dis 1984;130:59-63. 28. Liistro G, Stanescu D, Dooms G, Veriter C, Aubert-Tulkens G, Rodenstein D. Hypopharyngeal and neck cross-sectional changes monitored by inductive plethysmography. J Appl Physiol1990; 68:2649-55. 29. Salmi T, Telakivi T, Partincn M. Evaluation of automatic analysis of SCSB, airflow and oxygen saturation signals in patients with sleep related apneas. Chest 1989;96:255-61. 30. Warley ARH, Mitchell JH, Stradling JR. Evaluation of the Ohmeda 3700 pulse oximeter. Thorax 1987;42:892-6. 31. West P, George CF, Kryger MH. Dynamic in vivo response characteristics of three oxymeters: Hewlet-Packard 47201 A, Biox III and Nellcor N-100. Sleep 1987;10:263-71. 32. Bliwise D, Bliwise NC, Kramer HC, Dement We. Measurement error in visually scored e1ectrophysiological data: respiration during sleep. J Neurosci Methods 1984;12:49-56. 33. Cardwright RD, Diaz F, Lloyd S. The effects of sleep posture and sleep stage on apnea frequency. Sleep 1991;14:351-3. 34. Shepard JW, Thawley SE. Localization of upper airway collapse during sleep in patients with obstructive sleep apnea. Am Rev Respir Dis 1990;141:1350--5. 35. Smith PEM, Calverley PMA, Edwards RHT. Hypoxemia during sleep in Duchenne Muscular Dystrophy. Am Rev Respir Dis 1988;137:884-8. 36. Smith PEM, Edwards RHT, Calverley PMA. Ventilation and breathing pattern during sleep in Duchenne Muscular Dystrophy. Chest 1989;96: 1346-51. 37. Berry DTR, Webb WB, Block AJ. Sleep apnea syndrome: a critical review of the apnea index as a diagnostic criterion. Chest 1984;86:529-31. 38. Wittig RM, Romaker A, Zorick FJ, Roehrs TA, Conway WA, Roth T. Night-to-night consistency of apneas during sleep. Am Rev Respir Dis 1984; 129:244-6. 39. Knight H, Millman RP, Gur RC, Say kin AJ, Doherty JU, Pack AI. Clinical significance of sleep apnea in the elderly. Am Rev Respir Dis 1987; 136:845-50. 40. Onal E, Lopata M, O'Connor T. Pathogenesis of apneas in hypersomnia-sleep apnea syndrome. Am Rev Respir Dis 1982; 125: 167-74.
,~
,
.~
Respiratory Monitoring in Sleep Apnea Syndrome *tP. Levy, *J. L. Pepin, tB. Wuyam and
*D. Veale
*Sleep and Respiration
This review provides a critical analysis of current respiratory monitoring tec~ni.qu~s in diagnosis of sleep apnea syndrome. The correct analysis ofpolysomnography require~ knowledg~ of the h~ll1tatIons of the m~ans of recording used. These limitations, for invasive and noninvasive techmques, are dls.cus~ed III terms.of c.a1culatlOn, differentiation and scoring of respiratory events. Aims and means are stated for momtonng and sconng III research as well as in clinical practice. Key Words: Respiratory monitoring-Sleep apnea syndrome-Polysomnography.
Summary:
~.
::a..
~.
,.
There are a number of different techniques for assessing airflow and respiratory effort in patients with suspected sleep related breathing problems, including the sleep apnea syndrome (SAS). Although these techniques are less complex in terms of recording and analysis than analysis of the sleep electroencephalogram (EEG), they have not been standardized like the EEG with the international 10120 electrode system, which governs the placement of electroencephalographic (EEG) electrodes (l), and the Rechtshaffen and Kales criteria for EEG scores and staging sleep (2). Several different techniques for measurement of respiratory airflow and respiratory effort are in current use. There are insufficient data available, however, to permit recommendation of any single method for standard use (3). A number of requirements must be fulfilled in order to achieve satisfactory ventilatory monitoring in SAS. Techniques must be reliable with regard to measurement, recording and analysis and must also be capable of reliable calibration (4). Naturally the sensors must be sufficiently comfortable when placed so as not to disturb the patient while asleep (5). The primary objectives of recording respiratory parameters during sleep are first to record the number of respiratory events and second to define their nature, i.e. whether events are obstructive, central or mixed. Finally, we wish to evaluate the relationship of respiratory events to oxygen saturation. In this review, we aim to describe the methods of
monitoring respiration in clinical practice and in research. CALCULATION OF RESPIRATORY EVENTS
An apnea is defined as a cessation of airflow for more than 10 seconds (6). The quantitative recording of airflow can be by direct measurement of tidal volume (Vt) or by integration of airflow measured by a pne~ motachograph. Airflow measurement can also be semIquantitative or qualitative. The most reliable method for quantitative measurement of airflow is by means of a pneumotachograph. The major limitations concern patient tolerance rather than validity of the data recorded. There can, however, be leakages through the face mask, leading to underestimation of ventilatory volume. This technique is of limited use in sleep studies because the face mask leads to sleep disturbance, particularly in moderately severe SAS patients with mild hypersomnia (7). In addition, the tight fitting face mask may have reflex effects on ventilatory patterns (8). Krieger and Kurtz, however, studied 20 young subjects using this technique and did not find significant changes in sleep pattern or in ventilatory characteristics (9). Modifications of ventilation have not been studied in apneic subjects. Calibrated inductance plethysmography records variations in thoracic and abdominal movements. This technique is based on the principle that the respitatory system has two degrees of freedom, i.e. chest and abAccepted for publication July 1992. dominal motion. Under specific conditions of meaAddress correspondence and reprint requests to P. A. Levy, Department of Respiratory Disease, CHRU de Grenoble, BP 217 X surement and calibration this technique can give a semiquantitative estimation of ventilation and tidal 38043 Grenoble Cedex 9, France. S5
Downloaded from https://academic.oup.com/sleep/article-abstract/15/suppl_6/S5/2749348 by guest on 28 April 2019
Unit, Department of Respiratory Disease, (Pr Brambilla, Pr Paramelle), and Sleep Laboratory, CHRU Grenoble, 38043 Grenoble, France tPreta Laboratory, Department of Physiology, J. Fourier University, Grenoble, France +.Freeman Hospital, Newcastle upon Tyne, England
S6
P. LEVY ET AL.
DIFFERENTIAnON OF RESPIRATORY EVENTS
Cessation of breathing during sleep can be of two major types: a) central, with complete cessation of diaphragmatic activity and airflow and b) obstructive, in which diaphragmatic activity continues but airflow Sleep, Vol. 15, No.6, 1992
ceases. A combination of these two types of apneas is called mixed apnea. The characterization of the type of apnea can be made directly by measurement of muscular activity of the diaphragm or the intercostals with electromyography. By contrast, indirect estimation can be performed by assessing intrathoracic pressure changes associated with respiratory efforts by using an esophageal balloon to measure pleural pressure changes. Another indirect method of analyzing respiratory effort is the detection of thoracic and abdominal movements by induction plethysmography or magnetometry. Measurement of pleural pressure changes by esophageal balloons is a method of reference (22,23). This method is limited by patient tolerance of the esophageal balloon. Furthermore, the presence of a balloon catheter in the upper airway may modify dynamic airway collapse. This was the case when local anesthetic was applied to the throat in normals (24). Diaphragmatic muscle activity can be recorded by means of esophageal electrodes, but the invasive character of this method has led to a preference for surface electrode recording during sleep. In this situation there is interference with the signal by activity in other muscles besides the diaphragm, namely abdominal muscles. Furthermore, changes in pulmonary volume modify electric activity, particularly when there are large pulmonary volume changes at the time of apneas. Purely qualitative plethysmography can be used to record whether there is movement of the thorax or abdomen. Magnetometry can be used similarly (25). During changes in position, however, these two methods can become unreliable. Induction plethysmography is the more reliable of these types of qualitative recording. The obstructive nature of respiratory events can be confirmed in the majority of cases, but recordings can be inadequate in some very obese patients or in those in whom paradoxical movement ofthorax and abdomen is absent or in whom respiratory efforts are weak. This is the case in 15% of patients with sleep apnea syndrome (26). If absence of chest wall motion suggests that all apneas are central, then direct measurement of respiratory effort is necessary in order to exclude obstructive events (27). An alternative to the esophageal balloon technique is measurement of movement in the suprasternal notch by induction plethysmography at the surface (Psip). This gives an estimate of variations in pleural pressure with respiration (5,28). These data can be obtained by means of two small plethysmography bands around the neck, but these are often not well tolerated by the patient. The static charge sensitive bed (SCSB) (29) consists of a thin mattress placed under the bedclothes and can be used on an ordinary bed. Movement on the surface
Downloaded from https://academic.oup.com/sleep/article-abstract/15/suppl_6/S5/2749348 by guest on 28 April 2019
volume (10). Precision of measurement requires, however, that the coils not move, which is difficult to achieve during sleep, particularly in apneic subjects. Approaches to this problem include the use of a body jacket to which the coils are sewn or glued (12). Analysis requires multiple regression mathematics or use of the least squares method (10,11). The reference method for calibration of inductance plethysmography is the isovolume maneuver, but this is difficult to perform correctlly in a large number of patients. Sackner (5) has proposed an automatic calibration procedure, which utilizes data required during normal respiration. This procedure must be performed over a number of breathing cycles with minimal variability between cycles. Thermistors and thermocouples at the nose and mouth can achieve semiquantitative or qualitative measurement of airflow, but there may be variations in airflow between one nostril and another or between the nose and mouth (13,14). The amplitude of the recording signal is very dependent on the positioning of the recording electrodes in the airflow current. This is best achieved by mounting three thermocouples on nasal spectacles. The distinction of hypopnea from normal respiration is very difficult in the absence of reliable quantitative data on airflow (15). Furthermore, with thermistors one can have false positive recording of respiration with variations in temperature at the mouth without airflow. Impedance measurement of respiration relies on the principle that a linear relationship exists between thoracic electrical impedance and thoracic volume (16). This method of measurement is limited by interference from cardiac oscillations and body movements (5). If one uses four electrodes rather than two, however, one can record apneic events. These limitations to the use of impedance measurement can be overcome by using numeric filtering to smooth the curves, and peripheral movements can be detected by use of an actigraph (17). Tracheal or laryngeal sounds have been used to (:stimate airflow (18-20). More involved spectral analysis allows characterization of snoring and information at the site of airflow obstruction (21). Finally, measurement of expired CO 2 by a rapid analyzer has been found useful for semiquantitative estimation of airflow.
..
S7
RESPIRATORY MONITORING IN SAS
.'
EVALUATION OF THE CONSEQUENCES OF RESPIRATORY EVENTS Noninvasive measurement of oxygen saturation (Sa0 2) is now performed by pulse oximeters, which give a good correlation with intra-arterial measurements down to saturations around 65-75%. Nevertheless, there are differences among instruments with regard to dynamic characteristics. There are important errors associated with repeated oscillations in oxygen saturation in the sleep apnea syndrome if the sampling rate for Sa0 2 measurement is not high enough (30). Furthermore, when the sensor is applied to the finger there is a delay in response in the instrument associated with heart rate (31). The measurement of accurate oxygen saturation is particularly important in evaluating the severity of the sleep apnea syndrome.
SCORING OF RESPIRATORY EVENTS
.'
~.
In the absence of standardized methods of measurement, the description of respiratory events remains an incompletely resolved question. Although there is general agreement on the definition of apneas (cessation of airflow for more than 10 seconds), there have been numerous definitions as to what constitutes an hypopnea. These definitions vary according to the transducers used, as they may provide either quantitative or qualitative analysis. There is also disagreement about the criteria for defining a reduction in ventilation. These vary from a 50% to a 213 reduction compared to the proceeding respirations. There is also question as to whether one associates hypopnea with a desaturation or not. Thus, certain authors consider only apnea index, whereas others take into account hypopneas to define a respiratory disturbance index (RDI), which is the number of apneas and hypopneas per hour of sleep. It has now been established, however, that the consequences ofhypopneas are comparable to those of apneas, and this justifies the use of the RDI in scoring respiratory events in OSAS. The risk offalse
positive recording of central apneas is not inconsiderable with the use of nasal thermistors and strain gauges (32).
MONITORING AND SCORING IN CLINICAL PRACTICE In clinical practice, nasal and bucal thermistors ranged in series and recording of respiratory efforts by magnetometry or plethysmography are the most commonly used techniques. In theory, however, hypopneas and central apneas cannot be definitively established using these techniques. If there are very few such events throughout the whole recording, then taking them into account may not change the overall result substantially. By contrast, if central apneas dominate the recording, then a further monitoring of sleep may be required, taking into account measurement of respiratory efforts by esophageal recording in order to confirm this initial impression. Likewise, the diagnosis by qualitative methods of the hypopnea-type syndrome in sleep ought to be verified by measurement of ventilation using a pneumotachograph or calibrated inductance plethysmography. Finally, as body position may influence the results of qualitative or semiquantitative measurement methods, this may need to be systematically monitored (33). Such monitoring is important, as Shephard and Thawley have shown that position may influence the site of collapse in the upper airway (34). In patients with neuromuscular pathology, in whom inspiratory effort may be minimal, the signals may be particularly difficult to appreciate (35). Therefore, in some patients, who are unable to move because of muscle disease, the absence of positional change makes calibrated inductance plethysmography an excellent tool for studying ventilation during sleep (36).
MONITORING AND SCORING IN RESEARCH PRACTICE Many studies in the literature examining survival or the variability of respiratory events from one night to another or studying epidemiological aspects of OSAS have not precisely described the techniques for examining respiration or have used thermistors or strain gauges (37-39). This has contributed to a difficulty in establishing the level of respiratory events required as a treatment cutoff point. Thus, only studies trying to establish physiopathological mechanisms in respiratory events have systematically used quantitative recordings (15,40). Sleep. Vol. 15. No.6. 1992
Downloaded from https://academic.oup.com/sleep/article-abstract/15/suppl_6/S5/2749348 by guest on 28 April 2019
of this mattress induces a static charge distribution that creates a potential difference which may then be recorded. Thus, the SCSB can be used to estimate the amplitude of respiratory movement. This technique is usually used for screening purposes but can give some indication of breathing efforts during apneas. In practical clinical studies mercury type gauges are frequently used (3). There have been no studies, however, evaluating the usefulness of these types of gauges in sleep apnea, although they would seem to be very sensitive to changes in body position in bed.
S8
P. LEVY ET AL.
CONCLUSION
REFERENCES I. Jasper HH. The ten twenty electrode system of the International Federation. Electroencephalogr Clin Neurophysiol 1985; 10: 371-5. 2. Rechtshaffen A, Kales A. A manual of standardized terminology, technique and scoring system for sleep stages of human sleep. Brain information service, Brain Information Institute, University of California, Los Angeles, 1968. 3. Medical section of the American Lung Association. Indications and standards for cardiopulmonary sleep studies. Am Rev Respir Dis 1989; 139:559-68. 4. Boysen PG, Block AJ, Wynne JW. Sleep monitoring; an increase in demand and a decrease in invasiveness. Chest 1980;77:349-· 50. 5. Sackner MA. Noninvasive cardiorespiratory monitoring during sleep. Eur Resp J 1990;11:85s-105s. 6. Guilleminault C, Van Den Hoed J, Mitler M. Clinical overview of the sleep apnea syndromes. In: Guilleminault C, Dement W, eds. Sleep apnea syndromes. New York: Alan R Liss, 1978:1-· 12. 7. Guilleminault C, Stoohs R, Duncan S. Snoring: daytime sleep· iness in regular heavy snorers. Chest 1991 ;99:40-8. 8. Askanasi J, Silverberg PA, Foster RJ, Hyman AI, Milic Emili J, Kinney JM. Effects of respiratory apparatus on breathing pattern. J Appl Physiol 1980;48:577-80. 9. Krieger J, Kurtz D. Effects ofpneumotachographic recording of breathing on sleep and respiration during sleep. Bull Europ Phy· siopath Resp 1983;19:641-4. 10. Konno K, Mead J. Measurement of the separate volume changes of ribcage and abdomen during breathing. J Appl Physiol1967: 22:407-22. 11. Chadha TS, Watson H, Birch S, Jenouri GA, Schneider AW. Cohn MA, Sackner MA. Validation of respiratory inductive plethysmography using different calibration procedures. Am Rev Respir Dis 1982;125:644-9. 12. Spier S, England S. The respiratory inductive plethysmography: bands versus jerkins. Am Rev Respir Dis 1983;127:784-5. 13. Rao S, Potdar A. Nasal airflow with body in various positions. J Appl PhysioI1970;28: 162-65. 14. Hasagawa M, Kern EB. The human nasal cycle. Mayo Clinic Proc 1977;52:28-34. 15. Gould GA, Whyte KF, Rhind GB, Airlie MAA, Catterall JR, Shapiro CM, Douglas NJ. The sleep hypopnea syndrome. Am Rev Respir Dis 1988;137:895-8. 16. Goldensohn RA, Zablow L. An electrical spirometer. J Appl Physio/ 1959; 14:463-4.
Sleep, Vol. 15, No.6, 1992
Downloaded from https://academic.oup.com/sleep/article-abstract/15/suppl_6/S5/2749348 by guest on 28 April 2019
This review of the different monitoring systems used in the evaluation of respiration during sleep shows the need to know the limits of the materials in use. The correct interpretation of polysomnography requires knowledge of the limitations of the methods used. Conclusions need to take into account the function of the material used for monitoring ventilation. In research or pathophysiological studies, we need to use quantitative measurements of ventilation and respiratory effort. Monitoring of respiration in SAS will require increasing standardization in the future. This is particularly important with the evolution of automatic analysis systems.
17. Gumery P-Y. Non invasive monitoring in respiratory insufficiency. Conception and characterisation of sensors. Ph. D. thesis. Joseph Fourier University, Grenoble, France, 1992. 160 pp. 18. Krumpe PE, Cummiskey JM. Use oflaryngeal sound recordings to monitor apnea. Am Rev Respir Dis 1980;122:797-801. 19. Cummiskey JM, Williams TC, Krumpe PE, Guilleminault e. The detection and quantification of sleep apnea by tracheal sound recordings. Am Rev Respir Dis 1982;126:221-4. 20. Peirick J, Shepard JW Jr. Automated apnea detection by computer: analysis of tracheal breath sounds. Med BioI Eng Comput 1983;21 :632-5. 21. Dalmasso F, Benedetto G, Pogolotti R, Righini G, Spagnolo R. Digital processing of snoring sounds. Eur Respir J 1990; II :445505. 22. Milic-Emili J, Mead J, Turner JM, Glauser EM. Improved technique for estimating pleural pressure from esophageal balloons. J Appl PhysioI1964;19:207-11. 23. Baydur A, Berhakis PK, Zin WA, Jaeger M, Milic-Emili J. A simple method for assessing the validity of the esophageal balloon technique. Am Rev Respir Dis 1982;126:788-91. 24. McNicholas WT, Coffey M, McDonnell T, O'Regan R, Fitzgerald MX. Upper airway obstruction in normal subjects after selective topical oropharyngeal anesthesia. Am Rev Respir Dis 1987; 135: 1316-9. 25. Sharp JT, Druz WS, Foster JR, Wicks MS, Chokroverty S. Use of the respiratory magnetometer in diagnosis and classification of sleep apnea. Chest 1980;77:350--3. 26. Tobin MJ, Cohn MA, Sackner MA. Breathing abnormalities during sleep. Arch Intern Med 1983;143:1221-8. 27. Staats BA, Bonekat HW, Harris CD, Offord KP. Chest waH motion in sleep apnea. Am Rev Respir Dis 1984;130:59-63. 28. Liistro G, Stanescu D, Dooms G, Veriter C, Aubert-Tulkens G, Rodenstein D. Hypopharyngeal and neck cross-sectional changes monitored by inductive plethysmography. J Appl Physiol1990; 68:2649-55. 29. Salmi T, Telakivi T, Partincn M. Evaluation of automatic analysis of SCSB, airflow and oxygen saturation signals in patients with sleep related apneas. Chest 1989;96:255-61. 30. Warley ARH, Mitchell JH, Stradling JR. Evaluation of the Ohmeda 3700 pulse oximeter. Thorax 1987;42:892-6. 31. West P, George CF, Kryger MH. Dynamic in vivo response characteristics of three oxymeters: Hewlet-Packard 47201 A, Biox III and Nellcor N-100. Sleep 1987;10:263-71. 32. Bliwise D, Bliwise NC, Kramer HC, Dement We. Measurement error in visually scored e1ectrophysiological data: respiration during sleep. J Neurosci Methods 1984;12:49-56. 33. Cardwright RD, Diaz F, Lloyd S. The effects of sleep posture and sleep stage on apnea frequency. Sleep 1991;14:351-3. 34. Shepard JW, Thawley SE. Localization of upper airway collapse during sleep in patients with obstructive sleep apnea. Am Rev Respir Dis 1990;141:1350--5. 35. Smith PEM, Calverley PMA, Edwards RHT. Hypoxemia during sleep in Duchenne Muscular Dystrophy. Am Rev Respir Dis 1988;137:884-8. 36. Smith PEM, Edwards RHT, Calverley PMA. Ventilation and breathing pattern during sleep in Duchenne Muscular Dystrophy. Chest 1989;96: 1346-51. 37. Berry DTR, Webb WB, Block AJ. Sleep apnea syndrome: a critical review of the apnea index as a diagnostic criterion. Chest 1984;86:529-31. 38. Wittig RM, Romaker A, Zorick FJ, Roehrs TA, Conway WA, Roth T. Night-to-night consistency of apneas during sleep. Am Rev Respir Dis 1984; 129:244-6. 39. Knight H, Millman RP, Gur RC, Say kin AJ, Doherty JU, Pack AI. Clinical significance of sleep apnea in the elderly. Am Rev Respir Dis 1987; 136:845-50. 40. Onal E, Lopata M, O'Connor T. Pathogenesis of apneas in hypersomnia-sleep apnea syndrome. Am Rev Respir Dis 1982; 125: 167-74.
,~
,
.~
