tSleep Disorders Center of Southwest Florida, Naples, Florida, U.S.A.. Summary: Nasal continuous positive airway pressure (CPAP) "splints" the airway and ...
Sleep. 14(3):259-262
© 1991 Association of Professional Sleep Societies
Instrumentation, Technology, Equipment and Computer Systems Validation
A Comparison of Nine Nasal Continuous Positive Airway Pressure Machines in Maintaining Mask Pressure During Simulated Inspiration *M. C. Demirozu, *A. D. Chediak, *K. N. Nay, and tM. A. Cohn *Sleep Disorders Center at Mount Sinai Medical Center, Miami Beach, Florida; and tSleep Disorders Center of Southwest Florida, Naples, Florida, U.S.A.
Summary: Nasal continuous positive airway pressure (CPAP) "splints" the airway and prevents inspiratory collapse of the upper airway in patients with obstructive sleep apnea. Nine nasal CPAP machines were compared for their ability to maintain airway pressure at various simulated inspiratory flows. Each machine was connected to a vacuum system at 20, 40, and 60 Umin flow after it was initially set at test pressures of 5, 10, or 15 cm H 20 and the system or "mask" pressures were measured. In all machines, mask pressure fell during simulated inspiration and the declines in mask pressure were as high as 5 cm H 2 0. Because machines varied in their ability to maintain a test pressure, it is recommended that the nasal CPAP machine used in the home be the same as that which was tested in the sleep laboratory. If a different machine is used, it may require adjustment to assure efficacy. Key Words: Sleep apnea treatment-Nasal continuous positive airway pressure-Sleep disordered breathing.
Using its standard circuit, each machine was connected to a vacuum system to simulate inspiration (Fig. 1). For each test pressure the pressure in the system (termed mask pressure) was measured at 0, 20, 40, and 60 Llmin inspiratory flow produced with the vacuum system. The inspiratory flow was adjusted using a variable resistance valve and the flow rate measured with a pneumotachograph (Fleisch #1, Switzerland), which was connected to a differential transducer (Validyne DP 45, ±3 cm H 20, Northridge, CA). Pressure was measured just adjacent to the end of the nasal CPAP hose using a differential transducer (Validyne MP45, ±50 cm H 20, Northridge, CA). The signals from these gauges were processed through demodulators (Validyne CDI2, Northridge, CA) and recorded on a multichannel physiologic recorder (Grass 78D, Quincy, MA). Pressure was measured immediately after and at 1 min of simulated inspiration (bias flow) to assure stability of system pressure. Instantaneous declines in METHODS mask pressures were similar to those seen after allowWith the outlet port of each machine occluded (zero ing the system pressure to stabilize for 1 min, and the flow), the machine was initially adjusted by its CPAP latter were used for data analysis. Mask pressure was valve to deliver a test pressure of 5, 10, or 15 cm H 20. plotted against flow and a least squares fit line was drawn for comparison among machines. Statistical analyses of the effects of different end expiratory presAccepted for publication November 1990. sures (EEP) and increasing flow on declines of system Address correspondence and reprint requests to M. Cuneyt Demipressure were performed using a two-way analysis of rozu, M.D., Sleep Disorders Center, Mount Sinai Medical Center, 4300 Alton Road, Miami Beach, FL 33140, U.S.A. variance with multiple comparisons. When appropriUpper airway closure in the obstructive sleep apnea syndrome is thought to result from the negative pressure generated in the pharyngeal area during inspiration (1). Nasal continuous positive airway pressure (CPAP) is thought to prevent this collapse by acting as a pneumatic splint of the upper airway (1,2). Its success partly depends on the ability of the nasal CPAP system to deliver a sufficient flow of air to meet the inspiratory flow demand of the patient and thus maintain a predetermined amount of positive pressure in the mask and airway during the inspiratory portion of the respiratory cycle (3). The purpose of this study is to evaluate nine nasal CPAP systems (Table 1) in their ability to maintain positive pressure during simulated inspiration. In addition, we describe the relationship between increasing inspiratory flow rates and nasal CPAP mask pressure at various levels ofCPAP.
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TABLE 1. Nasal CPAP machines tested 1. 2. 3. 4. 5.
6. 7.
8. 9.
Sleep Easy I, Respironics, Monroeville, Pennsylvania Sleep Easy II, Respironics, Monroeville, Pennsylvania Companion 515, Puritan Bennett, Lenexa, Kansas Vital Flow III, System 2000, Inc., Kennett Square, Pennsylvania Aries, Mountain Medical Equipment, Inc., Littleton, Colorado C.M.T. Breathe Free, Medical Industries America, Inc., Des Moines, Iowa Night Bird, Bird Products, Palm Springs, California Sleep Easy III, Respironics, Monroeville, Pennsylvania Healthdyne CPAP 7001, Marietta, Georgia
ate, pairwise comparisons were made employing the Neuman-Keuls test. A p value of less than 0.05 was considered significant. RESULTS
Simulated inspiration produced a decline in mask pressure below the set test pressure in all machines and at all test pressures. The average fall in pressure from the baseline setting was approximately 1-2 cm H 20 pressure (Table 2). In some machines the drop in pressure from baseline was as great as 5 cm H 20. The individual values of measured mask pressure at a simulated inspiratory flow of 20, 40, and 60 Llmin at different test pressures may be found in Table 3. In general, the higher the inspiratory flow, the lower the effective mask pressure (p < 0.05) (Table 2). Howeve:r, the machines differed in their ability to maintain mask pressure as inspiratory flow was increased at different test pressures. Further, the degree of inspiratory declines in mask pressure does not appear to be related to the test pressure (p > 0.05) (Fig. 2). DISCUSSION
In these experiments inspiration was simulated using a continuous bias flow model. Some nasal CPAP machines (i.e., machine 9) incorporate a variable speed motor, and if given sufficient time the motor accelerates and may minimize declines in mask pressure produced by a continuous bias flow. Nonetheless, in all machines mask pressure declined during simulated inspiration and to a degree which may be clinically relevant (Table 2).
"P MASK"
CPAP MACHINE
VACUUM MOTOR
VALVE
FIG. I. For each nasal CPAP machine mask pressure (Pmask) was measured at zero flow and at 20, 40, and 60 Llmin inspiratory flow (Y). Flow was varied by using a variable resistance valve.
Nasal CPAP mask pressure fell to a greater degree as simulated inspiratory flow was increased from 20 to 40 to 60 Llmin. This observation suggests that flow limits the ability of nasal CPAP machines to maintain positive airway pressure. Machine 5 seemed to be most susceptible to this phenomenon. However, with the exception of machines 4 and 9, all of the machines demonstrated inspiratory decline in mask pressure of 2 cm or more at higher inspiratory flows (Table 3). Inspiratory flows of 20-30 Llmin are commonly described during normal sleep (4-6). Therefore, the higher flows tested may be of limited clinical relevance to an individual patient. Nonetheless, in selected patients such flow rates occur and may lead to greater nasal CPAP requirements or failure of nasal CPAP. There are several companies that manufacture nasal CPAP machines. When adjusted in the sleep laboratory all appear to work sufficiently to maintain airway patency and abolish sleep-related obstructive respiratory events. We determined that the fall in nasal CPAP mask pressure during inspiration differs among the machines and may be of sufficient magnitude to be clinically important. For example, an attempt to switch a patient tested with one machine to another without repeat testing might cause an undesirable increase or decrease in inspiratory mask pressure. If the patient is switched to a machine that results in a greater drop in mask pressure at a given inspiratory flow, the upper airway may not remain open and the device may be rendered ineffective. If switched to a machine that maintains a higher inspiratory mask pressure, the patient may receive a greater pressure, which might be uncomfortable and thereby worsen patient compliance.
TABLE 2. Mean and range of declines in mask pressure of all nasal CPAP machines at varying inspiratory flows and test pressures Test pressure"
20 Llmin b
40 Llminb
60 Llmin b
5
-0.8 ± 0.7 (-2.0 to 0) -0.4 ± 0.3 (-1.0 to 0) -0.6 ± 0.5 (-1.4 to 0)
-1.3 ± 0.8 (-3.0 to -0.4) -0.9 ± 0.9 (- 3.0 to 0) -1.2 ± 0.7 (-2.0 to -0.4)
- 2.0 ± 1.0 (-4.0 to -0.8) -1.6 ± 1.5 (-5.0 to 0) -1.9 ± 0.7 (-3.0 to -0.6)
10
15 " Test pressure in cm H 20. All data expressed as mean ± SD (range). b p < 0.05 compared to the other flows. Sleep, Vol. 14, No.3, 1991
COMPARISON OF NCPAP MACHINES
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7
o
8 0
~
0
f'\
IS
PPHARYNX
n
I ) rr+ 5 V ~'C 0 r\
I \
10
~ "
Ot1n
S
G
o
l/MIN
-50
INSPIRATION
2 0
20
80
FIG. 3. An example of mask pressure (Pmask), pharyngeal pressure (Ppharynx), and flow (V) waveforms during respiration in a subject breathing through a nasal CPAP system.
2A
11
10
~
0
8
0
8
... ~ 0
7 8 • IS
IS
"
0
20
40
80
18
2B
15 0
Normal breathing is accompanied by oscillations of airway pressure. A certain amount of pressure is required to overcome the resistance of the nose and suprapharyngeal airway; thus, respiratory swings of pharyngeal pressure are usually of greater magnitude than those seen in the nasal CPAP mask. In some instances, the pharyngeal pressure during inspiration may be less than atmospheric pressure despite nasal CPAP (Fig. 3). There is no standard method for choosing the mask pressure (end-expiratory, maximum expiratory, or minimum inspiratory) used to describe and prescribe nasal CPAP. If the laboratory prescribes a pressure
~
S
1"
CI
...
10
~
15
FIG. 2. For each nasal CPAP machine (machine number is the same as in Table I) each line represents a least square fit of the actual pressure measurements at each flow (0, 20, 40, 60 Umin). Figures 2A, 2B, and 2C are data at test pressure of 5, 10, and 15 cm H 20, respectively. Machine 5 minimum test pressure was 7 cm H 20. Machines 4, 6, and 7 minimum test pressure was 6 cm H 20. Other differences in the pressure at zero flow are produced by the least squares fit method of linear regression.
12 11 0
20
80
2C
FLOW (Uten/llln)
TABLE 3. Mask pressure of nasal CPAP machines at various flows and test pressures Test pressure (cm H 2O) 5
15
10
40a
60 a
20
40
60
20
40
60
4.4 4 2 4.8 4.4 3 5 4 b 4 4.4 4.4 4 5c 5 6b 5.4 5 7b 4.4 3.6 8 4.6 4 9 4.8 4.6 a Inspiratory flow (Umin). b Minimum test pressure 6 cm H 2 O. c Minimum test pressure 7 cm H 2O.
3.6 3.2 3 4.4 3 4 2.8 3.2 4.2
9.6 9.6 9.6 10 9 9.5 9.6 9.4 10
9.6 9 9.4 10 7 9.2 9.2 8.6 9.8
8.8 8 8.8 10 5 8.4 9 7.6 9.6
13.6 14 15 14 14 14 15 14.4 14.4
I3 13.6 14.6 14.4 I3 13 14 13.6 14.6
12.4 I3 13.8 13.6 12.6 12 13 12.8 14.4
Machine no. I
20 a
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M. C. DElvfIROZU ET AL.
without knowing in which part ofthe respiratory cycle it was measured, the patient may receive the incorrect pressure. Usually, nasal CPAP is adjusted while the patient sleeps in the laboratory. The pressure is gradually raised in 1-2.5 cm H 20 increments up to the minimum pre:ssure required to abolish snoring and obstructive events. Considering the differences detected among nasal CPAP machines and the fact that even small differences in pressure (1 cm H 20) may be clinically relevant, it would be desirable that the machine tested in the laboratory be the same as that prescribed for home use. This would likely reduce the error resulting from switching nasal CPAP machines; In summary, as inspiratory flow increases nasal CPAP mask pressure decreases, and different nasal CPAP machines vary in their ability to maintain positive airway pressure during inspriation. Because airway pressure declines are not uniform for the various nasal CPAP machines, the pressure (at end expiration) required to prevent airway closure during sleep may vary depending on the device used. Therefore, to assure efficacy either the nasal CPAP machine used at home should
Sleep, Vol. 14, No.3, 1991
be the same model as tested in the laboratory or if a different machine is used at home its pressure may need to be adjusted. In the latter circumstance, failure to properly correct the nasal CPAP may lead to therapeutic failure.
REFERENCES 1. Remmers JE, De Groot WJ, Sauerland EK, Anch AM. Pathogenesis of upper airway occlusion during sleep. J App/ Physiol 1978;44:931-8. 2. Popper RA, Leidlinger MJ, Williams AJ. Endoscopic observations of the pharyngeal airway during treatment of obstructive sleep apnea with nasal continuous positive airway pressure-'a pneumatic splint. West J Med 1986;144:83-5. 3. Sullivan CD, Issa FG, Berthon-Jones M, Eves L. Reversal of obstructive sleep apnea by continuous positive airway pressure applied through the nares. Lancet 1981;1:862-5. . 4. Rudgel DW, Maitin RJ, Johnson B, et al. Mechanics of the respiratory system arid breathing pattern during sleep in normal humans. J App/ PhYsioI1984;56:133-7. 5. Stradling JR, Chadwick GT, Frew AJ. Changes in ventilation and its components in normal subjects during sleep. Thorax 1985;40: 364-70. 6. Gothe B, Altose MD, Goldman MD, et al. Effect of quiet sleep on resting and CO 2 stimulated breathing in humans. J Appl Physio/1981;50:724-30.
