Inspiratory Work and Airway Pressure with Continuous Positive Airway Pressure Delivery Systems

Inspiratory Work and Airway Pressure with Continuous Positive Airway Pressure Delivery Systems·

Jeffrey A Katz, M.D.; Roger w Kraemer, C.R.T.T.; and G. Eric Gjerde, M.B.A, R.R.T.

To determine work-of breathing with continuous positive airway pressure (CPAP) delivery systems, we used a lung model to simulate spontaneous breathing. "Additional n work during tidal breathing was derived by comparing change in airway pressure with change in tidal volume. Seven demand-Bow CPAP delivery systems were compared with one continuous-Bow, 5-L reservoir-bag system (Bow of

Cmtinuous positive airway pressure (CPAP) systems are optimal when they do not increase work of breathing. Such systems fall into two basic categories: continuous How and demand flow In continuous-flow systems, a high flow of gas passes from an air-oxygen blender through a reservoir bag, humidifier, and Y-piece to the patient, who then exhales through a threshold resistor. During inspiration with demandflow systems, a regulating device on the inspiratory limb (often part of a mechanical ventilator) controls air How. In an ideal CPAP system, the delivered inspiratory flow should be sufficient-relative to the patient's inspiratory How-to keep airway pressure close to the end-expiratory level. Recently, Gibney et all showed that the work of breathing may increase 75 percent in healthy volun*From the Departments of Anesthesia, University of California, and San Francisco General Hospital, San Francisco. Presented in part at the Annual Meeting, Society of Critical Care Medicine, San Francisco, May 1984. Manuscript received December 10; revision accepted March 28. Reprint requests: Dr. Katz, Department of Anesthesia, 1001 Potrero

Avenue, San Francisco 94110

60 Umin to maintain positive airway pressure), It was

concluded that demand-Bow CPAP delivery systems vary widely in the amount of additional work required of a patient. When a lung model is used, some demand-Bow systems perform as well as, or better than, a continuousBow reservoir-bag system.

teers breathing through demand-How systems (Puritan-Bennett MA-2 and Bear ventilators) rather than continuous-How systems. Indeed, some clinicians no longer use demand-How CPAP systems in patients recovering from acute respiratory failure because of the possible additional work of breathing. Our clinical impression is that not all demand-How systems function as poorly as the two studied by Gibney et al. I Using a lung model to simulate spontaneous breathing, we compared the performance of seven demand-How CPAP systems with that of a standard continuous-How system. METHODS AND MATERIALS

A lung model was developed using a Michigan Instruments "training test lung" (li'L) to simulate spontaneous breathing (Fig 1). The TIL has two compartments, each of which has adjustable compliance and resistance. The two compartments were connected externally by a mechanical link so that when one compartment (which simulated respiratory muscles) was mechanically ventilated with an Emerson ventilator, the other compartment (which simulated the lung) was passively displaced. The simulated lung was

LUNG MODEL

volume pressure

Inspiratory 1mb

X-y Recorder

of CPAP device

Expiratory limb

FIGURE 1. Illustration of a lung model using a "training test lung" (TTL) to simulate spontaneous breathing, and of the measurements that were made. CHEST I 88 I 4 I OClOBER, 1985

518

allowed to exhale independently of the simulated respiratory muscles. As CPAP changed the resting functional residual capacity (FRC) of the simulated lung, the FRC of the other compartment (ie, the simulated respiratory muscles) was increased similarly by external application of counterbalancing weights. The Emerson ventilator allowed standardization of spontaneous breaths. All simulated breaths were of a sine-wave flow pattern and produced a tidal volume of 500 ml at 20, 40, and 60 Umin peak inspiratory flow. For the simulated lung, compliance was set at 50 mV em H.O and airway resistance at 5.45 em H.O/L s at a flow of gas of 1 Us (which simulates the airway resistance associated with a 7-mm endotracheal tube). Each CPAP system was studied at end-expiratory pressures (EEP) of 0, 10, and 20 em H 20 and was adjusted fOr maximum sensitivity. All settings were verified repeatedly. We evaluated the following CPAP systems: (1)a 5-L reservoir-bag system with a continuous flow of 60 Umin; (2)an Emerson demandvalve system; (3) a Siemens Servo 900C ventilator; (4) a PuritanBennett 7200 ventilator; (5) a Siemens Servo 900B ventilator; (6) a Bear 2 ventilator; (7) an Engstrom Erica ventilator; and (8) a Medishield CPU 1 ventilator. Each system had attached to it a standard patient circuit (with humidifier); when applicable, pressure and flow transducers were calibrated to manufacturer's specifications. The reservoir-bag system was tested without a unidirectional valve, at a continuous flow of60 Umin. The Emerson demand-valve system was tested at no continuous flow and at purposefully overpressurized levels to create a continuous flow of 30 Umin and 60 Umin at each level ofEEE We found that changes in airway pressure and additional work were 45 percent greater during no continuous flow when compared with the data during continuous flow of either 30 or 60 Umin. Increasing the flow from 30 Umin to 60 Umin did not alter changes in airway pressure and work; thus, the data for a continuous flow of 30 Umin are reported. An Emerson water-column positive end-expiratory pressure (PEEP) valve was used to generate EEP in the reservoir-bag, Emerson demand-valve, and Siemens Servo 900B systems. The remaining systems incorporate their own resistor.

Ffd ADDITIONAL WORK

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REDUCED WORK

- - -- EXPIRATION

,i

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/

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EMERSON

!

DEMAND VALVE

1

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SIEMENS SERVO

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Airway Pressure (cmH 2 0 )

Pressure-volume curves for each CPAP delivery system at 10 em H.O end-expiratory pressure at an inspiratory flow of 40 FIGURE 3.

500

Umin.

Tidal Volume

(mil

o

,~

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Statham PM 131 TC pressure transducer (calibrated with a water - - INSPIRATION

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Airway pressure was measured at the endotracheal tube with a

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Airway Pressure

(cmH2 0 ) FIGURE 2. The pressure-volume curves generated during continuous positive airway pressure are of two types. In one type (A), airway pressure is consistently less than the pressure at end-expiration. Therefore, the work of breathing with the CPAP device increases. In the second type (8), after an initial decrease, airway pressure increases to above the pressure at end-expiration as inspiratory flow exceeds demand. This increase in airway pressure to above the pressure at end-expiration means that work is perfonned by the CPAP device on the simulated lung and is called "reduced work."

column). A Fleisch No 2 pneumotachograph (calibrated with a Fischer Porter flowmeter) was used to measure flow and volume at the same point. udal volume was obtained by integration of the flow signal and wascalibrated with a 500-ml syringe. Recordings of airway pressure, flow, and tidal volume were made on a Gould strip chart recorder. A Hewlett-Packard X-Y recorder was used to generate pressure-volume curves. The pressure-volume curves generated on the X-Y recorder were of two basic types. In one type (Fig 2, A), airway pressure during inspiration was consistently below the end-expiratory level. In the second type (Fig 2, B), airway pressure during inspiration exceeded the level at end-expiration. Inspiratory work was represented by the area of the pressurevolume curve recorded during inspiration. Any area recorded at pressures less than the EEP represented "additional work" necessary to inspire from the CPAP system (Fig 2). Any area recorded at pressures above EEP represented work done by the device on the simulated lung and was reported as "reduced work" (Fig 2). Inspiratory work was reported in kg-mlL and as a percentage change from a predetermined "normal" value (0.05 kg-mlL) for workof-breathing. I Mean inspiratory airway pressure relative to EE~ is Inspiratory \\Cork and AJrwtIfI Pressure with CPAP(KlItz, Kraemer, Gjerde)

RESERVOIR BAG

EMERSON DEMAND VALVE

SIEMENS SERVO 900 C

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" MEAN AIRWAY PRESSURE ~ MINIMUM AlfMAY PRESSURE IZJ

AT 20,40, AND 60 L/MIN PEAK INSPIRATORY FLOW FIGURE 4. Mean airway pressure and minimum airway pressure for each CPAP delivery system at inspiratory Hows of 20, 40, and 60 Umin, respectively.

the measured inspiratory work (ie, additional work minus reduced work) divided by the measured tidal volume." The amount of EEP that was flow-dependent in the Emerson PEEP valve was measured with increasing gas Hows (20, 40, and 60 Umin) through the water column at EEP settings of 0, 10, and 20 em

H 20 . Mean expiratory pressure and flow were computed by measuring the area under the pressure-time and flow-time curves during expiration. Mean expiratory resistance was determined by dividing the mean expiratory pressure by the mean expiratory H
Table l-Ainooy Pnmure and lrupiratory Work with Varioua CPAP Delivery SyatemI at 10 em H,O End-E%pirtJtory Pnmure

CPAP System

Inspiratory flow =20 Umin 5-L Reservoir bag Emerson demand valve Siemens Servo 900C Puritan-Bennett 7200 Siemens Servo 900B Bear 2 Engstrom Erica Medishield CPU 1 Inspiratoryflow=40 Umin 5-L Reservoir bag Emerson demand valve Siemens Servo 900C Puritan-Bennett 7200 Siemens Servo 900B Bear 2 Engstrom Erica Medishield CPU 1 Inspiratory flow =60 Umin S-L Reservoir bag Emerson demand valve Siemens Servo 900C Puritan-Bennett 7200 Siemens Servo 900B Bear 2 Engstr6m Erica Medishield CPU 1

Paw. (em HaD)

Mean Airway Pressuret (em HaD)

8.7 8.9 8.2 8.9 7.4· 6.1· 3.3· 5.2·

9.1 9.3· 10.3· 9.4· 12.4· 8.4· 6.3· 5.9·

7.6 8.0· 6.6· 7.4 4.0· 5.4· 1.8· 3.5·

8.2 8.5· 10.3· 9.1· 12.2· 7.7· 6.6· 4.5·

6.1 6.6· 3.4· 5.3· -0.5· 3.3· -1.4· 1.4·

7.1 7.4· 9.9· 8.5· 11.1· 7.5· 6.~

2.8·

Work % Additional

% Reduced

18 13· 3·

0 0 10· 0

1~

1· 32· 74· 82·

48·

36

0 0 14· 4·

31· 9· 21· 3·

48·

0 0 0

48·

3·

67·

0 0

11~

0 0

58

51· 18· 4()4l

17·

63· 84·

143·

11· 9·

38·

13· 4 0

·p<.OI (as determined by one-way analysis of variance) compared with reservoir bag at that inspiratory Howrate. tPawl = Minimum inspiratory airway pressure; mean inspiratory airway pressure; additional and reduced work reported as a percentage of a selected normal value (0.05 kg-mIL). CHEST I 88 141 OCTOBER. 1985

521

All measurements were made in duplicate and averaged to give the representative value for that level. In all variables, the difference between the first and second measurements was not significant (as determined by Students t-test). The mean of the first measurements was within 1 percent of the mean of the second measurements. Data were analyzed using one-way analysis of variance; differences between the demand-flow systems and the reservoir-bag system were analyzed for statistical significance using Dunnetts test. A p value of <0.01 was considered significant. RESULTS

Figure 3 illustrates pressure-volume curves for each system at an EEP of 10em H 20 and an inspiratory How demand of 40 Umin. For all systems, airway pressure during inspiration decreased initially from the end-expiratory level. The minimum airway pressure below the end-expiratory level (PawJ decreased progressively as inspiratory flow demand increased (Fig 4 and Table 1). At all inspiratory flows, Pawlwas significantly lower for the Siemens 9OOB, Bear 2, Engstrom Erica, and Medishield CPU 1 system than for the reservoir-bag system. In addition, Pawl was significantly lower for the Siemens 900C at 40 and 60 Umin and for the Puritan-Bennett 7200 at 60 Umin (Fig 4 and Table 1). At inspiratory flows of 40 and 60 Umin, Pawlwas significantly greater for the Emerson demand-valve when compared with the reservoir-bag system. No difference in mean airway pressure was found for the reservoir-bag, Emerson demand-valve, and Puritan-Bennett 7200 systems at an inspiratory flowof 20 Umin. At inspiratory flows of 40 and 60 Umin, mean airway pressure was closer to the end-expiratory level when the Emerson demand-valve and Puritan-BenRESERVOIR BAG

160

EMERSON DEMAND VALVE

SIEMENS SERV0 900 C

nett 7200 systems were used. In addition, at all inspiratory flows, mean airway pressure was significantly closer to the end-expiratory level when the Siemens 900C system was used than when the reservoir-bag system was used (Fig 4 and Table 1). The reservoir-bag, Emerson demand-valve, Engstrom Erica, and Medishield CPU 1 systems kept airway pressure below the end-expiratory level during the entire inspiratory phase (Fig 3). As a result, work of breathing increased (Fig 5 and Table 1). Additional work was significantly less for the Siemens 9OOB, 900C, Puritan-Bennett 7200, and the Emerson demand-valve systems than for the reservoir-bag system. The Engstrom Erica, Medishield CPU 1, and Bear 2 ventilators all incurred significantly more additional work than did the reservoir-bag system (Fig 5). During inspiration with the Siemens 9OOB, 900C, Puritan-Bennett 7200, and Bear 2 systems, airway pressure exceeded the end-expiratory level, thereby reducing work-of-breathing. This "overshoot' was pronounced in the Siemens 900B system (Fig 3 and Table 1). For the Siemens 9OOB, 900C, and Puritan-Bennett 7200, the delivered inspiratory How slightly exceeded the demand inspiratory flow, which accounts for a delivered tidal volume of slightly more than 500 ml. Thus, in these systems, a delivered inspiratory flow greater than demand flow occurs, possibly related to inaccuracies regulating flow to maintain the CPAP level. For all CPAP delivery systems except the Engstrom Erica, and Bear 2, increasing the level ofEEP resulted in minor changes in Pawl and mean airway pressure (Fig 6), as well as in additional work (Fig 7). For the

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40 WORK IMPOSED BY CPAP SYSTEMS AT 20, 40, AND 60 L/MIN PEAK INSPIRATORY FLOW FIGURE 5. Additional and reduced work for each CPAP delivery system at 10 em H 20 end-expiratory pressure for inspiratory flows of 20, 40, and 60 Umin, respectively.

522

Inspiratory Workand Airway Pressure with CPAP (Katz,Kraemer, Gjerde)

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MEAN AIRWAY PRESSURE ~

*

MINIMUM AIRWAY PRESSURE [ill

AT 0, 10, AND 20 cmH2 0 END-EXPIRATORY PRESSURE FIGURE 6. Mean airway pressure and minimum airway pressure for each CPAP delivery system at an inspiratory flow of 60 Umin for increasing levels of end-expiratory pressure (EEP) of 0, 10, and 20 em H 20, respectively. End-expiratory pressure levels normalized to 0 cm H 20. For each level of EE~ data from inspiratory flow of20, 40, and 60 Urn in were analyzed using one-way analysis of variance; differences between levels of EEP were determined using the Newman-Keuls test. Asterisk indicates p
Engstrom Erica, increasing the level of EEP resulted in greater changes in airway pressure and in additional work. For the Bear 2, increasing the level of EEP resulted in smaller changes in airway pressure, although additional work did not change. At 20 em H 2 0 EE~ 160

RESERVOIR BAG

EMERSON DEMAND VALVE

SIEMENS SERVO 900C

the airway pressure changes and additional work were comparable to the reservoir-bag system. The Fleisch pneumotachograph added resistance to the inspiratory line in our lung model. When only the Fleisch pneumotachograph was attached to the airway,

PURITANBENNETT 7200

SIEMENS SERVO

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ENGSTROM ERICA

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WORK IMPOSED BY CPAP SYSTEMS AT 0, 10, AND 20 cmH20 END-EXPIRATORY PRESSURE FIGURE 7. Additional and reduced work for each CPAP delivery system at an inspiratory flow of60 Umin for increasing levels of end-expiratory pressure (EEP) of 0, 10, and 20 ern H 20, respectively. For each level of EE~ data from inspiratory flows of 20, 40, and 60 Umin were analyzed using one-way analysis of variance; differences between levels of EEP were determined using the Newman-Keuls test. Asterisk indicates p
523

Table I-AinDtJfl Preaure Above the EEP Level in the Emerson PEEP Valve· End-Expiratory Pressuret (em H 20)

Continuous Flow (Umin)

20 40

60 ·EE~

sure.

o

10 0.6

0.4 1.0 2.0

end-expiratory pressure; PEE~

20

1.0 2.0 3.0

1.2

2.1

positive end-expiratory pres-

tInitial EEP set at 3 Umin continuous ftovv.

the measured pressure drop with increasing sine-wave flow rates (20, 40, and 60 Umin) was 0.3, 0.6, and 1.0 cm 8.0, respectively. The Emerson PEEP valve does not function as a perfect threshold resistor. When the constant Bowrate was increased, airway pressure progressively increased above the set end-expiratory level (Table 2). In addition, this ftow-dependent increase in pressure was affected by the level of EE~ Increasing the flow rate from minimal (3 Umin) to 60 Umin increased EEP 2.0 em H.O when EEP was set at 0 and 10cm H 20, and 3.0 cm 8.0 when EEP was set at 20 cm H 20. This study was not designed specifically to examine the expiratory resistance of various CPAP delivery systems. However; we found expiratory pressures and flows to vary (Fig 3 and Table 3). In the reservoir-bag system, the peak expiratory pressure was least. Although peak expiratory pressure was higher with the Emerson demand-valve system than with the reservoir-bag system, mean expiratory resistance did not differ significantly. The demand-flow systems that were incorporated into mechanical ventilators varied in their expiratory resistance. Only the Puritan-Bennett 7200 had a mean expiratory resistance that was equivalent to the reservoir-bag system. DISCUSSION

The major findings of our study are that CPAP systems vary widely in the amount of additional work they impose, and that some demand Bow systems impose less additional work than a continuous Bow reservoir-bag system. The ideal CPAP system would

impose only minimal change in airway pressure from the end-expiratory level, during both inspiration and expiration. A continuous-flow reservoir-bag system has both a patient breathing circuit and a humidifier; each of which produces air-flow resistance and nonelastic work. However, when gas flow is greater than the patient's inspiratory flow, the flow resistance of the inspiratory circuit should be negligible. Any decreases in airway pressure during inspiration would then result from the flow-dependent component in the PEEP valve and/or from a poorly compliant reservoir bag. 3 In addition, in our lung model, the Fleisch pneumotachograph added air flow resistance to the simulated lung. An example of how the activity of these components can be quantified can be seen by studying the 5-L reservoir-bag system at an inspiratory flow of 40 Umin and 10 cm H 20 EE~ Under these conditions, we measured an airway pressure drop of 2.4 em H 20 (Table1). Of this, the flow-dependent component of the PEEP valve was 1.5 cm H 20 (2.1-0.6 cm H 20 from Table 2), and the pressure drop due to the pneumotachograph was 0.6 em H 20. Therefore, 0.3 cm H 20 of the measured pressure drop was due to compliance characteristics of the reservoir bag. Some investigators advocate the use of an elastic loaded reservoir bag to increase the capacitance in the circuit and to reduce further the variation in airway pressure during inspiration.1.4.5 We chose not to load our reservoir bag in this way, as it is not common clinical practice. Also, the above data do not implicate the reservoir bag as a large component of the pressure drop during inspiration. Elastic loading of the reservoir bag would be important when gas Bowis less than the patient's inspiratory flow5 In demand-Bow CPAP systems, the decrease in airway pressure on inspiration is not related to the flowdependent component in the PEEP valve. In these systems, the airway pressure must decrease to below the sensitivity level of the demand valve before any gas Bow is initiated. Such an event is represented in the pressure-volume curves as a decrease in pressure without a change in volume. Following initiation of gas flow, if the regulation of demand flow does not instantane-

Table 3-MetJn E%piratory BaiBtance, Peak E%piratory Preaure, and Flow with VariouB CPAPDelivery S".."..· CPAP System

Mean Resistance (em H 2O/Us)

Peak Expiratory Pressure (em H 2O)

Peak Expiratory Flow (Umin)

5-L Reservoir bag Emerson demand valve Puritan-Bennett 7200 Bear 2 Medishield CPU 1 Siemens Servo 900C Siemens Servo 900B Engstrom Erica

2.7 2.5 2.5 5.0 5.2 5.3 8.7 8.7

12 15 15.5 16 15.5 16 16

48

20

44

52 48 48 42 42 36

*Measured at an inspiratory flow of 40 Umin and 10 em H 20 end-expiratory pressure. Inspiratory Yt40rk and Airwft.J Pr8ssur8 with CPAP (Kslz, Kraemer, Gjerde)

ously match patient effort, then airway pressure will decrease further, In addition, if demand How is less than that dictated by patient effort, then air How resistance due to the inspiratory circuit and humidifier will also contribute to the decrease in airway pressure. Demand-How systems that can match inspiratory effort instantaneously, during the entire breath, will produce a minimal drop in airway pressure. In contrast to recent studies by Gibney et aI, I Op't Holt et al," and Henry et al,"we found that, in the lung model, some demand-How systems perform as well as, or better than, the standard continuous-How reservoirbag system. Regarding decreases in airway pressure and additional work, the Emerson demand-valve system showed small but significant improvements when compared with the reservoir-bag system. The Siemens 900C and Puritan-Bennett 7200 ventilators produced less additional work and had mean airway pressures that were closer to the end-expiratory level than did the reservoir-bag system. These findings resulted from rapid increases in air How once the demand valves were open. In this study, the pressure support modes available in the Siemens 900C and Puritan-Bennett 7200 were not used. The Emerson demand valve is an inexpensive device that performs well if properly adjusted. The valve functions similarly to a pressure regulator, A single control knob regulates output pressure into the patient circuit. The pressure can be set to exceed the endexpiratory level, thus creating a continuous flow The advantages of using an Emerson demand-valve system rather than a reservoir-bag system are that there are fewer parts that can become disconnected, there is less need for adjustment if the patient's condition changes, and there is less use of oxygen. The Siemens Servo 900B also required less additional work than did the reservoir-bag system. However, the minimum airway pressure was much larger and the pressure increase above the end-expiratory level much greater at all rates of How than when other devices were used. Cox and Niblett" measured the changes in airway pressure when a healthy male volunteer breathed through a sealed mouthpiece connected to various CPAP systems. Their results regarding changes in airway pressure in the Siemens 900B and 900C were similar to ours. However, they concluded that the marked decrease in airway pressure and the time delay before initiation of gas How with the 900B must increase the work of breathing. In our lung model, the computed additional work was less with the 900B than with the reservoir-bag system. However; our clinical impression is that some patients appear to breathe more easily when breathing with either a continuousHow system or the 900C than with the 9OOB. The short period of airway obstruction at the beginning of in-

spiration and before initiation of gas How may cause energy utilization of the inspiratory muscles not measured as work. For example, during a maximum inspiratory force maneuver against an occluded airway, marked decreases in airway pressure occur without a change in volume. Decreases in airway pressure were much greater with the Medishield CPU 1, Engstrom Erica, and Bear 2 systems than with the reservoir-bag system. Causes for the large decrease in airway pressure are poor inspiratory How or pressure sensing, and/or a slow response time, and/or inadequate How generation. The magnitude of the additional work as a percentage of a normal value (0.05 kg-m/L) varied according to the CPAP delivery system and inspiratory How demand (Fig 5). For example, with the reservoir-bag and Emerson demand-valve systems, at an inspiratory How of 40 Umin (a How rate comparable to that measured in patients recovering from acute respiratory failure)," additional work of 30 percent to 40 percent of normal would be required. However, when mechanical work on the lungs during inspiration was measured in patients recovering from acute respiratory failure and compared with breathing through aT-tube, 9 breathing with CPAP (at 6, 12, or 18 em H 20) generated by an Emerson demand valve resulted in a net decrease in work of approximately 0.02 kg-m/L or 40 percent of the normal value. The decreased inspiratory work was a result of both an increase in lung compliance and a decrease in nonelastic resistance. The decrease in inspiratory work might have been greater if the CPAP system were more ideal, te, if airway pressure during inspiration had not decreased below the end-expiratory level. However, despite the overall beneficial effect of CPAP on inspiratory work with the Emerson demand-valve systems, the Medishield, Engstrom Erica, and Bear 2 ventilators produced a greater percentage of additional work. This increase might cancel any decrease in work resulting from CPAE Gibney et all measured work of breathing in normal subjects using the Bear 1 and MA-2ventilators with the work incurred by two continuous-How devices. They found that 0.022 kg-m/L more additional work was required with the Bear 1 ventilator, In our study, the Bear 2 ventilator required 0.024 kg-m/L additional work when the peak inspiratory How was 40 Umin. The comparison of our data to that of Gibney et all suggests that our lung model is a valid tool for measuring the additional work of breathing with various CPAP delivery systems. Some clinicians have abandoned the use of CPAP produced by some demand-How systems because of the additional work they may impose on the patient recovering from acute respiratory failure. In some circumstances, breathing through a T-tube at ambient airway pressure is preferred to breathing via CPAE CHEST I 88 I 4 I OClOBER, 1985

525

This preference is based on the use of equipment with faulty design. The preponderant data indicate that pulmonary mechanics and oxygenation are improved in patients when breathing via CPAP vs breathing through aT-tube. 9-11 In addition, some clinicians using intermittent mandatory ventilation have bypassed the demand valve by incorporating a parallel circuit that maintains CPAP with continuous flo~ When using the more technologically advanced ventilators (Siemens 900C and Puritan-Bennett 72(0), bypassing the demand valve during spontaneous breathing is unnecessary and of no advantage. In conclusion, demand-flow CPAP delivery systems vary widely in the amount of additional work they impose. In a few demand-How systems, airway pressure decreases greatly before air How begins. This decrease mayor may not result in a large increase in additional work. However; in a patient, the short period of airway obstruction may cause additional energy utilization and distress that is not quantified when using our lung model. While the percentage of additional work and airway pressure changes observed in the CPAP delivery systems differed significantly, clinical differences may be seen only in patients with compromised respiratory mechanics and/or high inspiratory demand. REFERENCES 1 Gibney RTN, Wilson RS, Pontoppidan H. Comparison ofwork of

2 3 4

5 6

7 8 9 10

11

breathing on high gas Bowand demand valve continuous positive airway pressure systems. Chest 1982; 82:692-95 Otis AB. The work of breathing. In: Fenn WO, Rahn H, eds. Handbook of physiology: section 3: Respiration. Volume 1. Washington, DC: American Physiological Society, 1964:469 Culpepper J, Snyder J, Pernock B, Pinsky M. Effect of PEEP valve resistance on airway pressure and inspiratory work (abstract). Crit Care Med 1983; 11:220 Gherini S, Peters RM, Virgilio RW Mechanical work on the lungs and work of breathing with positive end-expiratory pressure and continuous positive airway pressure. Chest 1979; 76: 251-56 Zebrowski ME, Geer In: Low Bow continuous positive airway pressure with a modified fresh gasreservoir. Crit Care Med 1981; 9:106-08 Op't Holt TB, Hall M~ Bass JB, Allison RC. Comparison of changes in airway pressure during continuous positive airway pressure (CPAP)between demand valve and continuous Bowdevices. Respir Car 1982; 27:1200-09 Henry WC, West GA, Wilson RS. A comparison of the oxygen cost of breathing between a continuous-Bow CPAP system and a demand-Bow CPAP system. Respir Care 1983; 28:1273-81 Cox D, Niblett DJ. Studies on continuous positive airway pressure breathing systems. Br J Anaesth 1984; 56:905-11 Katz JA, Marks JD. Inspiratory work during spontaneous breathing with and without CPAE Anesthesiology (in press) Annest SJ, Gottlieb M, PalaskiWH, Stratton H, Newell JC, Dalton R, et ale Detrimental effects of removing end-expiratory pressure prior to endotracheal extubation. Ann Surg 1980; 191: 539-45 Quan SF, Falltrick Schlobohm RM. Extubation from ambient or expiratory positive airway pressure in adults. Anesthesiology 1981; 55:53-56

m:

Chest Radiology-Update and Review The Chest Division, Department of Radiology, University of California San Diego School of Medicine, will present this course December 2-6 at the Hotel Del Coronado. For information, contact Ms. Dawne Ryals, PO Box 610203, DFW Airport, Texas 75261-0203(214): 659-9590.

526

Inspiratory Work and AJrway Pressure with CPAP(Katz, Kraemer, Gjerde)