Comparison of three CPAP systems used in EMS Summary Continuous positive airway pressure (CPAP) is used in emergency medical services (EMS) with increasing frequency to treat a variety of respiratory conditions.1-8 There are several devices available for clinicians to choose from when providing CPAP in pre-hospital care. As with most products, variations in device performance exist and clinicians should be aware of these differences in performance as they may impact desired outcomes. Four commercially available EMS oxygen CPAP systems from three manufacturers were bench-tested to determine performance characteristics, including pressure stability and work of breathing. Each manufacturer's device performed differently at the same CPAP setting on a mechanical test lung when given the same breathing patterns. It is important for the clinician to understand how each CPAP device works as therapy may need to be adjusted to achieve therapeutic objectives. CPAP has been a widely accepted modality for treating multiple respiratory conditions in the hospital since the 1970s. The ability to treat a patient noninvasively helps to prevent the complications associated with intubation, provide rapid improvement in oxygenation (SpO2), and allow for easy initiation and discontinuation of therapy. As in the hospital, CPAP – a type of noninvasive positive pressure ventilation (NPPV) – has been gaining popularity in EMS for nearly a decade. However, despite numerous articles and literature supporting the efficacy of CPAP in providing respiratory support, there are still many EMS systems in the United States that have not adopted NPPV as an option for treating respiratoryimpaired patients. CPAP principles CPAP is constant positive pressure applied throughout the respiratory cycle to the spontaneously breathing patient. With CPAP, the pressure in the lungs is always positive and does not return to ambient pressure. CPAP provides a specified amount of positive pressure to the airways to act as a pneumatic splint in an effort to open the alveoli, maintain airway patency and increase functional residual capacity (FRC). This can accomplish several objectives: •Maintain airway patency and recruit alveoli that may be full of fluid • Improve oxygenation measured via pulse oximetry (SpO2) •Decrease work of breathing (WOB) and respiratory rate •Decrease systolic blood pressure 2 CPAP applied noninvasively requires equipment capable of generating sufficient flow to meet patient demand and maintain stable pressure. CPAP is delivered through a circuit and an appropriate mask interface. Issues In the pre-hospital environment, opinions vary on when and how to apply CPAP. Each EMS system has its own guidelines as to which respiratory conditions may be treated with CPAP. In hospitals, oxygen for CPAP is not a concern due to the availability of piped-in oxygen systems. In EMS, however, oxygen supply is limited and conserving oxygen is an issue. Other pre-hospital clinical concerns relating to CPAP are the patient’s WOB and the ability to maintain a stable pressure throughout the respiratory cycle. There are several commercially available CPAP systems intended for EMS use. Each device has a unique method of delivering oxygen CPAP therapy. The Boussignac CPAP System uses the direct flow from an oxygen cylinder to create pressure in the mask. The PORTO2VENT ™ CPAPOS device features a demand valve that delivers oxygen during inhalation only. Therapy pressure for the WhisperFlow ® devices are set by attaching a CPAP valve to the mask. A fixed tension spring built into the valve regulates pressure inside the mask. The objective of this testing was to determine oxygen consumption times, patient work of breathing, and pressure stability of these three oxygen CPAP units commonly used in EMS. 2 Units tested WhisperFlow, Respironics Inc. •WhisperFlow Fixed CPAP Generator (one unit tested) •WhisperFlow Fixed Low Flow CPAP Generator (one unit tested) Boussignac CPAP System, Vitaid (two units tested) PORTO2VENT ™ CPAPOS , Emergent Respiratory Products (two units tested) Method To ensure impartial and accurate testing, each system must be evaluated in the same way. On all of the following tests, each device’s accompanying mask was sealed to a thin sheet of clear plastic. The plastic sheet was drilled to fit a pressure line adapter from which pressure data was sampled. The pressure line was also sealed to the plastic to prevent unintentional leak. Each device and its accompanying mask was connected to a Hans Rudolph Series 1101 breathing simulator. Each device was tested under the same breathing pattern, lung condition, and CPAP pressure to enable direct performance comparisons. Devices were compared on several performance characteristics including oxygen consumption, pressure delivery characteristics (deviation, stability), and patient WOB. Results Oxygen run-out time on a full D-size oxygen cylinder was calculated by operating each device at a common therapeutic pressure of 10 cmH2O. The breathing simulator represented the patient. Average time to run-out on a D-size oxygen cylinder WhisperFlow Fixed WhisperFlow Low Flow CPAPOS Boussignac 33:20 45:10 43:59 13:11 0:00 6:00 12:00 18:00 24:00 30:00 36:00 42:00 48:00 Time (mm:ss) Figure 1: Using a D-size oxygen cylinder, the Boussignac CPAP System had the shortest run time (13:11 minutes) while the WhisperFlow Low Flow System had the longest run time. 3 Work of breathing (WOB) In the context of this evaluation, WOB is a measure of the energy a patient would expend while breathing on a CPAP device that is not delivering ideal CPAP therapy. Ideal CPAP is defined as delivering a fixed, stable pressure throughout the breathing cycle. When pressure is below set therapy pressure during inhalation or above therapy pressure during active exhalation, these conditions are considered work for the patient because patient energy is required to overcome the differential. The goal is to reduce WOB as much as possible for the patient. Figure 2 illustrates this principle. D TOT Pressure PT Time WOB values are reported in Joules per liter. For this test, both pressures below baseline (during inhalation) and above baseline (during exhalation) were used for the calculation of the WOB value. Figure 2: Imposed WOB is defined by the pressure time product (PTP). PTP is made up of both P T and D TOT, where P T is the pressure drop from baseline and D TOT is the total time spent The same breathing patterns used in, and the resulting data collected from, the pressure delivery characteristics test (see page 5) were used for the WOB calculations. below baseline. Anything that increases P T or D TOT increases WOB. Average work of breathing per breath (Joules/liter) Inspiratory Boussignac 1 flow Therapy pressure 5 cmH20 30 l/min 0.16 60 l/min 0.28 Therapy pressure 7.5 cmH2O 30 l/min 0.20 60 l/min 0.43 Therapy pressure 10 cmH2O 30 l/min 0.22 60 l/min 0.48 Boussignac 2 CPAPOS 1 CPAPOS 2 WhisperFlow Low Flow WhisperFlow Fixed 0.17 0.36 0.44 0.66 0.42 0.53 0.08 0.12 0.12 0.17 0.20 0.42 0.43 0.65 0.39 0.51 0.06 0.12 0.10 0.15 0.24 0.48 0.44 0.65 0.23 0.38 0.08 0.28 0.08 0.12 Figure 3: The WhisperFlow Low Flow and WhisperFlow Fixed devices had the lowest WOB values when compared to the Boussignac CPAP System and the CPAPOS . The WhisperFlow device’s stable pressure delivery is the main contributing factor to the differences in WOB values when compared to the other devices in this evaluation. 4 Pressure delivery characteristics Each device in this evaluation was tested at CPAP therapy pressures of 5.0, 7.5 and 10.0 cmH2O. The parameters in the chart below were entered into the breathing simulator. Pattern 30 l/min 60 l/min Resistance 5 5 Compliance 80 80 Rate 20 20 The next section presents the data collected for all devices at a CPAP therapy pressure of 7.5 cmH2O, 60 l/min peak inspiratory flow and with no intentional mask leak. Only 7.5 cmH2O of CPAP and 60 l/min are displayed in the graphs below because substantially equivalent results were seen at CPAP pressures of 5 cmH2O and 10 cmH2O, and at 30 l/min. The numerical and graphical data presented below note the following characteristics: Amplitude 5.6 11.7 Slope 40 40 % Inhale 34 34 VT ~350 ~750 •Pressure deviation – the total peak-to-peak pressure swing during a given breath •Peak inspiratory flow – the peak flow value during inhalation •Total tidal volume (VT) – the total volume inhaled •Input flow (patient demand) vs. actual flow – Input Flow is the flow profile used by the breathing simulator for each breath; •Actual flow is the flow profile that is recorded while the device is connected to the breathing simulator •Pressure delivery profile – the pressures recorded throughout a given breath •Ideal CPAP – the ideal pressure delivery profile for a given CPAP device (as marked by the red line in each of the graphs on page 7 and 8) 5 Boussignac: 7.5 cmH2O therpay pressure 60 l/min peak inspiratory flow/no leak Pressure deviation BN 1: 4.5 cmH2O BN 2: 3.9 cmH2O Peak inspiratory flow BN 1: 50.0 l/min BN 2: 50.0 l/min Total tidal volume BN 1: 620 ml BN 2: 626 ml Figure 4: The actual flow (solid gray line) does not meet the input flow representing patient demand (dotted line). Pressure is below 7.5 cmH2O throughout inspiration (A). During exhalation target pressure is above 7.5 cmH2O for the active part of exhalation (B). Both the Boussignac 1 and the Boussignac 2 devices had identical performance. CPAPOS : 7.5 cmH2O therapy pressure 60 l/min peak inspiratory flow/no leak Pressure deviation CPAPOS 1: 8.6 cmH2O CPAPOS 2: 8.6 cmH2O Peak inspiratory flow CPAPOS 1: 55.1 l/min CPAPOS 2: 64.7 l/min Total tidal volume CPAPOS 1: 564 ml CPAPOS 2: 609 ml Figure 5: The simulated patient needed to pull a negative 6 cmH2O to trigger oxygen delivery (A). There was a delay in the onset of flow from the CPAPOS device: the arrow (B) indicates when the patient initiated the breath and the arrow (C) indicates when flow from the CPAPOS device begins. Pressure remains below baseline for all of inspiration and above baseline for the initial phase of exhalation. The CPAPOS 1 and CPAPOS 2 devices did not have identical performance. 6 WhisperFlow: 7.5 cmH2O therpay pressure 60 l/min peak inspiratory flow/no leak Pressure deviation WFL: 2.2 cmH2O WFF: 2.5 cmH2O Peak inspiratory flow WFL: 59.2 l/min WFF: 64.1 l/min Total tidal volume WFL: 724 ml WFF: 704 ml Figure 6: Pressure remained reasonably stable throughout the breath cycle (A). Actual flow and input flow (patient's demand) are equal (B). The WhisperFlow Fixed and WhisperFlow Fixed Low Flow had identical performance. Discussion This testing showed performance differences in three CPAP devices that are currently used in EMS. Each device had unique methods of delivering CPAP therapy. When each device was tested, with the same simulated patient characteristics, there were varying results. In EMS, oxygen run-out time may be a consideration in choosing a device. The faster a device depletes an oxygen cylinder, the more cylinders will be required to sustain a patient while CPAP is delivered. Depending on transport time, the ambulance’s oxygen system may be an alternative to compressed gas, however, this is not always an option. EMS caregivers using the devices tested in this evaluation should be aware of the oxygen run-out times associated with use of each of these products. The goals of a CPAP generator are to provide a pressure curve that is as flat as possible. Achieving a flat pressure curve means the device is maintaining therapeutic pressure throughout the breath cycle and reducing the patient’s work of breathing. Therapeutic pressure and WOB are clinical issues that may have an impact on patient care. These need to be known and understood when selecting and using any CPAP device. This evaluation observed notable differences in the pressure delivery characteristics between the three manufacturers’ devices tested. Persons administering oxygen CPAP using any of these devices need to select the device that best fits their patient’s therapeutic requirements. 7 Conclusion CPAP is a valuable tool for providing respiratory assistance to patients in the EMS environment. The clinician needs to understand the application, capabilities and limitations of the devices. Understanding the CPAP devices’ performance capabilities is also important because performance of individual units can vary. This testing documented the variability in static therapeutic pressure and work of breathing between four commercially available EMS CPAP systems from three manufacturers. More research is necessary to determine if this variability. Written by Robert W. McCoy, BS, RRT, FAARC; Ryan Diesem, BA Valley Inspired Products, Apple Valley, MN References 1. Sullivan R. Prehospital use of CPAP: positive pressure = positive patient outcomes. Emerg Med Serv. 2005 Aug;34(8):120, 122-4, 126. 2. 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Cross A, Cameron P, Kierce M, Ragg M, Kelly A-M. Non-invasive ventilation inacute respiratory failure: a randomised comparison of continuous positive airway pressure and bi-level positive airway pressure. Emerg Med J 2003;20:531–534. 8. Bersten AD, Holt AW,Vedig AE, et al. Treatment of severe cardiogenic pulmonary edema with continuous positive airway pressure delivered by face mask. N Engl J Med 1991; 325:1825–30. © 2009 Koninklijke Philips Electronics N.V. All rights are reserved. PORTO2VENT is a trademark of Emergent Respiratory Products. WhisperFlow is a trademark of Respironics, Inc. and its affiliates. Philips Healthcare reserves the right to make changes in specifications and/ or to discontinue any product at any time without notice or obligation and will not be liable for any consequences resulting from the use of this publication. Philips Healthcare is part of Royal Philips Electronics www.philips.com/healthcare healthcare@philips.com fax: +31 40 27 64 887 Printed in the U.S.A. 4522 962 56461 * OCT 2009 Philips Healthcare Global Information Center P.O. Box 1286 5602 BG Eindhoven The Netherlands