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Overview

Continuous Positive Airway Pressure (CPAP) has become a standard treatment for congestive heart failure (CHF) and a number of other respiratory distress situations and has significantly reduced the need for endotracheal intubation (ETI) in these patient groups. [read more=”Read more” less=”Read less”] CPAP devices work by providing a constant positive airway pressure to the patient to splint open the airways during exhalation and, to reduce the patient’s work of breathing(WOB) work of breathing is the energy expended by the patient to breathe). CPAP is designed to reduce that WOB by “over delivering” a flow of air or oxygen during the inspiratory phase. Increased WOB leads to exhaustion and respiratory depression. The PEEP pressure during the expiratory phase is supplied to splint open the airways and to force accumulated fluid back into the interstitial spaces. In addition to the improvement in the patient’s condition and the reduction in the need for intubation and drug interventions, it has been shown that the use of CPAP with these patients’ (in the prehospital environment) also reduces the length of hospital stay and the need for admittance to the Intensive Care Unit (ICU).

Discussion

The efficacy of the delivery of CPAP pressures by the devices offered for the prehospital market differs significantly. To deliver a “continuous” (and this is the keyword in CPAP) positive airway pressure, during the inspiratory phase, the device must deliver a flow rate in excess of the flow rate demanded by the patient. Some devices on the market, while being very novel in their approach to delivering a PEEP pressure, do not provide a continuous positive pressure by providing the flow demanded by the patient during the inspiratory phase and therefore may be incorrectly classified as CPAP devices.

Other devices utilize a constant gas flow and a pressure balancing valve on the facemask to maintain the CPAP pressure. These tend to produce high sound levels which can be annoying to the patient. Their efficacy is considered to be good.

The last group of CPAP devices uses an internal pressure control circuit to actuate a demand valve and supply CPAP pressure. Some of these devices, however, have a significant delay in delivery of flow when the patient initiates the inspiratory cycle. This is due to the high triggering pressure of the demand valve which generates the inspiratory flow and creates a “hiccup” in flow delivery which can be disturbing for the patient. In addition, some have a high peak pressure at the commencement of the inspiratory phase. Any significant drop below the CPAP baseline during inspiration and airway
pressure spike at the beginning of expiration reduce the efficacy of the CPAP and adds to the patient’s WOB.

As can be seen from the following graph, the volume of the shaded area indicates when the pressures are above or below the baseline CPAP pressured set during one respiratory cycle. This indicates the additional work imposed on the patient
(Fig. 1).

Fig. 1. Pressure waveform showing high inspiratory and expiratory pressures of two,
commonly available, pressure control/demand valve devices – CPAP pressure 7.5 cm H2O

Gas consumption is an important issue in the pre-hospital emergency Review/Update market. On a “D” size oxygen cylinder of the gas flow/pressure balancing valves ranges between 35 – 45 minutes while the internal pressure control devices last between 45 – 55 minutes. The novel PEEP devices have a significantly shorter operating time of between 15 – 20 minutes (based on a CPAP pressure of 7.5 cmH2O) (Fig. 2).

Fig. 2. Cylinder duration on a standard “D” size Oxygen Cylinder

Conclusion

Prehospital CPAP has been clinically proven to assist patients in respiratory distress from CHF and a number of other conditions. The following statements are taken from the conclusions drawn during a number of clinical studies. These comments validate the use of CPAP in the prehospital environment.

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