The importance of the circuit capacity in the administration of CPAP

A high flow turbine CPAP system

A continuous high flow CPAP system incorporating a turbine blower is described. The system achieves inspiratory flow rates of 150 l/min or more by means of reticulated gas flow and inspired oxygen fractions of 0.21-0.95. Positive airway pressure is provided by weighted disc valves and a modified aviation-type CPAP face mask provides electronic communication with the patient. The mobility of the system also enables its use as an intermittent physiotherapy aid. Work of breathing of the system, as assessed by total pressure fluctuations is at a minimum.

A continuous high flow intermittent mandatory ventilation system incorporating a Downs venturi device and a modified Nuffield Series 200 ventilator

Systems for respiratory support are becoming increasingly expensive and complex. Many systems suffer inadequacies when used for spontaneous ventilation. Some modes on newer ventilators are rarely used because of the complex controls and settings. There is no truly universal ventilator that satisfies every intensivist's wishes. CPAP/IMV is becoming accepted as the standard management of many patients with acute respiratory failure and there would be few intensive care units where CPAP/IMV is not used for part of a patient's respiratory support. We describe a cost-effective system that may be used for respiratory support in the spontaneously breathing mode. This system combines a high flow venturi, an efficient humidifier and an inexpensive reliable ventilator that can be used for adult and paediatric patients. The system, primarily for use in patients breathing spontaneously, functions well in patients requiring full ventilation.

Pressure characteristics of the Ambu CPAP system and the Servo ventilator 900C in CPAP mode

Spontaneous breathing was stimulated in the Ambu continuous positive airway pressure (CPAP) system and the Servo Ventilator 900C by means of a lung model programmed to mimic the respiratory flow patterns of a healthy volunteer and a patient in severe respiratory distress. Changes in airway pressure, flow and volume were recorded during "breathing" with CPAP at 0.5, 1.0 and 1.5 kPa. In the Ambu system, the airway pressure decreased during inspiration and increased during expiration, while the mean airway pressure was close to the pre-set CPAP value. The pressure changes were minimal when the fresh gas flow was increased from 15 to 25 1 X min-1. The higher fresh gas flow is recommendable during deep or rapid breathing. In the Servo ventilator 900C, there was a short initial inspiratory pressure drop, succeeded by a pressure rise above the CPAP value. The expiratory airway pressure was somewhat higher than CPAP. Both systems were found to be recommendable for clinical use.

The importance of the balloon reservoir volume of a CPAP system in reducing the work of breathing

We have previously reported, that the work of breathing in spontaneously breathing patients on CPAP could be significantly reduced by increasing the volume of the balloon reservoir in the circuit of a CPAP system from 3 to 23 l. We now report a study designed to determine the optimum balloon reservoir volume for the minimization of the work of breathing. Twenty intubated, spontaneously breathing patients were connected to a CPAP system with interchangeable balloon reservoirs. In each patient the work of breathing was measured for reservoir volumes of 3, 6, 12, 18, and 24 l attached in random order, while the positive airway pressure was held constant at 10 cm H2O. The balloons were constructed of the same material and had similar compliance. Rebreathing was prevented with use of one-way valves. Significant (p less than 0.001) decreases in the work of breathing were found on increasing reservoir volumes from 3 to 6, 6 to 12, and 12 to 18 l. A less significant (p less than 0.01) decrease in the work of breathing was found between reservoirs of 18 and 24 l. Rebreathing did not occur with significantly (p less than 0.001) lower flow rates when large reservoirs were used. We conclude that a balloon reservoir of 18 l represents the best compromise between reduction in the work of breathing, utilization of low source flow, and convenience of clinical use.