To the Editor:—
The recent study by Stevenson et al. 1compared the performance of the AV2+ ventilator of the Narkomed GS anesthesia workstation (North American Draeger, Telford, PA) with the Divan ventilator of the Narkomed 6000 workstation (North American Draeger) using an infant lung model. Both of these ventilators are designed to work with a circle anesthesia circuit. Among its many features, the Divan ventilator is designed to ensure more accurate delivery of set tidal volume to the airway of the patient than a traditional anesthesia ventilator by compensating for the compliance of the breathing system and patient circuit. In their article, Stevenson et al. 1state that ‘. . . the compensation for a decrease in lung compliance by the 6000 ventilator system is incomplete‘ and that ‘the clinical advantage of this compliance feature needs further evaluation.‘ Their findings and the clinical implications of compliance compensation as implemented on the Narkomed 6000 can be understood by examining the manner in which the compliance compensation feature is designed.
During setup, the Divan ventilator is designed to determine an external compliance compensation factor (Cext) for those components external to the patient, which include the machine components of the breathing system (Cbs) and the hoses of the breathing circuit (Ccirc).
Therefore, the total compliance (CTot), including the compliance of the patient’s lungs, (CL) is as follows:
For the set tidal volume (VT) to be delivered to the airway of the patient, the volume the ventilator must deliver (Vvent) is the tidal volume plus sufficient volume to compensate for the compliance of the system external to the patient (Vext).
During mechanical ventilation, the volume delivered is the product of peak inspiratory pressure (Pinsp) and compliance:
Therefore, equation 3can be rewritten in the following manner: During each mechanical breath, the ventilator continuously measures inspiratory pressure and uses the known external compliance compensation factor (Cext) and the set tidal volume to find the total volume that must be delivered to ensure that the set tidal volume is delivered to the airway.
Because compliance compensation involves increasing the volume delivered by the ventilator, there are safety features incorporated into the ventilator design that limit the amount of volume that can be added in an attempt to compensate for compliance. For example, it is possible that a user could place a low-compliance circuit, such as a pediatric circuit, on the machine without executing the compliance test. If the last circuit tested had a greater compliance, the ventilator could deliver excessive volumes to the patient. To prevent this from occurring, when tidal volumes of less than 200 ml are selected, the Divan will only use the measured circuit compliance if it is 0.8 ml/cm H2O or less. If the measured circuit compliance exceeds 0.8 ml/cm H2O, a default value of 0.6 ml/cm H2O is used as the circuit compliance compensation factor.
In their study, Stevenson et al. 1tested the ability of the Divan ventilator and the AV2+ to deliver tidal volumes of 50, 100, and 200 ml to an infant test lung when lung compliance was changed from 3 ml/cm H2O (normal) to 1 ml/cm H2O (low). Their results showed more accurate volume delivery by the Divan ventilator because a greater minute ventilation was delivered by the Divan than by the AV2+ ventilator, which does not compensate for compliance. However, the Divan did not compensate completely when lung compliance was reduced because the minute ventilation delivered to the test lung was less than the initial value.
The relevance of these findings to patient care can be understood by putting the test conditions into a clinical perspective. At the low lung-compliance setting (1 ml/cm H2O), the tidal volumes of 100 and 200 ml would generate respective peak pressures of 100 and 200 cm H2O, which exceed clinical conditions. Therefore, the Divan ventilator is not designed to provide compliance compensation under those conditions.
For the 50-ml tidal volume at the low lung-compliance studied by Stevenson et al., 1the peak pressure would be 50 cm H2O—within the clinically possible range. In this case, the Divan would need to deliver 50 ml plus enough additional volume to compensate for circuit and breathing system compliance. We obtained a sample of the Pediatric King breathing circuit (King Systems Corp., Noblesville, IN) used in the study and found the compliance to be approximately 1.6 ml/cm H2O. As explained, the Divan would substitute a circuit compliance compensation factor of 0.6 ml/cm H2O for the actual hose compliance of the circuit. The internal breathing system compliance of the Divan is about 2.2 ml/cm H2O. Therefore, the external compliance compensation factor used by the ventilator would be 2.8 ml/cm H2O, whereas the actual external compliance of the experimental setup would be 3.8 ml/cm H2O. Because the actual external compliance was greater than the compensation factor used by the ventilator, the volume (Vext) that was added to the set tidal volume would not compensate completely for the actual external compliance (see equation 5).
To obtain optimum performance from the Divan ventilator, users are instructed to execute the simple compliance test each time the circuit is changed. Users also are advised to use pediatric circle circuits when volume mode is used to deliver tidal volumes of less than 200 ml. North American Draeger does not describe the compliance compensation limits in the specifications published in the user’s manual for the Narkomed 6000. Therefore, Stevenson et al. 1were not aware of the manner in which this feature was designed. We congratulate these investigators on a thorough and well-presented study. We hope this additional information will provide the appropriate clinical perspective on their results. As a result of this study, North American Draeger is updating user manuals to include more detailed information on compliance compensation by the Divan ventilator. North American Draeger also is pleased to help customers to select circuits that will ensure the performance they require to meet their clinical needs.