How to Measure Vital Capacity on a Servo-i Ventilator
To measure vital capacity on a Servo-i ventilator, use the end-expiratory airway occlusion technique by utilizing the ventilator's end-expiratory occlusion hold option, which can be activated via the manual operating button on the Servo ventilator. 1
Step-by-Step Procedure
Ensure the patient is relaxed and not actively breathing against the ventilator to obtain accurate measurements; brief sedation may be necessary if the patient is actively breathing 2
Access the end-expiratory occlusion hold option on the Servo ventilator, which is available as a manual operating button (e.g., on the Siemens Servo 900C or 300) 1
Perform the occlusion during the last 0.5 seconds of expiration to capture the full vital capacity 1
The occlusion should be maintained until a plateau in airway pressure is observed, indicating the end of expiratory flow 1
For patients with spontaneous breathing activity, measurement may require insertion of an esophageal balloon to estimate changes in pleural pressure for more accurate results 1
Technical Considerations
When using the end-expiratory occlusion technique, be aware that gas compression in the ventilator tubing may affect measurements 1
Alternatively, a pneumatic valve can be used for rapid occlusion at the airway opening, which avoids problems related to gas compression in the ventilator tubing 1
The timing of the occlusion is important but small changes in expiratory time are not expected to substantially affect measurements, especially for expiratory times around 3 seconds 1
Potential Challenges and Solutions
In patients with active expiratory muscle contraction, the decrease in pleural pressure at end-expiration could be due to expiratory muscle relaxation rather than inspiratory muscle contraction, leading to inaccurate measurements 1
For patients with spontaneous breathing activity, some clinicians have been able to relax patients sufficiently to obtain an apparent end-expiratory plateau 1
Patient-ventilator asynchrony can lead to inaccurate readings, so ensure the patient is not actively breathing during measurement 2
Check for leaks in the ventilator circuit that could lead to falsely low pressure readings 2
Clinical Applications
Vital capacity measurements help assess respiratory muscle strength and overall pulmonary function 1
These measurements can be used to guide ventilator settings, assess lung compliance, and monitor response to interventions such as bronchodilators 2, 3
Monitoring vital capacity is valuable for evaluating patients' readiness for weaning from mechanical ventilation 1, 3
The Servo-i ventilator's ability to measure respiratory parameters helps clinicians set appropriate levels of support while maintaining adequate ventilation 3, 4
Remember that the Servo-i ventilator has been shown to maintain more accurate volume, FiO2, and PEEP settings compared to other portable ventilators, making it reliable for these measurements 5.