Hospitals & Services
This mode of ventilation controls the inspired flow rate, pattern, time and volume. Providing the inspired pressure limits are not exceeded (30 cm H2O) it will deliver a preset set tidal volume. Patients are unable to breathe against the ventilator during the inspiratory phase. This may result in patient-ventilator dyssynchrony. If the respiratory demand exceeds the delivered inspiratory flow rate, this causes flow starvation, and a negative pressure will develop in the circuit. This is distressing for the patient; increases risk of micro-aspiration and may result in haemodynamic disturbances. When this occurs, the patient should be switched to another mode to allow breathing during all phases of ventilation. Typically spontaneous modes; ASB (assisted spontaneous breathing), or BiLevel (controlled pressure) are used. Alternatively the patient may be paralysed if they continue to have significant respiratory distress.
Open to view diagram of SIMV volume control
BiLevel Pressure control
This mode provides set inspiratory and expiratory pressures, and allows the patient to breath at any time during the respiratory cycle. Generally speaking, patients find this more comfortable as flow starvation is rare and they can determine their own respiratory patterns. However, in all pressure controlled modes, the tidal volume is the dependent on the inspiratory resistance, and elastances (compliances-1) of the lung, chest wall and abdomen. It is therefore important the tidal columns of patients on pressures adjusted as necessary. The equation below shows this relationship:
Paw = EVt + Q.Res insp
Where: Paw = Peak airway pressure, E = Lung and chest wall elastances, Q = inspiratory flow rate, and Res insp = inspiratory resistance