What is the impact of vital capacity (VC)-CMV on the patient's work of breathing (WOB)?

• A. Increases by 10-20%
• B. Increases by 33-50%
• C. No effect on WOB
• D. Decreases the WOB

Answer: (B)

Vital capacity (VC) in controlled mechanical ventilation (CMV) is critical because it influences how effectively the ventilator supports the patient's breathing efforts. When VC is adjusted such that it significantly exceeds the patient's natural capabilities, the patient's work of breathing (WOB) can increase due to the mismatch between ventilation demand and ventilator support.

In this context, when vital capacity is not set optimally—too high or too low—patients may need to expend additional effort to initiate breaths or match the required tidal volume. If vital capacity is set inappropriately high, which could happen when a ventilator is configured to deliver larger tidal volumes than necessary, the patient may experience an increased WOB by 33-50%. This is because patients may struggle against the ventilator settings and have difficulty coordinating their own respiratory efforts with those delivered by the machine.

Thus, an increase in the WOB due to a high VC in CMV reflects the physiological stress placed on the respiratory muscles as they have to work harder, leading to potential fatigue and increased oxygen consumption. Balancing ventilator settings with the patient's needs is crucial to avoid this unnecessary increase in effort.