Volume Control Ventilation




Anesthesiologists use mechanical ventilators in the operating room, on patients who are paralyzed to facilitate surgery. Most of these are “bag in bottle” mechanical bellows which are controlled by three factors: 1) tidal volume, 2) respiratory rate, 3) I:E ratio. The I:E is the ratio of time spent in inspiration versus expiration. Inspiration is active. Expiration is passive and thus requires more time to allow alveolar units to empty. If expiration is of insufficient duration, gas is trapped in the alveoli (at end expiration), a process known as “auto-PEEP” (click here). If the patient is to receive ten breaths per minute, then the duration of the cycle is 6 seconds. A conventional I:E is 1:2, so 2 seconds are set aside for inspiration and 4 for expiration. PEEP is rarely used in the OR, and the combination of reduced FRC (due to tracheal intubation) and monotonous ventilation leads to a considerable amount of atelectasis in many patients.

Conventional anesthesia ventilator: the patient is delivered mandatory breaths from a “bag in bottle” ventilator. He can also draw unsupported spontaneous breaths from an in-line reservoir bag: intermittent mandatory ventilation is constructed from this mechanism.

The main advantage of volume controlled ventilators is guaranteed minute ventilation. This is particularly important in the operating room, where lung compliance may be influenced by the type of surgery involved (abdominal or chest surgery), and in the ICU or in transit if patient’s tidal volumes are not being continuously monitored.

Early intensive care ventilators represented a continuation of operating room techniques, where the patient was heavily sedated and paralyzed until the disease process resolved. The problem, though, was how to get the patient off the ventilator before their muscles atrophied. This required some form of patient-ventilator interaction.

There is a considerable difference between mandatory and spontaneous breaths. In mandatory ventilation the patient is a passive object receiving gas as determined by the ventilator at a set rate and volume (or pressure). A spontaneously breathing (awake) individual demands gas at a flow and rate of their own choosing. Assisted ventilation thus requires a triggering device and a flow of gas to match the patient’s peak inspiratory demand (30 to 60 liters per minute). The two methods developed to overcome these problems were assist-control ventilation and intermittent mandatory ventilation.





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