![]() This is the percentage of oxygen in the air mix delivered by the ventilator during each respiratory cycle. The amount of negative pressure needed to trigger a breath is called the trigger sensitivity and is usually set up by the respiratory therapist. The negative pressure is transmitted to the airways and then to the ventilator tubing, where sensors detect the change in pressure and deliver a breath to the set tidal volume. When the diaphragm contracts, the intrathoracic pressure becomes more negative. The ventilator in AC mode is programmed to sense changes in the system pressure when a patient initiates a breath. A caveat on this is that by increasing the RR, the dead space is also increased, so increasing RR may not be as effective as increasing VT in improving ventilation. As with VT, increasing RR will increase minute ventilation and decrease the patient’s blood CO2. ![]() The ventilator will only deliver breaths at the set RR if the patient does not trigger it faster. This is called patient-triggered breaths. ![]() For example, if a patient is breathing at 20 bpm and the ventilator is set at 15 bpm, the ventilator will follow the patient and deliver 20 bpm (one each time the patient initiates a breath). In AC, this set rate can be overturned by the patient, meaning that if the patient inhales, the ventilator will sense the drop in pressure and deliver that breath, even if the patient is breathing above the set rate. For example, if the set rate is 15, then the delivery is 15 bpm or 1 breath every 4 seconds. This is the set rate for delivering breaths per minute (bpm). Changing the VT will, in turn, change the minute ventilation (VT x RR) an increase in minute ventilation will result in a decrease in carbon dioxide (CO2), by the same token, a decreased VT will result in a decreased minute ventilation and increase in the patient’s blood CO2. This is the set amount of volume that will be delivered with each breath. When AC mode is selected in the ventilator, four parameters may be quickly modified: The VT delivered by the ventilator in AC always will be the same regardless of compliance, peak, or plateau pressures in the lungs. It works by setting a fixed tidal volume (VT) that the ventilator will deliver at set intervals of time or when the patient initiates a breath. ĪC ventilation is a volume-cycled mode of ventilation. ![]() Assist-control (AC) mode is one of the most common methods of mechanical ventilation in the intensive care unit. Mechanical ventilation is a lifesaving procedure that is often performed when patients require respiratory support. ![]()
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