Assist Control Ventilation (ACV) is a primary volume-cycled mode of mechanical ventilation used extensively in intensive care settings. For this reason, it is included last in our discussion. This mode functions by delivering a predetermined fixed tidal volume (VT) to the patient either at set time intervals or when the patient initiates a breath themselves. This ensures that the same volume of air is provided with each ventilated breath, regardless of variations in the lung's compliance, or the peak and plateau pressures.
Parameter Adjustments in ACV
In ACV, the clinician sets four main parameters: Tidal Volume (VT), Respiratory Rate (RR), the Fraction of Inspired Oxygen (FiO2), and Positive End-Expiratory Pressure (PEEP). These settings allow for precise control over the breathing process, accommodating the patient’s needs and enhancing comfort by allowing spontaneous breathing efforts to trigger the ventilator. This aspect is particularly beneficial in managing patient CO2 levels effectively, which is crucial for correcting respiratory acidosis or alkalosis and reducing the work of breathing for the patient.
Physiological Implications and Ventilation Pressures
A critical aspect of ACV is understanding how it interacts with lung compliance. In patients with very compliant lungs, low plateau pressures are generated, making it easier to ventilate the patient. Conversely, in conditions like pulmonary edema, ARDS, pneumonia, or pulmonary fibrosis, where lungs are stiffer, higher pressures are needed to achieve the same tidal volume, which can increase the risk of ventilator-induced lung injury or barotrauma. It is important to monitor these pressures closely to mitigate such risks.
Advantages and Disadvantages of ACV
ACV increases patient comfort by allowing them to influence the timing and effort of breaths. This can significantly enhance patient interaction with the ventilatory support, providing a more natural breathing experience. However, there are notable disadvantages to be aware of. The potential for barotrauma is a concern in patients with less compliant lungs, as the fixed volume can lead to high intra-thoracic pressures. Additionally, problems such as breath stacking and auto-PEEP might occur if the patient becomes tachypneic or if the exhalation time is insufficient. This can compress the chest cavity and reduce venous return, leading to hypotension. Proper sedation and careful monitoring of ventilation settings are essential to prevent complications like respiratory alkalosis from hyperventilation.
Conclusion
In conclusion, Assist Control Ventilation offers a controlled method of supporting patients in critical care with the ability to maintain consistent tidal volumes. Despite its benefits, the use of ACV requires vigilant monitoring and adjustment of ventilation parameters to avoid potential complications associated with fixed volume delivery.
| Mode | Clinician sets | Suited to |
|---|---|---|
| PCV-VG | A target tidal volume, which the ventilator delivers by adjusting inspiratory pressure on each breath | Surgeries characterized by variable lung compliance, where it provides consistent minute ventilation |
| SIMV | Tidal volume (SIMV-VCV) or PIP (SIMV-PCV), minimum respiratory rate, I:E ratio, PEEP, Pmax, pressure support, trigger sensitivity | Patients capable of spontaneous breathing who still need support, including recovery from muscle relaxation and weaning |
| PSVPro | Pressure support and PEEP; the patient controls initiation, rate, timing and tidal volume | General anesthesia in unparalyzed children, where it reduced propofol use by almost 20% and shortened emergence time |
| ACV | Tidal volume, respiratory rate, FiO2, PEEP | Intensive care use; watch for barotrauma in stiff lungs and for breath stacking and auto-PEEP |
