Mechanical ventilation is a form of life support used when a person’s ability to breathe independently is compromised. It pushes air into the lungs to ensure adequate oxygenation and carbon dioxide removal. Volume Control Ventilation (VCV) is a foundational and commonly used mode of support. VCV operates on the principle of a guaranteed breath size, meaning the ventilator is programmed to deliver a specific, fixed volume of air with every mechanical breath. This set volume is delivered regardless of the pressure required, making the resulting airway pressure the variable that must be closely monitored.
The Mechanics of Volume Delivery
The breath begins when the ventilator pushes air into the patient’s lungs at a constant, predetermined rate, known as the flow rate. This flow continues until the total programmed breath size, called the Tidal Volume (\(V_T\)), has been fully delivered.
During the inspiratory phase, airway pressure rises steadily as the lungs inflate and resist the incoming flow. Once the set Tidal Volume is reached, the ventilator ends inspiration, sometimes pausing briefly to measure static lung pressure. The final phase is exhalation, a passive process where the chest wall and lungs recoil naturally, pushing the air back out until the next breath begins. The fixed nature of the delivered volume makes VCV a volume-targeted and flow-controlled mode of ventilation.
Key Settings Programmed by Clinicians
Clinicians must input several parameters. Tidal Volume (\(V_T\)) is the precise amount of air delivered with each inspiration. This volume is typically calculated based on the patient’s height and weight to ensure a lung-protective size, often 6 to 8 milliliters per kilogram of ideal body weight.
The Respiratory Rate (f or RR) dictates how many mandatory breaths the machine will deliver per minute. Positive End-Expiratory Pressure (PEEP) is also set, which maintains pressure in the lungs at the end of exhalation to prevent the collapse of small airways. Another element is the Flow Pattern, which describes how the gas is delivered over time, usually as a constant flow until the target volume is reached.
Monitoring Peak Pressure and Lung Safety
While the volume is guaranteed in VCV, the required pressure is not fixed and can vary widely. The ventilator must generate a Peak Inspiratory Pressure (PIP) to overcome resistance. If the patient’s lung stiffness (compliance) decreases, or if the airway resistance increases, the resulting Peak Inspiratory Pressure will climb.
Because excessive pressure can damage lung tissue, clinicians must monitor two pressure values closely. Peak Inspiratory Pressure (PIP) reflects the maximum pressure reached during the breath, including the pressure needed to overcome airway resistance. The Plateau Pressure (\(P_{plat}\)) is measured during a brief inspiratory hold when airflow ceases, reflecting only the pressure distending the alveoli (lung stress).
A high Plateau Pressure, generally above 30 \(cmH_2O\), indicates significant tension on the alveoli and signals a high risk of ventilator-induced lung injury. To mitigate this danger, a maximum pressure limit (High-Pressure Alarm) is always set. If the Peak Inspiratory Pressure reaches this alarm limit, the breath is automatically terminated early, even if the full programmed Tidal Volume has not been delivered, prioritizing lung safety over volume delivery.
Clinical Rationale for Choosing Volume Control
VCV is selected to ensure predictable and consistent air exchange per minute. Since the Tidal Volume and the Respiratory Rate are both fixed, the product of these two settings—the Minute Ventilation (\(V_E\))—remains stable. Minute ventilation is the total volume of air moving in and out of the lungs each minute and is the primary factor controlling the clearance of carbon dioxide.
This guaranteed minute ventilation provides clinicians with precise control over the patient’s carbon dioxide levels. When a patient’s metabolic demands are stable, or when tight control over blood gas parameters is necessary, VCV allows for accurate prediction of the ventilatory effect. The fixed volume delivery minimizes the risk of inadvertently under-ventilating the patient, which could lead to a buildup of carbon dioxide in the bloodstream.