Breathing is a fundamental process, involving the movement of air into and out of the lungs. The volume of air exchanged with each normal, quiet breath is known as “tidal volume” (VT or TV). In healthy adults, this typically measures around 500 milliliters. In medical settings, particularly for patients requiring mechanical ventilation, controlling tidal volume becomes a precise strategy. Low tidal volume refers to a specific approach where the amount of air delivered with each breath is intentionally reduced from a typical spontaneous breath. This controlled adjustment is made to support the patient’s lung function and manage various medical conditions.
Understanding Tidal Volume
Tidal volume represents the quantity of air that enters and exits the lungs during a single breath. This measurement is a key indicator of respiratory function. In healthy adults, this typically ranges from 400 to 500 milliliters, or approximately 7 milliliters per kilogram of body mass. When a patient is placed on a mechanical ventilator, healthcare professionals precisely control the tidal volume delivered with each machine-assisted breath.
Low tidal volume ventilation involves a deliberate reduction from these normal values. This controlled volume is calculated based on the patient’s predicted body weight, not their actual weight, because lung size correlates more closely with height than with overall body mass. For patients without specific lung injuries, a target tidal volume often ranges between 6 to 8 milliliters per kilogram of predicted body weight. For individuals with acute lung conditions, this volume is further reduced, usually to 4 to 6 milliliters per kilogram of predicted body weight.
Why Low Tidal Volume is Used
Lung Protective Ventilation
Low tidal volume ventilation primarily protects the lungs from damage during mechanical support, a concept known as lung protective ventilation. Historically, ventilators often delivered larger tidal volumes, sometimes exceeding 10 milliliters per kilogram of predicted body weight, which was later found to cause harm. This practice led to what is termed ventilator-induced lung injury (VILI). Low tidal volume ventilation mitigates VILI by preventing two main types of damage: volutrauma and atelectrauma.
Types of Lung Injury
Volutrauma occurs when lung tissue is overstretched by excessive air volumes, damaging the delicate air sacs (alveoli). Limiting the volume per breath avoids this overdistension. Atelectrauma results from the repetitive collapse and reopening of small airways and alveoli, causing shear stress and inflammation. While low tidal volume directly addresses volutrauma, it is often combined with other ventilator settings, like positive end-expiratory pressure (PEEP), to keep these structures open.
Historical Impact
Research, especially in the context of Acute Respiratory Distress Syndrome (ARDS) in the late 1990s, profoundly demonstrated that reducing tidal volumes significantly improved patient survival and outcomes. This evidence solidified low tidal volume ventilation as a standard of care.
Its Role in Patient Care
Low tidal volume ventilation is a fundamental strategy in critical care, widely applied for patients with Acute Respiratory Distress Syndrome (ARDS) and other acute lung injuries. In these vulnerable lung conditions, traditional ventilation settings could worsen the patient’s state. By delivering smaller, controlled breaths, this strategy helps stabilize respiratory status while minimizing further harm to delicate lung tissues.
Healthcare professionals continuously monitor patients to ensure effectiveness and safety. This monitoring includes assessing arterial blood gas levels to check oxygen and carbon dioxide, and measuring plateau pressures, which reflect the pressure inside the small airways at the end of inspiration. Keeping plateau pressures below a certain threshold, typically 30 centimeters of water, is a goal, as higher pressures indicate potential overstretching. Low tidal volume ventilation has improved patient outcomes, reducing complications and mortality rates associated with mechanical ventilation in critically ill individuals. This approach represents a significant advancement in respiratory care.