What is Tidal Volume
Tidal volume (VT) is the volume of air inhaled or exhaled with each breath, expressed in milliliters or liters at body temperature and pressure, saturated with water vapor (BTPS). 1
Definition and Measurement
Tidal volume represents the volume of a single breath, which can refer to either a particular breath or the average breath volume measured over a specified time period, typically one minute. 1
VT is conventionally expressed in units of milliliters (mL) or liters (L) under BTPS conditions (body temperature and pressure, saturated with water vapor). 1
During mechanical ventilation, tidal volume can be estimated non-invasively using electrical impedance tomography (EIT), where impedance changes strongly correlate with lung volume changes, though conversion to absolute volumes requires calibration. 1
Clinical Significance in Mechanical Ventilation
For lung-protective ventilation, tidal volumes should be set at 4-8 mL/kg predicted body weight (PBW) to prevent ventilator-induced lung injury, with volumes closer to 6 mL/kg PBW preferred for patients with ARDS or acute lung injury. 2
Predicted body weight is calculated as 50 + 2.3 × (height in inches - 60) for males, ensuring that tidal volume correlates with lung size based on height rather than actual body weight. 2
During CPR, tidal volumes of approximately 500-600 mL (6-7 mL/kg) are sufficient because cardiac output is reduced to 25-33% of normal, decreasing oxygen uptake and CO2 delivery to the lungs. 1
Each rescue breath should be delivered over 1 second with sufficient tidal volume to produce visible chest rise, avoiding excessive ventilation which increases intrathoracic pressure and decreases cardiac output. 1
Relationship to Other Respiratory Parameters
Tidal volume is a component of the VD/VT ratio (dead space to tidal volume ratio), which represents the proportion of tidal volume made up of physiologic dead space and serves as an index of ventilation-perfusion mismatching. 1
Minute ventilation (V̇E) is calculated from tidal volume and respiratory rate, representing the total volume of expired air exhaled from the lungs in one minute, expressed in liters per minute (BTPS). 1
Driving pressure (plateau pressure minus PEEP) reflects the ratio of tidal volume to respiratory system compliance, making it a superior predictor of outcomes in ARDS patients compared to tidal volume alone. 3
Common Pitfalls
Avoid setting tidal volume based on actual body weight in obese patients, as lung size correlates with height, not weight—this leads to excessive volumes and ventilator-induced lung injury. 2
Excessive tidal volumes (>8 mL/kg PBW) can cause pulmonary inflammation even in patients without pre-existing lung disease, as demonstrated by increased TNF-alpha and IL-8 levels in bronchoalveolar lavage fluid. 4
Transport ventilators may deliver inconsistent tidal volumes when lung compliance or airway resistance changes, predisposing patients to hypoventilation during transport if not continuously monitored. 5