From the Guidelines
Functional residual capacity (FRC) refers to the volume of air in the lungs after a normal expiration. Closing volume (CV) refers to the point at which there is insufficient air left in the lungs to maintain lung inflation. When FRC falls below CV, distal lung patency cannot be maintained leading to atelectasis (collapse of part or all of a lung) 1. This relationship is crucial in understanding respiratory mechanics, particularly in clinical settings where conditions like obesity significantly reduce FRC, leading to atelectasis and shunting in dependent lung regions 1. Key points to consider include:
- FRC represents the equilibrium point between the elastic recoil of the lungs inward and the chest wall outward.
- When FRC decreases below the closing volume, small airways begin to collapse because there isn't enough air to keep them open, leading to atelectasis.
- Conditions that reduce FRC, such as obesity, increase the risk of atelectasis, as noted in the peri-operative management of obese surgical patients 1.
- Understanding these concepts is vital for clinicians to implement strategies like positive end-expiratory pressure (PEEP) during mechanical ventilation to maintain adequate FRC and prevent lung collapse.
From the Research
Lung Volumes and Capacities
- The volume of air in the lungs after a normal expiration is referred to as Functional residual capacity (FRC) 2, 3, 4, 5.
- The point at which there is insufficient air left in the lungs to maintain lung inflation is referred to as Closing volume (CV) 3, 4.
Effects of FRC and CV on Lung Function
- When FRC falls below CV, distal lung patency cannot be maintained, leading to atelectasis (collapse of part or all of a lung) 3, 4.
- Positive end-expiratory pressure (PEEP) can increase FRC and prevent airway closure, thereby reducing the risk of atelectasis 2, 3, 4, 5.