From the Guidelines
Supine is the worst position for a patient's functional residual capacity (FRC). When a patient lies flat on their back, the abdominal contents push upward against the diaphragm, reducing the volume of the lungs and decreasing FRC by approximately 25-30% compared to the upright position, as noted in studies such as 1. This reduction occurs because gravity causes the abdominal organs to exert pressure on the diaphragm, limiting its downward movement during respiration. Standing is actually the best position for FRC, followed by sitting, then side-lying, with supine being the worst.
The evidence from 1 suggests that a head and trunk inclined (semi-Fowler) position may improve functional residual capacity (FRC) and this is consistent with the findings of 1 which showed that the transition to a semi-sitting position with the head elevated at 30° allows a significant increase in FRC. In contrast, the supine position is associated with a decrease in FRC, particularly in patients with respiratory compromise, as noted in 1.
Key points to consider include:
- The supine position reduces FRC by approximately 25-30% compared to the upright position
- Standing is the best position for FRC, followed by sitting, then side-lying
- The semi-Fowler position may improve FRC and is a recommended position for patients with respiratory compromise
- The evidence from 1, 1, and 1 supports the use of upright positioning to optimize lung volumes and improve oxygenation in patients with respiratory distress.
From the Research
Positional Effects on Functional Residual Capacity (FRC)
The effect of body position on functional residual capacity (FRC) has been studied in various populations, including healthy individuals, overweight and obese subjects, and patients with respiratory and cardiac diseases.
- Studies have shown that FRC decreases when changing from a sitting to a supine position 2.
- In morbidly obese patients, the supine position has been found to have a lower FRC compared to other positions such as the reverse Trendelenburg position 3.
- The beach chair position has been found to have no significant difference in FRC compared to the supine position in morbidly obese patients 3.
- In healthy subjects, FRC values are higher in more erect positions, such as sitting and standing, compared to the supine position 4.
Comparison of Positions
Comparing the different positions:
- Supine position: has been found to have a lower FRC in various studies 4, 2, 3.
- Sitting position: has been found to have a higher FRC in healthy subjects and some patient populations 4.
- Standing position: has been found to have a higher FRC in healthy subjects 4.
- Side-lying position: has been found to have a lower FRC in some studies, although the evidence is not as strong as for the supine position 4, 5.
Worst Position for FRC
Based on the available evidence, the worst position for FRC appears to be the supine position, as it has been consistently found to have a lower FRC compared to other positions in various populations 4, 2, 3.