Sitting verus supine vital capacity

Whilst working in Mt Gambier recently one of our Respiratory Physicians, Dr Borta asked me to measure a patient’s supine vital capacity which is one of our less common noninvasive respiratory function test measures.

Evaluation of diaphragm strength can be accomplished by measuring the vital capacity in an upright or sitting position followed by a measurement made in the supine position.

Here Lisa, one of our Respiratory Nurses demonstrates the supine and sitting positions.

It is well established in healthy volunteers that lung volume and vital capacity decrease after moving from the upright to supine position. This phenomenon is thought to be due to the shifting of blood to the pulmonary vasculature, changes in the position of the diaphragm, and the weight of the abdominal viscera pressing against the diaphragm. In healthy adults, the FVC falls by approximately 7.5 ± 5.7%.This change is exaggerated in many patients with severe diaphragmatic weakness, though it has not been well evaluated in Amyotrophic Lateral Sclerosis (ALS) patients, the assessment of ΔFVC has been suggested as a screening test for diaphragmatic weakness.
Interpreting an increased reduction in vital capacity in the supine position as diaphragm dysfunction should be made cautiously if the patient’s body mass index is greater than 45 kg/m² as a reduction may not indicate diaphragm dysfunction, but rather an increase in the resistance to diaphragm descent.

Lechtzin et al. studied 25 patients with ALS and demonstrated that supine FVC is an excellent measure of diaphragmatic strength in patients with ALS. It is apparent that the diaphragm becomes weak well before the upright FVC is reduced but using the supine FVC < 75% predicted as a cutoff is a highly sensitive and specific measure of diaphragmatic weakness. Longitudinal studies need to be performed to determine whether supine FVC is a better predictor of future outcomes than measures currently in use. It is not known whether early detection of respiratory muscle weakness and intervention leads to improved outcomes in ALS but it will allow interventions to be developed that target mild respiratory impairment.

Vanessa