When fluid in tissues is displaced from the lower body into the upper body, the circumference of the neck grows, which may hinder the flow of air, Canadian researchers report. These findings may help explain why the prevalence of obstructive sleep apnea (OSA) is increased in patients with diseases characterized by “fluid overload,” like heart failure and kidney failure, for example.

People with sleep apnea stop breathing temporarily and repeatedly while asleep causing them to gasp for breath. Although obesity and a thick neck are known risk factors for the disorder, they account for only about one third of variability in the apnea-hypopnea index - a measure of the frequency of slow or stopped breathing episodes at night.

Dr. T. Douglas Bradley, from the Toronto General Hospital, and his team theorized that fluid accumulation in soft tissues in the neck might cause narrowing or blockage of the pharynx - the tube that connects the mouth and nasal passages with the esophagus.

Experiments in 11 healthy non-obese subjects with no symptoms suggesting OSA support this line of thinking.

In order to measure airflow resistance, investigators inserted two open catheters, one into the nose and the other advanced beyond the base of the tongue.

To force fluid redistribution from the lower to the upper body, to simulate effects in patients with fluid overload, the subjects wore medical anti-shock trousers that exert pressure on the legs when inflated.

In the control condition, the subjects just rested lying down face up with the trousers deflated while measurements were made. During the test condition, the trousers were inflated causing displacement of fluid.

While lower leg pressure was increased, airflow resistance rose by a significant 40 percent over at 1 minute and a full 102 percent at 5 minutes. Neck circumference also increased significantly from baseline at these time points.

The investigators say that the change in neck circumference is in keeping with displacement of fluid into nearby blood vessels and soft tissues as a cause of the increase in breathing difficulties.

They note that in patients with fluid overload, when lying down, excess fluid in the lower extremities is displaced, and conclude that this may increase obstruction of the pharynx in patients predisposed to obstructive sleep apnea.

SOURCE: American Journal of Respiratory and Critical Care Medicine, December 15, 2006.