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Post-Polio Health (ISSN 1066-5331)

Vol. 6, No. 3, Summer 1990

The Diagnosis of Underventilation Following Polio*

G.T. Spencer, FFARCS, (retired) London, England

*Note: This was the first of two papers presented by Dr. Spencer at the International Symposium on Poliomyelitis in Munich on April 7-9, 1988.

The diagnosis of underventilation after polio is not very different from the diagnosis of any other condition, but maybe it helps to go through it in order, because it can be confused easily with the underventilation and respiratory difficulties which occur in chronic obstructive airways disease. The distinction is important because the treatment is very different and the prognosis, given proper treatment, is very much better.

Firstly, you must be aware that the risk of underventilation after polio exists and that any other unrelated illness, operation or anesthetic may cause trouble. Secondly, you must listen very carefully to the story. If I ever had to make do with only one diagnostic method, this is the one I would choose to keep. It usually gives more information than any other single indicator.

Symptoms

I am going to list the various symptoms of underventilation that I have seen in 150 patients with polio over 20 years. Many of them were thought by the subjects who experienced them and even by their medical advisors to be caused by quite different things.

Of course not everybody has all these symptoms and people can have some of them from other causes. Anyone who has more than two or three has grounds for suspicion and further investigation.

Signs

The signs of underventilation include quiet speech with fewer words per breath when speaking, or difficulty in speaking for more than a short time. A reduction in breath-holding time and the obvious use of unusual muscles when breathing, for example, the head, shoulders or arms. Difficulty in or a dislike of lying flat is particularly associated with paralysis of the diaphragm (that is the sheet of muscle between the chest and abdomen during breathing), and the special sign of scoliosis. Cyanosis (blueness of the lips and finger nails) is a very late sign of underventilation and its absence should not be regarded as sufficient reassurance that underventilation is not occurring.

Measurements

Now, at last and quite low down the diagnostic list, we get to measurements.

Forced Vital Capacity: Undoubtedly the most important is the forced vital capacity which has to be measured in several positions, for example, lying, sitting, and standing where that is possible. Serious underventilation is unusual if the forced vital capacity is over three litres, though it can happen if other problems are present.

Pulmonary Function Tests: Unfortunately, in people with weak muscles and low vital capacities following polio, formal lung function tests can be seriously misleading. Many of the measurements, for example FEVI, are designed to measure the severity of chronic obstructive airways disease and not underventilation due to a restrictive defect such as muscle weakness.

Polycythaemia: This term indicates an increase in the red cells of circulating blood and is commonly part of the body's adaptation to chronic underventilation particularly at night. It is not dissimilar to the polycythaemia seen in mountaineers who acclimatize to life at high altitude and it is interesting that we have seen several patients with mild post-polio underventilation who got into very serious trouble after spending even a single night at high altitude while on holiday, or on a long overnight aircraft flight where cabin pressure can be reduced to the equivalent of around eight thousand feet.

Measurement of the Tension of Oxygen and Carbon Dioxide in the Arterial Blood: While this is the most direct measurement of ventilation, the levels can often be normal during wakefulness by day, only becoming abnormal during sleep. Indeed, as is widely known, underventilation after polio occurs primarily during sleep and to prove that this is happening requires an overnight study of breathing during sleep. This must include a measurement of carbon dioxide tensions which is more difficult to measure reliably from the skin surface than is oxygen tension or saturation and many purported sleep studies can be misleading if both oxygen and carbon dioxide tensions are not measured repeatedly and regularly during the night.