Fainting is the most common complication of Spirometry. Although it rarely results in injury, it is disturbing to both subject and technician and may be hazardous in some instances. For safety of the subject, piece of mind of the technician or nurse, and for optimal test quality, it helps to understand fainting and how to prevent it.

The Forced Vital Capacity (FVC) maneuver, the basic maneuver performed during Spirometry, requires an inhalation to total lung capacity following by a maximal blasting effort which is then sustained to blowout all of the subject’s air as rapidly and completely as possible. This maneuver is demonstrated with the flow volume curve. The initial blast produces the sharp peak at the beginning of the maneuver; continued effort causes expulsion of air at the maximal achievable flow

Rate. Sustained effort results in complete exhalation. If you monitor the pressure in the chest during the FVC maneuver, you will find a very high intra‑

Thoracic pressure during the earliest part of the maneuver. Some subjects may sustain this at a high level. The effect of this increased intra‑thoracic pressure is to form a large pressure gradient for blood, which is returning to the heart through the veins. If the intra‑thoracic pressure is high enough, it can markedly reduce venous return to the heart, which results in a fall in cardiac output and blood pressure. The reduced blood pressure or cardiac output can cause a drop in

Blood supply to the brain and other organs. This drop in blood pressure, if large enough and sustained long enough, can cause a loss of consciousness or fainting.

Why doesn’t this happen all of the time? Although it is necessary to develop a high intra‑thoracic pressure to achieve peak flow, it is not necessary to maintain such a high pressure to maintain maximal flow during the latter phase of the expiratory effort. For most subjects, intra‑thoracic pressure is high for only 13 seconds. After that, pressure falls to a low value during the late ~ expiratory effort. In some subjects, particularly muscular subjects with airways obstruction

(Chronic bronchitis, emphysema, and asthma) the presence of airflow obstruction prevents the fall in intra‑thoracic pressure. The more muscular subjects are capable of maintaining a high pressure for a prolonged period of time. These are the subjects who are prone to fainting. How can you prevent fainting? Since a high intrathoracic pressure is needed only for the first 13 seconds of the maneuver, it is possible to coach subjects to avoid lightheadedness or fainting. For

Subjects who have had previous experience with lightheadedness, or in whom you see evidence that they might faint, you can coach them to do a more relaxed expiratory effort during the latter phase of the forced vital capacity maneuver (e.g. “O.K. BLAST!” “Now keep blowing, keep blowing…o.k. Now keep blowing but not so hard, keep blowing but not so hard…) With practice you can coach your subjects through this maneuver without lightheadedness and fainting. How can you be sure you are getting good results? Since maximal flow is limited during most of the expiratory maneuver, a less forceful effort in late expiration can still produce maximal flow. You can compare the flow volume curves of sequential efforts to be sure that maximal flow has been reached (the flow volume curve, FVC and FEV1 should be very comparable). In summary, fainting is due to high intrathoracic pressure during the forced vital capacity maneuver. It occurs most commonly in muscular subjects with airflow obstruction. Cautiously observing your subject and coaching them to do a slightly less forceful, but can avoid it

Still sustained, effort during the late portion of the forced vital capacity maneuver.

Courtesy, Mayo Foundation.