The "fainting lark" consists of squatting in a full knee bend and hyperventilating. The subject then stands up suddenly and performs a forced expiration against a closed glottis. The procedure combines the effects of acute orthostatic stress, straining and hyperventilation. The latter doubles skeletal muscle blood flow and halves cerebral blood flow. The maneuver provokes a precipi- tous fall in finger arterial pressure and diastolic cerebral blood flow velocity suddenly drops to zero (Fig. 1). The subject rapidly loses consciousness and collapses with- out warning symptoms. In the supine position systemic blood pressure and cerebral blood flow velocity are rapidly restored and the subject regains consciousness. The "fainting lark" has been used by children, high school students and recruits as entertainment for their friends (Howard, 1951), but also as a research tool to study the sequence of clinical events during syncope (Lempert, 1994). The time course of the hemodynamic events involved in the "fainting lark" is much more abrupt than it is in spontaneous vasovagal syncope.
