Initial orthostatic hypotension induced by rising from squatting: Difference between revisions

A 37-year old female patient, an enthusiastic horse rider underwent cardiovascular reflex assessment after losing consciousness. The episode occurred after she squatted to bandage the legs of her horse (a routine equine procedure to support the distal tendons in the extremities) and then stood up. The patient reported that before the loss of consciousness she saw black spots and felt lightheaded.  She was unconscious for less than one minute and was well oriented when she regained consciousness [1].

Based on this reproducible blood pressure change induced by the squat-stand manoeuvre and the patient’s recognition of symptoms similar to those she had experienced spontaneously, initial orthostatic hypotension was identified as the cause for her loss of consciousness [1].  Leg crossing with muscle tensing immediately after standing diminished the fall in blood pressure (Fig. 2).  

Rising erect from squatting is a considerable hemodynamic, i.e. orthostatic stress [2]. On average blood pressure in healthy young adults falls by  60 mmHg systolic and  40  mmHg diastolic with a nadir  about  7 s after rising (Fig. 3) [3,4]. Mild symptoms of transient lightheadedness are often present.

[[File:Casusrisingfromsquatting_Fig3.jpg | thumb | left | Figure 3. Average intra-arterial blood pressure and heart rate changes induced by squatting in four 21-40 years old healthy males. From reference  3.]]

Two factors are likely to be involved in the rapid fall in pressure. First, a sudden decrease in total peripheral resistance in the legs due to the ischemic effect of squatting enables a very rapid inflow of arterial blood in the legs. Secondly, marked pooling of blood in the venous vessels in the legs and abdomen, which have been squeezed in the squatting position results in a decrease in venous return and thereby in cardiac output. The combination of the two factors results in a rapid translocation of a large amount of arterial blood from the chest to the distensible venous capacitance system below the diaphragm. The accelerative force during rapid standing up may play an additional role in the fall in pressure [5].   

Rising to erect from squatting is a recognised trigger for a faint in daily life (see Case 2 in this section).  Squatting in combination with hyperventilation and straining can induce a faint in almost everybody (fainting lark) (see Case in this section 4). In the 1960-70’s  rising to erect from the squatting position was widely reported in the German literature to assess initial orthostatic adjustments [6,7]. The squat-stand test was introduced recently in the English literature as an orthostatic stress test [8,9].  

Treatment for symptomatic initial orthostatic hypotension is largely unknown. An increase in salt intake has been advised (see case 1 in this section). The present case and additional recent observations [10] strongly suggest that lower body muscle tensing immediately after standing up is effective to prevent the abnormally large initial fall in pressure upon standing.