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Physical countermeasures to increase orthostatic tolerance

2014; Wiley; Volume: 277; Issue: 1 Linguagem: Inglês

10.1111/joim.12249

1365-2796

Wouter Wieling, Nynke van Dijk, Roland D. Thijs, Frederik J. de Lange, C. T. Paul Krediet, John R. Halliwill,

Cardiovascular and exercise physiology

Standing upright challenges the cardiovascular system as the pull of gravity displaces about 70% of the circulating blood volume to below heart level, much of it to the compliant veins of the dependent limbs and the pelvic organs. In patients with autonomic failure due to neurodegenerative diseases, the normal cardiovascular adjustments to this challenge are impaired, and symptomatic orthostatic hypotension becomes a common risk on standing or even sitting quietly. These patients learn to sway and shift, so that the pumping action of the muscles can be utilized to counter gravitational displacement of blood by squeezing venous blood from the legs upward. Augmentation of venous return in the upright posture can also be achieved by deliberate tensing of lower limb and abdominal muscles 1, 2, as depicted in Fig. 1. From 2; reproduced with permission. These clinical observations were the basis for physical countermeasures, which are taught to patients with autonomic failure to combat symptomatic orthostatic hypotension 3-5. Physical counterpressure manoeuvres specifically generate a counterpressure to oppose gravitational venous pooling (e.g. a single bout of lower-body muscle contraction to translocate blood centrally and sustained tensing of the same muscles to prevent subsequent peripheral pooling in the legs and abdomen). More recently, it has been shown that physical counterpressure manoeuvres are also effective interventions in otherwise healthy subjects with episodic orthostatic syncope due to neurally mediated (i.e. vasovagal reactions) 6, 7 or postexercise syncope 8. In this narrative review, we will primarily consider these physical counterpressure manoeuvres. Secondarily, we will describe the broader category of physical countermeasures that include breathing manoeuvres and other physical methods, to oppose orthostasis. Existing external devices, which operate through some of the same physiological principles as these manoeuvres, will only be discussed for proof of principle. The defining characteristic of the manoeuvres described in this review is the fact that they can be employed by patients when a faint is imminent. This is in contrast to devices such as bandages and abdominal belts, which require ongoing use to be effective. We will discuss both early studies in patient with primary autonomic failure due to neurodegenerative diseases, as well as more recent experience obtained in patients with neurally mediated syncope. The physiology and pathophysiology of orthostatic blood pressure control and perfusion of the brain are key factors in understanding how physical countermeasures work. These topics have been reviewed extensively 2, 9-12 and will only be discussed here briefly. It has been reported that intramuscular pressure is related to orthostatic tolerance 2. Henderson et al. demonstrated that intramuscular pressure measured in the relaxed biceps muscle was decreased after prolonged bed rest (38%), following surgery (35%), during voluntary hyperventilation (28%) and in the absence of air movement over the skin (31%) 13, 14. These conditions are strongly associated with decreased orthostatic tolerance and a tendency to faint 2, 15. In addition, intramuscular calf pressure has been shown to be 15–24 and 6–9 mmHg, respectively, in those without and with a tendency to faint during the head-up tilt test using a tilt table with a saddle and suspended legs (Fig. 1) 16. Although these interesting results from studies performed in the 1930s and early 1940s have never been confirmed, it is highly likely that any increase in muscle tension will function to augment intramuscular pressure. Intramuscular pressure can be thought of as a pressure opposing that within the veins. As such, venous distension is determined by the difference in the opposing pressures on each side of the venous wall (i.e. the venous transmural pressure). Increasing pressure outside the vein will therefore reduce venous distension, displacing blood back towards the heart 2. During quiet standing, the body behaves more or less as an inverted pendulum that sways about the ankles. The static increase in tone of the antigravity muscles that are involved in maintaining upright posture also function to oppose venous pooling in lower limb veins, thereby protecting central blood volume, i.e. the amount of blood available for the heart to pump 13, 14, 17-19. It is considered that postural sway during quiet standing is able to compensate for otherwise poor orthostatic tolerance 20, 21. Along these lines, Amberson 22 suggested the possibility of a connection between arterial baroreceptors and skeletal muscle tone, which could serve to increase muscle tensing during orthostasis. Although the precise neural pathway has not been established 2, recent work by Bernardi et al. demonstrated that carotid baroreflex modulation influences postural sway 23. The first reports of the application of skeletal muscle tensing to prevent fainting reactions were from psychologists interested in the prevention of fainting reactions due to haemophobia. In the 1980s Öst and Sterness reported that ‘applied tension’ could be used as a behavioural method for treatment of this phobia 24, but the physiological mechanisms underlying its effect remained poorly understood due to the lack of haemodynamic measurements. However, the development in the early 1990s of the Penaz-Wesseling volume-clamp method, combined with the computation of stroke volume by pulse wave analysis, commercially available as the Finapres device, enabled clinical researchers to combine the experiences of individual patients with continuous noninvasive measurements of beat-by-beat changes in arterial pressure 25, 26. As a result, the underlying haemodynamics of a wide range of movements that simulated every day activities could be investigated, first in patients with symptomatic orthostatic hypotension due to autonomic failure 1, 3 and in recent years as a countermeasure to avert an impending vasovagal faint. Single case reports were published at first 1, 27-31. Figure 2 shows an example of such work, in which the combination of leg crossing and leg muscle tensing is effective in counteracting an impending vasovagal syncope 32. From 32; reproduced with permission. Further evidence came from a study by Krediet et al., which included 20 patients 6. This work confirmed that the combination of leg crossing and leg muscle tensing depicted in Fig. 2 is highly effective. A rise in blood pressure was observed in all 20 subjects, and the vasovagal reaction was averted in five of these individuals. The remaining 15 subjects were able to postpone the faint by an average of 2.5 min. Patients who could completely abort the faint started the manoeuvre at a significantly higher blood pressure level than those patients who could not (79/51 vs. 61/41 mmHg). In a study focusing on the underlying haemodynamic mechanism, Krediet et al. 33 demonstrated that physical counterpressure manoeuvres such as leg crossing, muscle tensing, squatting and the crash position are effective against vasovagal reactions solely through increases in cardiac output as shown in Fig. 3. From 33; reproduced with permission. During the manoeuvres involving muscle tensing, cardiac output increased by a factor of 1.3–1.7 from the low levels during presyncope and was restored to 95–104% of the stable values recorded in the head-up position in the first few minutes of tilt 33. Systemic vascular resistance responses varied, but remained largely unchanged. Because lower-body muscle tensing is accompanied by a threefold increase in leg blood flow 34, a counteracting presumably reflex-mediated vasoconstriction must occur in other parts of the circulation, such as the nonworking muscle, kidney and splanchnic vascular beds. The rise in cardiac output during muscle tensing is largely attributed to mechanical and not to autonomic effects. The change in cardiac output as produced by leg crossing with muscle tensing is strikingly similar to that which is produced by inflation of an antigravity suit, which is similarly effective at aborting an impending vasovagal faint (see Fig. 4) 35. This reinforces the notion that the physiological effects of muscle tensing are mainly mechanical. From 35; reproduced with permission. However, an instantaneous increase in heart rate (see Figs 2 and 3) was also observed during muscle tensing. This indicates that autonomic effects are present as well. The instantaneous increase in heart rate at the onset of muscle tensing is a reflex effect produced by a combination of the muscle mechanoreflexes and central command with inhibition of cardiac vagal tone 26. Such chronotropic changes at the onset of exercise are generally associated with concurrent increases in cardiac contractility, which may contribute to the increased cardiac output 33. It is worth noting that forceful arm tensing manoeuvres, i.e. hand gripping at maximal voluntary force using a rubber ball and arm tensing by gripping one hand with the other and abducting both the arms at the same time 7, 36, 37, are also effective if they are accompanied by whole-body muscle tensing and thereby by an increase in cardiac output. Isometric arm exercises without tensing of large lower-body muscle groups are far less effective and cannot prevent an impending vasovagal faint 6, 38. Activation of the muscle venous pump of the legs during tiptoeing or walking, in the presence of competent venous valves, pumps blood back to the heart and partially restores cardiac filling pressure. The leg muscle pump can be considered as a ‘second heart’ 2 and is capable of translocating blood against a substantial pressure gradient (e.g. >90 mmHg). Manoeuvres that use skeletal muscle pumping are heel raises (i.e. plantar flexion; rising on the toes using calf muscles to raise heels off the floor) and repeated knee flexion (i.e. marching in place) 4, 16, 39. However, their effects on standing blood pressure in patients with autonomic failure vary. The variable responses may stem from differences in the degree of sympathetic vasomotor failure in these patients 40, 41. Knowledge that bending forward can mitigate orthostatic hypotension dates back to the 1930s 42 i.e. to the time of the first description of patients with idiopathic orthostatic hypotension in the English literature by Bradbury and Eggleson. It is a useful manoeuvre for patients with autonomic failure to increase blood pressure in the upright posture, as has been reported by many investigators 1, 41, 43 and is shown in Fig. 5. From 1; reproduced with written informed consent of the patient and permission from the publisher. The beneficial effect of bending forward in patients with autonomic failure can be ascribed to pronounced abdominal compression and to lowering the head to heart level. Abdominal compression squeezes blood from the compliant splanchnic venous pool towards the heart, resulting in an increase in cardiac output and thereby in arterial pressure 44, 45. Additionally, lowering the head to heart level shortens the hydrostatic column between the heart and the brain instantaneously by 25–30 cm corresponding to a hydrostatic pressure increase of 15–20 mmHg in mean blood pressure 11. In patients prone to vasovagal syncope, bending forward is also reported to be a useful manoeuvre to increase orthostatic tolerance. Treatment of fainting patients traditionally consists of lowering the head between the knees whilst sitting (Fig. 6) 46-49. Likewise, bending forward with hands on knees appears to be a preferred position for many athletes during recover from vigorous physical activity. From 49; reproduced with permission. The beneficial effect of leg crossing in patients with autonomic failure (Fig. 7) 1, 43, 50, 51 has been attributed to mechanical compression of the veins in the legs, buttocks and abdomen, which displaces gravitationally pooled blood towards the heart and increases thoracic blood volume 39, 52, 53. This results in an increase in cardiac filling pressure, stroke volume and cardiac output, effectively correcting the symptom-causing reductions in systemic arterial pressure and cerebral blood flow. From 32; reproduced with written informed consent from the patient and permission from the publisher. When leg crossing is practiced routinely, standing systolic/diastolic blood pressure can be increased by ~20/10 mmHg in patients with autonomic failure 3, 4, 9, 39, 43. Even such a small rise in upright blood pressure may be clinically important, as it may shift mean arterial pressure from just below to just above the critical level of perfusion of the brain 10. Larger increases of ~30/15 mmHg can be seen when leg crossing is combined with the additional tensing of the leg musculature, thighs and buttocks. Leg crossing improves orthostatic tolerance in healthy subjects as well as in patients with vasovagal fainting 27, 54-56. When standing for prolonged periods, healthy humans who have a tendency to faint often unknowingly utilize this leg crossing countermeasure (i.e. the ‘cocktail party posture’ serves a physiological purpose). By sitting down, the orthostatic load due to gravitational displacement of blood is decreased, resulting in increases in venous return, stroke volume and cardiac output and thereby blood pressure is increased 57, 58. Portable chairs have been shown to be quite useful for patients who are severely incapacitated by their orthostatic symptoms 59. We have shown that the beneficial effect of sitting is greater, i.e. blood pressure increases more, when using lower portable chairs 60. A chair height of about 40 cm may be optimal for many patients, being effective in raising blood pressure and yet not so low as to cause difficulty in rising, although this may be more of a concern for patients with neurodegenerative diseases with motor disability 9, 59. Leg crossing can increase seated systolic blood pressure considerably in patients with autonomic failure (Fig. 7) 5, 60, 61, whereas the effects in healthy normotensive subjects (on average <2 mmHg) and patients with hypertension (on average 65 years of age 36; however, the number of patients involved was small and, under laboratory conditions, the pressor effect of muscle tensing in fit elderly subjects is at least as great as in young subjects 54. Squatting was not part of the counterpressure manoeuvres in the PC trial, but its great effectiveness under laboratory conditions is clear. Thus, an additional large prospective randomized controlled trial is not needed 88. The subtle but significant effects of physical counterpressure manoeuvres, such as leg crossing or squatting, on a low standing blood pressure are difficult to monitor by sphygmomanometer. A continuous (ambulatory) noninvasive blood pressure device, such as Finapres 25, 26, enables quantification of their effects in detail. The changes in blood pressure can be demonstrated immediately to a patient by showing the finger blood pressure tracing on a video screen in the doctor's surgery. This biofeedback will demonstrate to patients the effectiveness of their manoeuvres and will also help to select effective manoeuvres and to relate symptoms to actual blood pressure readings. Patients can thereby practise applying the manoeuvre effectively whilst being coached, possibly by a specialist nurse practioner 4, 32. Physical countermanoeuvres often need to be modified specifically for individual patients depending on their abilities. They may be hard to perform in patients with multiple system atrophy. In the elderly, crossing the legs and pressing them together may lead to destabilisation, causing them to fall over. However, the buttock-clenching manoeuvre is often possible in the elderly 3. In our experience, this manoeuvre is effective to combat initial orthostatic hypotension. Patients should be instructed in how to perform muscle tensing without raising intrathoracic pressure, as raising intrathoracic pressure impedes venous return to the heart and may cause blood pressure to drop and lead to lightheadedness in patients with neurogenic orthostatic hypotension 11. Patients should also be advised to avoid deep breathing and consequent hypocapnia during physical manoeuvres, because hypocapnia causes vasodilatation in skeletal muscle and vasoconstriction in the cerebral vessels both in patients with autonomic failure and in those with a tendency to vasovagal fainting 84, 85, 89. Close observation whilst practising the manoeuvres may be useful to alert patients to this habit. A great advantage of physical countermanoeuvres is that they can be applied instantaneously at the moment of symptomatic low upright pressure. They thereby give the patient the opportunity to regain self-confidence in provocative situations. Gradual exposure to specific provocative conditions may be of use to regain self-confidence. Patients may benefit from practising leg and lower-body muscle tensing whilst standing motionless each morning as part of their daily routine 32. A video demonstrating useful counterpressure manoeuvres and illustrating the direct effect they have on blood pressure is available on the patient website www.stars.org.uk. We have found the following practical patient recommendations to be helpful (Table 1). First, apply leg crossing or skeletal muscle pumping using heel raises or marching in place as a preventive measure. Leg crossing has the advantage that it can be performed casually without much effort and without drawing attention to oneself. With proper instruction and practice, many patients will begin to automatically apply leg crossing in daily life to prevent the feeling of lightheadedness or faints during quiet standing. Leg crossing can also be used to prevent these symptoms in the sitting position in patients with reflex syncope (Wieling and Krediet, unpublished observations). Secondly, when leg crossing is insufficient to prevent symptoms, patients can try adding leg muscle tensing and buttock clenching. Whole-body muscle tensing, for example with arm tensing by gripping one hand with the other and abducting both the arms at the same time, can also be used. Buttock clenching is also very effective to combat orthostatic hypotension upon standing (initial orthostatic hypotension). Finally, squatting, which is the most effective physical manoeuvre to increase blood pressure, can be used as an emergency measure to prevent losing consciousness when fainting symptoms develop rapidly. Likewise, bending over as if to tie shoe laces has similar effects to squatting and is simpler to perform by elderly patients 1, 18. When arising again from the squatted position, patients should immediately sit down or begin lower-body muscle tensing to prevent the return of symptoms (Fig. 8 68). The beneficial effect of physical countermeasures, based on the keen observations of astute clinicians in the first half of the 20th century, is an excellent example of how therapies that help many patients may be based on clinical observations in small groups or even individual patients 90, 91. In summary, physical countermeasures are simple, inexpensive techniques that have a strong biological rationale based on experiments conducted in the physiology laboratory. These techniques can be applied instantaneously at the moment of symptomatic low upright pressure. Furthermore, they are clinically effective evidence-based interventions without side effects that improve quality of life in patients with orthostatic intolerance. There are no conflicts of interest.