Integrating physiologic principles into the comprehensive management of cardiopulmonary dysfunction.

Integrating Physiological Principles into the Comprehensive Management of Cardiopulmonary Dysfunction Hislop originally proposed that pathokinesiology is the underlying scientific discipline of physical therapy. [1] Using such an approach, she described the three principal goals of physical therapy as follows: 1) to restore motion homeostasis to the person or to the person's subsystems, 2) to enhance the adaptive capacities of the person to impairment, and 3) to prevent the disruption of motion homeostasis. She proposed that physical therapy attempts to nullify the effects of disruptive forces or potential disturbances that are manifested as motion disorders. Recently, Zadai discussed the clinical implications of Hislop's framework with respect to cardiopulmonary physical therapy. [2] The purpose of this article is to illustrate how the integration of specific physiological principles can further extend the contributions of Hislop and Zadai to cardiopulmonary physical therapy.

The movement or transport of oxygen in the body depends on optimal pulmonary and cardiovascular function including ventilation of the alveoli, diffusion of gases across the alveolar capillary membrane, perfusion of blood to the lungs, and gas transport to the tissues. [3] When one or more of these components fails, arterial hypoxemia, the hallmark of cardiopulmonary dysfunction, is precipitated. [4] A particularly common cause of arterial hypoxemia is mismatching of ventilation and perfusion in the lung. [4]

Considerable evidence exists supporting the beneficial effect of specific body positioning and mobilization on oxygen transport, in particular on ventilation-perfusion matching, thus arterial oxygenation. [5-15] Both ventilation and perfusion increase from the top to the bottom of the upright lung. Thus, the lowermost lung zones are both better ventilated and perfused than the uppermost lung zones. Optimal ventilation-perfusion matching occurs in the midzone of each lung. During exercise, however, both ventilation and perfusion increase, and regional differences are less pronounced. Thus, oxygen transport and ventilation-perfusion matching throughout the lungs are both increased. We propose that these physiological principles can be integrated into cardiopulmonary physical therapy to enhance treatment efficacy.

Specifically, this article discusses the role of body positioning and mobilization as therapeutic interventions to directly enhance oxygen transport in patients with cardiopulmonary dysfunction. Body positioning refers to positioning an individual to optimize oxygen transport. Mobilization, in the context of cardiopulmonary care, refers to the administration of progressive exercise to elicit hemodynamic responses to gravity and to stimulate increases in ventilation, thereby increasing oxygen transport. The negative consequences of immobility, in the form of bed rest, are presented first to provide a framework for the discussion on body positioning and mobilization. We have selected clinical examples related to the critically ill individual to illustrate the effects of body positioning and mobilization on impaired oxygen transport. The physiological principles described, however, can be applied to any patient with cardiopulmonary dysfunction.

Consequences of Immobility

Until the 1940s, when the negative effects of immobility were better understood, many disorders involving the cardiovascular system were commonly treated with prolonged periods of bed rest. At that time, Harrison emphasized that activity should be limited only to the point of reducing patients' symptoms resulting from their particular pathological condition. [16] An important consequence of immobility on the cardiovascular system is orthostatic intolerance resulting from the shift of body fluids into the thorax from the extremities and the loss of the stimulus of gravity needed to maintain hemodynamic status in the upright position. [13,17-19] This fluid shift, which occurs after only six hours of bed rest, is more instrumental than reduced physical activity in promoting orthostatic intolerance. [13] For example, it has been shown that although bed exercises during a period of bed rest prevented loss of muscle strength in healthy subjects, orthostatic intolerance was not affected. [19,20] Chase et al observed that vigorous exercise performed at 75% of maximum oxygen consumption ([VO.sub.2 max]) for 45 minutes a day also did not prevent orthostatic intolerance. [21] In addition, Miller et al reported that the exercise tolerance of healthy subjects who performed daily supine cycling exercises was comparable to that of subjects who had not exercised, when tested in the upright position following a period of bed rest. [22] Thus, Winslow concluded that measures to counteract fluid shift directly are more important than exercise alone in minimizing the negative effects of bed rest on cardiovascular hemodynamics. [13] The results of these studies strongly support the use of mobilization involving a gravitational stimulus in patients immobilized by bed rest. This form of mobilization is particularly beneficial for acutely ill patients for whom the consequences of orthostatic intolerance may be severe.

With respect to the pulmonary system, immobility promotes monotonous tidal ventilation, which compromises the ventilation of the dependent lung fields. Consequently, airway closure, atelectasis, secretion retention, and interstitial fluid accumulation may occur. The specific distribution of these changes within the lung is affected by the body positions that the patient assumes. [6,8,10-12] Crosbie and Myles, for example, investigated the effect of the slumped Fowler's position commonly assumed by hospitalized patients on the lung volumes of healthy subjects and concluded that this position could precipitate pulmonary complications. [23] Craig et al reported significant decreases in lung volumes with closure of inferior airways in the posterior lung fields when patients changed from the upright to the supine position. [24] Froese and Bryan further studied the distribution of ventilation in the dependent lung zones of an anesthetized paralyzed subject in the supine position. [8] Their conclusions were significant in that effective ventilation of inferior regions was achieved only by manipulating body position. Considering the effect of body position on oxygen transport, the body position between treatments is clearly as important as that during treatment.

In addition to its marked effect on the cardiopulmonary system, immobility has other systemic consequences including decreased total blood volume and decreased hemoglobin concentration, [25] increased resting heart rate, [26] and decreased [VO.sub.2 max]. [25,27] Immobility promotes fluid stasis in the kidneys, which may lead to kidney stones and infection. [28] Furthermore, increased calcium excretion, [20] musculoskeletal changes, [29] and emotional and behavioral disorders [30] have been associated with prolonged bed rest. The cardiopulmonary system is unique in that its function affects all other body systems, which in turn influence overall cardiopulmonary efficiency. Thus, the multisystem sequelae of immobility are of considerable clinical significance because of their potential to cause or exacerbate cardiopulmonary dysfunction.

Role of Body Positioning

and Mobilization

Body positioning alone can be an effective means of enhancing oxygen transport via improving ventilation-perfusion matching in the lung. The physiological principles underlying the use of body positioning specifically for optimizing ventilation-perfusion matching and arterial oxygenation are well established [6] and thus will not be discussed further. Body positioning, however, may also be applied to address each component of ventilation-perfusion matching. Murphy et al, for example, studied the effects of postural drainage positions on the pulmonary function of patients with cystic fibrosis. [31] These investigators observed that postural drainage positions alone had a beneficial effect on peak expiratory flow rate, forced expiratory volume, and forced vital capacity and that this effect was not augmented with other physical therapy techniques such as manual percussion, mechanical percussion, and forced expiratory maneuvers. In addition, Chulay et al studied the effect of routine body positioning on the incidence of pulmonary complications following coronary artery bypass surgery. [5] One group of patients remained in the supine position for the first 24 hours postsurgery, and a second group of patients was turned to alternate sides every two hours during this time period. Turning resulted in a significantly decreased incidence of postoperative fever and a 32% reduction in length of stay in the intensive care unit. Despite the beneficial effects of routine turning in this study, these effects may have been enhanced if position selection was specific to the underlying. pathophysiology for each individual. Although the effect of body positioning on pulmonary perfusion has been well established, [5-15] specific clinical studies to examine the therapeutic use of body positioning to improve pulmonary perfusion are lacking.

In addition to the specific pulmonary pathophysiology, other factors must be considered when using body positioning to enhance pulmonary function. Hurewitz et al, for example, reported a decrease in ventilation in the dependent lung fields of a patient with an obese abdomen. [32] An analogous situation occurs in patients undergoing peritoneal dialysis or following major abdominal surgery.

The normal aging process also effects the distribution of ventilation via a loss of elastic recoil and thus the normal negative intrapleural pressure that acts as an expansive force on the alveoli. This altered intrapleural pressure results in greater ventilation to the apex rather than the base of the upright lung because of compression of dependent airways. [33] As a consequence, ventilation-perfusion matching is compromised. Furthermore, the influence of age on the distribution of ventilation is magnified by the body positions the individual assumes. Leblanc et al, for example, reported that airway closure occurred in the dependent airways of a 44-year-old subject positioned supine andin a 65-year-old subject sitting upright. [10] Breathing at low lung volumes also decreases the elastic recoil forces and thus has the same effect as age on the distribution of ventilation. [3] In addition, factors such as the individual's multisystem status and medical interventions will limit the positioning alternatives. Thus, the effective application of body positioning to improve pulmonary function challenges the physical therapist to address these various factors in identifying optimal body positioning for each patient.

Although selective body positioning can significantly affect oxygen transport, it will not fully address the negative consequences of immobility for which mobilization is essential. The goal of mobilization is to elicit cardiopulmonary responses sufficient to increase minute ventilation and cardiac output while maintaining normal hemodynamic responses. A singularly important consequence of such graded exercise for the immobilized critically ill individual is the recruitment and distension of lung zones with low ventilation and those with low perfusion. [3] As a result, these zones have improved ventilation-perfusion matching and, therefore, a greater contribution to oxygen transport. [15,34] These effects can only be achieved if the exercise stimulus is sufficient to elicit an adequate cardiopulmonary response and is not in excess of the patient's oxygen transport capacity.

Relatively few studies have investigated the responses of patients with acute and critical cardiopulmonary conditions. Dull and Dull investigated the effect of early mobilization on cardiopulmonary status in patients following coronary artery bypass surgery. [7] They concluded that the beneficial effect of early mobilization was not enhanced by the addition of maximal inspiratory breathing exercises or incentive spirometry. Wolff et al studied the effects of exercise and eucapnic hyperventilation on bronchial clearance in healthy subjects using radioactive isotopes. [14] They reported that exercise significantly increased secretion clearance in the exercise group as compared with the control group, which performed quiet breathing exercises at rest. In addition, this effect was greater in the exercise group than in the group performing resting eucapnic hyperventilation. These studies provide support for the beneficial effects of mobilization on cardiopulmonary function.

Guidelines for the prescription of mobilization for the acutely ill patient, however, are needed. Physical therapists have had to rely on their clinical judgment to define the limits of exercise intensity for acutely ill patients. With advancements in technology and the availability of sophisticated monitoring equipment, such as telemetry and arterial saturation monitors, acutely ill patients can be mobilized effectively and safely. Research is warranted to refine clinical guidelines for progressively mobilizing these patients to further improve the safety and efficacy of this procedure.

Implifications

Extending current definitions of cardiopulmonary physical therapy practice to emphasize all aspects of oxygen transport is consistent with the scientifically based approach to practice advocated by Hislop [1] and Zadai. [2] A physiological approach to cardiopulmonary physical therapy as described may also have an important role in explaining some of the apparent limitations of current methods of practice reported in the literature. For example, ventilation is often the focus of treatment with less attention given to overall oxygen transport. Thus, impaired ventilation is frequently attributed to alterations in breathing pattern or mucous retention, which are considered to be the basis for the primary pathophysiology and, therefore, the focus of treatment. [35,36] The physiological approach, however, places greater emphasis on the pathological mechanisms underlying these clinical signs. This emphasis can be exemplified by the incidence of pulmonary complications in 23% of patients who have received conventional cardiopulmonary physical therapy after upper abdominal surgery. [37] The high incidence of postoperative complications seen in these patients has been explained by reflex phrenic nerve inhibition, and blocking this reflex or increasing phrenic nerve output has been suggested. [38,39] We could infer, therefore, that selective body positioning may prevent encroachment of the viscera on the thorax (because of the loss of the diaphragmatic barrier) and thereby directly optimize ventilation.

Some evidence exists that does not support the efficacy of physical therapy techniques, such as percussion, in achieving secretion clearance and optimizing ventilation. [31,35,40-44] In part, this evidence may reflect deficiencies in the experimental designs of reported studies. For example, variables such as body position and treatment outcome measures have been poorly standardized. [35] Thus, Kirilloff et all concluded that although postural drainage can be effective in improving the status of patients with cardiopulmonary dysfunction, no convincing data exist showing that this effect is enhanced with the addition of percussion and vibration techniques. [35] Furthermore, vibration and percussion have been shown to compromise cardiopulmonary function in some instances. [44,45] Connors et al, for example, reported a significant drop in arterial oxygen pressure in patients with acute cardiopulmonary dysfunction treated with postural drainage in combination with percussion and vibration in the presence of minimal sputum production. [44] They attributed this effect to impaired ventilation-perfusion matching and suggested that these techniques were potentially hazardous in this patient population. Campbell et al reported an immediate decrease in forced expiratory volume in one second ([FEV.sub.1]) during postural drainage combined with percussion and vibration that did not occur with postural drainage alone. These investigators attributed the decrease in [FEV.sub.1] to bronchospasm, which they concluded was induced by percussion. [45] Further research and application of the physiological approach will elucidate its role in addressing these limitations of current cardiopulmonary practice.

Conclusions

Integrating physiological principles to optimize oxygen transport extends the pathokinesiologic perspective of practice advanced by Hislop [1] and Zadai. [2] Furthermore, this approach is directed at the underlying pathophysiology and thus may enhance the efficacy of cardiopulmonary physical therapy. The application of this approach demands an integration of the patient's multisystem status so that treatment can be prescribed to optimize oxygen transport as a whole. Continuing research is necessary, however, to further refine the application of this approach and explore its full potential.

References

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J Ross, BSR, is Clinical Instructor, School of Rehabilitation Medicine, University of British Columbia, 2211 Wesbrook Mall, Vancouver, British Columbia, Canada V6T 1W5, and Physical Therapist, Critical Care, Vancouver General hospital, 855 W 12th Ave, Vancouver, British Columbia, Canada V5Z 1M9.

E Dean, PhD, is Assistant Professor, School of Rehabilitation Medicine, University of British Columbia.

Address correspondence to Ms Ross at School of Rehabilitation Medicine, University of British Columbia, 2211 Wesbrook Mall, Vancouver, British Columbia, Canada V6T 1W5.

This work was supported in part by the Canadian Lung Association.

This article was submitted April 6 1988; was with the authors for revision for eight weeks; and was accepted November 9, 1988.