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Home , Respiratory system

Prosthetics and Orthotics International, 1991, 15, 193-198

Physiological considerations in seating

C. P. U. STEWART

Dundee Limb Fitting Centre, Broughty Ferry, UK

Abstract In the normal , atmospheric Physiological changes occur with change of diffuses into the and thereafter to cellular posture. mitochondria as the partial of oxygen Seating imposes significant effect on the (pO2) in the cells is lower than in the blood. If the cardiovascular, respiratory, abdominal, renal and arterial pO2 falls, cellular pO2 will also fall with a neurological systems. consequential rise of pCO2 (partial The presence of severe skeletal deformities can pressure of ). The oxygen balance significantly alter the physiological responses of in the body is dependent on both the ventilation the individual to changes in posture. In the case of and the metabolic demands of the tissues. severe kyphoscoliosis profound haemodynamic In situations where occurs, changes may occur. Lung has been a) there is an increase in alveolar and arterial

shown to be posture dependent and the pCO2 imposition of a specific seated position may have b) there is a reduction in the alveolar, arterial and

profound effects. This may compound existing cellular pO2 lung problems for example , which In these circumstances the patient can become is not uncommon in these individuals, leading to hypoxic. hypoventilation. For efficient , ventilation and Abdominal compression which can occur with alveolar perfusion must be balanced. Ventilation­ the patient in a flexed position can exacerbate a perfusion is the ratio of alveolar ventilation over hiatus hernia, which can be both uncomfortable pulmonary blood flow and is used as a measure of for the patient and may lead to feeding ventilation and blood flow changes in various difficulties. The flexion at the hips of the lower situations. If, for example, ventilation and blood limbs may also lead to problems of renal drainage flow are mismatched, then the ventilation- especially where there is a catheter or other perfusion ratio alters giving an indication of the drainage appliance. oxygen and carbon dioxide changes that can be Seating significantly affects many neurological expected, which may have a profound effect on reflexes. For example the presence of an extensor the body. pattern can be helped by the adoption of a flexed The lung has been found to have considerable position. The presence of can also influence regional differences in gas exchange. the neurological response to a specific position. Those providing seating systems must consider the physiological effects that occur and compromise between these and the other requirements.

Cardiovascular and respiratory systems Normal ventilation consists of an equilibrium between air flow through the and perfusion of the lung parenchyma with blood allowing gaseous exchange to occur. Oxygen is removed from the inspired air and carbon dioxide laden air is expired. Fig. 1. Distribution of ventilation and blood flow down the upright lung. Ventilation-profusion ratio (VA/Q) AU correspondence to be addressed to Dr. C. P. U. decreases down the lung. (From J. B. West, Stewart, D.L.F.C., 133 Queen Street, Broughty Ferry, Ventilation: blood flow and gas exchange/3rd edition. — Dundee DD5 1AG, UK. Oxford: Blackwell.)

193 194 C. P. U. Stewart

Fig. 2. Oxygen and carbon dioxide diagram, showing that p02 and pC02 () of all lung units have different ventilation perfusion ratios. (From West, 1987.) Fig. 4. Carbon dioxide dissociation curve for blood of different . Note that oxygenated Ventilation increases from the lung apex to the blood carries less CO- for the same pCO,. (From West, 1985.) lung base primarily due to the anatomical shape of the lung, there being more lung parenchyma at the is obtained by increasing the ventilation, but in base than at the apex. Similarly, pulmonary blood normal situations ventilation itself has little effect flow increases rapidly from lung apex to lung base on pO2 level (Fig. 5). due partly to the anatomical shape and also to the The regional ventilation of the lung can be hydrostatic and gravitational acting on that demonstrated to vary according to the posture blood. As a result the ventilation-perfusion ratio adopted. As described above, ventilation in the is high (> 1.0) at the lung apex and low (< 1.0) at standing position increases from lung apex to base the lung base (Fig. 1). whereas in the supine position these differences This in turn results in different blood gas disappear with apical and basal ventilation being in different regions of the lung. At similar. In the lateral position the dependent lung the lung apex where the ventilation-perfusion becomes the better ventilated (James, 1976). ratio is high the pO2 is high while pCO2 is low. In deformed chests, for example kyphoscoliotic This is because the relative blood flow is low and chests, the anatomical gains greater gaseous exchange can take place at a more importance as does the residual volume. The leisurely pace. The opposite is true of the lung anatomical dead space is "the volume of base (Fig. 2). These regional differences can be exclusive of alveoli" (Fig. 6). quite marked (Fig. 3). In comparison with The deformed chest cannot expand normally and oxygen, carbon dioxide levels have less regional thus lessens the amount of gas exchanged. The differences and are more affected by ventilation. effective aveolar ventilation also becomes This can be seen by reference to the "dissociation reduced with less fresh inspired air available for curve" of carbon dioxide (Fig. 4). Increase in gaseous exchange. In severe deformity, the ventilation raises the carbon dioxide output with effective can be reduced dramatically high or low ventilation ratios. This contrasts with leading to the development of pulmonary the oxygen dissociation curve which shows that emphysema. This is found frequently in with low ventilation-perfusion ratios some benefit kyphoscoliosis (Bergofsky et al., 1959).

Fig. 3. Regional differences in blood gases in different Fig. 5. Oxygen dissociation curve. (Haemoglobin parts of the lung, (From West, 1987.) of 14.5g/100ml). (From West, 1987.) Physiological considerations in seating 195 may follow. This may lead to right ventricular loading with the imbalance of ventilation and blood perfusion, with carbon dioxide retention (Bergofsky et al., 1959; James, 1976). The resulting development of chronic hypoventilation may ultimately lead to cor pulmonale. However this complication is rare under the age of 20 (James, 1959). Later, there may develop a reduced ventilation response to carbon dioxide further reducing the efficiency of ventilation. Increasing scoliosis reduces lung vital capacities. In addition increased cage rigidity has been found to occur with increasing age (Caro and Du Bois, 1961) and may be compounded in spinal disease, such as ankylosing spondylosis and kyphoscoliosis. This coupled with the evidence that chest wall compliance rises with age may Fig. 6. Anatomical dead space. (The volume of the suggest an increased difficulty with , respiratory system exclusive of the alveoli.) requiring more effort. Increasing scoliosis has also The deformed chest has, as a consequence of been shown to result in a fall in the total lung the deformed skeletal shape, a lower than normal capacity and . These changes are lung volume. This can be as much as 63% of the significantly greater if a paralytic scoliosis has predicted level (James, 1976). The vascular occurred. Despite increasing kyphoscoliosis with resistance may be raised due to the extra-aveolar the haemodynamic ventilation changes that vessels becoming narrowed. This results in a follow as deformity progresses, many do not linear reduction in blood flow which can be very develop problems until later. "Young patients poor at the apex. When supine the apical blood with idiopathic kyphoscoliosis are often symptom flow improves with no great change in the basal free and remain so until middle age when those flow. There may be some change in the antero­ with severe deformity may become breathless on posterior plane with posterior (dependant) blood exertion and develop respiratory, or less often, flow exceeding the anterior flow. The changes are cardiac failure" (Dollery et al., 1965). During the explained by the hydrostatic differences of blood early years the lungs can cope with the deficiency in the blood vessels compounded by the increased but as individuals grow older pulmonary vascular resistance. resistance increases and some regional Dollery and his colleagues (1965) reported that hypoventilation may occur and this may be in the some of the changes with regard to the differences scoliotic concavity (Dollery et al., 1965). in ventilation from apex to the base were not There are some for whom respiratory function found in ten studied cases of kyphoscoliosis. The becomes severely compromised as a result of ventilation was found to be fairly even although in bronchiectasis. Bronchiectasis is an abnormal and some older patients ventilation reduction was irreversible dilation of the bronchi, usually found in the lower zone. In the younger cases, associated with inflammation and can be caused ventilation was found to be remarkably even, by the blocking of a by either mucous or despite gross skeletal deformity (Bake et al., inhaled matter. This often occurs in individuals 1972; Dollery et al., 1965). X-rays of with severe skeletal deformity and mental kyphoscoliotic chests showed translucent lungs deficiency. This plug occludes part of the on the concave side suggesting more efficient bronchus trapping air and secretion leading to lungs. The anteroposterior diameter of the lung recurrent , ultimately resulting in on the concave side was found to be reduced and dilation of the bronchi. of the lung occurs thus so also was the lung volume. with recurrent infection leading to some arterial- No evidence of air entrappment occurs but venous shunting. This may be as a result of the poorly ventilated spaces have been described. haemodynamic changes occurring with Blood flow is abnormal not due to the kinklng in enlargement of the bronchial and the the blood vessels but probably due to the development of bronchopulmonary anasto­ foreshortened chest reducing the total height of moses. This loss of vital functional lung tissue the lung tissue. Under-perfusion of lung tissue may be crucial and in those with severe handi­ may however occur and over a period of time, cap it may be extensive, producing an extra­ pulmonary vascular resistance may rise and ordinarily inefficient lung. 196 C. P. V. Stewart The posture achieved by a seating system can have a direct effect on the cardio-respiratory system. Posture can be seen to affect the physiology of the respiratory system. The maintenance of a good respiratory shape by controlling spinal curvature is in the interests of maintaining a good cardio-respiratory function and long term future (Zorab, 1976). In addition, the seated position may be important in the case of severely handicapped individuals in whom normal physiological and coughing reflexes may be abnormal. If the individual Fig. 7. Herniation of the through the chokes on food it may become inhaled leading to diaphragm resulting in hiatus hernia. the development of bronchiectasis which can physiological changes are not to occur since hasten the development of cardio-respiratory chronic constipation, and possibly spurious distress. diarrhoea can be a severe physiologically In the case of paralytic scoliosis, the patient's disturbing problem. ability to ventilate is significantly reduced, both The seated posture itself does not directly affect due to severe deformity and respiratory muscle the digestive system, but reflux oesophagitis may weakness. In these circumstances postural be encouraged by a supine posture. This appears problems can lead to respiratory distress to be common in the severely disabled individual. requiring extremely careful positioning. This is due both to the neuromuscular imbalance A patient that has been cared for in a supine and the severe skeletal deformities which may position will find that postural hypotension may distort the diaphragm, encouraging herniation of occur if they are suddenly placed in the sitting or the abdominal contents into the (Fig. 7). standing position. Gradual elevation is therefore The supine position encourages reflux of gastric needed to counteract this effect, thus allowing the contents into the oesophagus which is extremely sympathetic time to readjust to uncomfortable and if a chronic problem can lead the new position. to peptic ulceration, oesophageal stricture or The cardio-respiratory system maintains a oesophageal carcinoma. The reflux of gastric delicately balanced equilibrium which has contents may lead to aspiration of this material considerable reserves. However in the event of causing either or bronchiectasis skeletal deformity significant changes may occur which, as described above, can have serious which can lead to severe cardiopulmonary consequences. This problem may be helped by disease and premature death. The posture of the careful positioning which may require a individual can be shown to influence directly compromise. some of the haemodynamic factors involved and Ileostomy or colostomy and drainage can play an important role in maintaining normal bags require special consideration. In much the cardio-respiratory integrity. same way as described in constipation, obstruction of these can lead to severe Abdominal system physiological changes. Drainage of these stomas The contains three main must occur unimpeded and thus the posture physiological systems, adopted is worthy of thought. Ileal conduits which 1. Alimentary tract may have been provided to control urinary 2. Hepatic system incontinence require the same consideration 3. Renal system since poor drainage can lead to hydronephrosis, recurrent infection and the potential for serious Alimentary tract renal damage. The alimentary tract is a self regulating Immobility can lead to obesity exacerbating the digestive system and its function is not problems of transferring in and out of a seat. The significantly affected by posture. Physical very large or very weak require careful inactivity, imposed by a sedentary or wheelchair positioning if the independence of the individual lifestyle, encourages constipation and the is to be maintained. In addition the required hip difficulty of transferring can compound this flexion needed to sit can be a problem in very problem making bowel evacuation difficult. obese people since increased intra-abdominal Bowel habits with regular evacuation is the norm pressure may compromise the respiratory for fit and healthy adults and disabled individuals excursion of the diaphragm, encouraging the require to develop similar regularity if severe development of a hiatus hernia. Physiological considerations in seating 197 Hepatic system . The presence of an extensor pattern of No significant changes appear to occur in this reflexes which appears to be the most frequent system in relation to seating and therefore no can often be overcome by flexion in the hips. further comment will be made. However considerable flexion may be required and this in time may encourage the development Renal system of permanent flexion contractures, which may or Incontinence is a major social and hygienic may not be important depending on the person's problem. For the wheelchair dependent circumstances. individual the problems are aggravated by the Flexor patterns may conversely require a more difficulties of access and transfer into and out of extended position. The presence of primitive the wheelchair. Inability to transfer may make the reflexes such as the asymmetrical tonic reflex, incontinence worse since the individual may be may require very specific location to reduce them aware of the need to micturate but by the time to allow both comfort and function to be they have got out of the chair it may be too late and achieved. involuntary emptying of the bladder may have Involuntary movements present great diffi­ occurred. Childrens' nappies and incontinence culties for those individuals requiring special pads can be socially acceptable. These are seating. These may be athetoid in nature or may however bulky, often unhygienic, and present the present in the form of epilepsy. with an environment which encourages Epilepsy imposes its own problems since rapid bacterial and/or fungal proliferation and, with extraction of the individual from the chair may be the moist environment, may lead to skin necessary. This requires rapid release fasteners if maceration and breakdown. The efficiency of an restraints are necessary. Obviously the seat design intimately fitting seat can be lessened by the bulky must avoid potential danger to the occupant if the incontinence pad which interferes with the seat epilepsy is to be adequately managed. function and limits its usefulness. The intimacy of Pain can severely limit an individual's lifestyle. the seat and problem of skin breakdown require This pain may be skeletal in origin due to skeletal consideration. These problems can be collapse (e.g. osteoporosis or spinal disease) or compounded by anaesthetic skin and pressure pressure on (e.g. osteophyte on loading which may encourage the development of spinal nerves) or it may be due to damage pressure sores. (e.g. subluxation of the hip). Cutaneous damage Those patients with a bladder catheter, as seen in a pressure sore, may lead to severe pain, urodomes or other incontinence devices require where the dressings themselves may interfere with careful positioning. Free drainage of these devices the seating system. Positioning must take account is essential if urinary obstruction is to be avoided. of potential pressure points. The severely There must not be any "uphill" stretches of disabled often find it difficult to explain to those conduit as these could compromise bladder caring for them that they are in pain. The carer in drainage leading to potentially damaging urine turn however may learn a person's responses to reflux or hydronephrosis. discomfort. Consequently, the carer should be Consideration should be given to the individual closely involved in the provision of seating since who has to empty a drainage bag independently the seating team may not readily be aware of the since his posture may make this task difficult or individual's problems. impossible thus reducing his level of The presence of can be independence. debilitating and interfere with the quality of life. The position imposed by a seating system may Central nervous system result in the occupant remaining in a A very significant number of individuals predetermined position for many hours. This requiring special seating have some form of being so, the seating team must be alert to the cerebral inability whether congenital, possibility of painful postures and these should be biochemical or post-natal. The wide range of avoided whenever possible. abnormalities presenting with seating problems Positioning of an individual in specialised are found with considerable variation in clinical seating can have a profound effect on that findings and difficulties. individual. The position may affect the tone of the The various neurological deficits and ensuing body and the ability of the individual to function reflexes are discussed by Trefler and Taylor in this independently. Therefore great consideration is supplement. needed when supplying a chair. With modern The problems however that require special technology a significantly increasing number of seating often require special since the individuals have communication aids. The link pathology imposes constraints on the ultimate with the therapist and engineer is crucial if this 198 C. P. U. Stewart technology is to be utilised to its maximum BERGOFSKY E H, TURINO G M, FISHMAN A P(1959). potential. Positioning of the individual must take Cardio in kyphoscoliosis. the above into consideration as well as many of (Baltimore) 38, 263—317. the problems mentioned earlier in this text if the CARO C, Du Bois A B(1961). Pulmonary function in person requiring the special seat is to obtain the kyphoscoliosis. Thorax 16, 282—290. maximum benefit from the system supplied.

Dollery C T, Gillan P, Hugh-JONES P, Zorab P A Conclusion (1965). Regional lung function in kyphoscoliosis. The objectives of seating may involve conflicts Thorax 20, 175-181. with the many medical considerations. Successful provision of special seating requires JAMES J I P(1976). Scoliosis/2nd edition. - the identification of the priorities of these Edinburgh: Churchill Livingstone. conflicting requirements and subsequent WEST J B(1985). Respiratory physiology: the selection of the compromise to achieve a practical essentials/3rd edition. — Baltimore: Williams and solution. Williams.

REFERENCES WEST J B(1987). Pulmonary pathophysiology/3rd edition. — Baltimore: Williams and Williams. BAKE B, BJURE J, KASALICHY J, NACHEMSON A(1972). Regional pulmonary ventilation and perfusion ZORAB P A(1976). The medical aspects of scoliosis. distribution in patients with untreated idiopathic In: Scoliosis/2nd edition. Ed. J. I. P. James. — scoliosis. Thorax 27, 703-712. Edinburgh: Churchill Livingstone.