Mechanical inefficiency ofthethoracic cage inscoliosis

Themechanism ofimpairment ofventilatory function inidiopathic scoliosis hasbeen studied in23children, allgirls, andcompared with27normalchildren and24normal young adult females. Thevital capacity, FEV1,gastransfer factor, andthemaximumstatic expiratory airway pressurewereallsignificantly reduced. Total lungcapacity andthemaximuminspiratory pressure werelower thaninthenormal subjects, butthedifference was notsignificant. Restriction ofthoracic cagemovementbyabelt showed that thethorax inthechildren withscoliosis was as mobile as inthe normal subjects. Theresults areexplained intermsofthecharacteristic deformity inscoliosis which causesonehemi-thorax tobecomerelatively smaller thantheother. Itisconcluded thatthis causes aninherent mechanical inefficiency ofventilation whichislikely tocontribute torespiratory failure in these subjects. Themechanisms ofimpairment of ventilatory function inidiopathic scoliosis arebynomeansclear. Standard pulmonary function tests inadults with idiopathic scoliosis showdiminution ofairway flow rates andreduction oflungvolume compartments.'-6 Thevital capacity (VC)isreduced, thetotal lung capacity (TLC)maybereduced,7-9 andgastransfer capacity hasbeenreported low9orhigh.'0 Despite deformity ofthethoracic cageandlimitation oflung volume, therelaxation pressure curve ispreserved in thechild andyoungadult, butappears tobecome severely restricted inlater life." Bergofsky eta13 reported anincrease ofelastic resistance whichthey regarded asmainly responsible foralarge increase in theworkofbreathing. Nosatisfactory explanation forthese observations hasbeensuggested. Denison etal'0 asserted that the diminution inlungvolume compartments could not beexplained theoretically on thebasis oflateral flexion orintermsofchest walldeformity, and suggested that elevation ofthediaphragm caused by theassociated kyphosis mustbetheexplanation. This assumes that thekyphotic element ofthedeformity inscoliosis iscomparable withforward flexion ofthe thoraco-lumbar spine inthenormal subject, which of course isnottrue. Kyphosis inscoliosis isassociated withcompensatory curves sothatusually thehead andneckarenotflexed withrespect tothepelvis.