Large airway response to cemented arthroplasty: relationship to hypoxaemia. An experimental study in dogs

Alteration in airway smooth muscle tone has been implicated in the mechanism of hypoxemia, pulmonary hypertension and changes in dynamic thoracic compliance after total hip arthroplasty (THA). We used the pressure within a water-filled cuff of an endotracheal tube as a continuous measure of changes in tracheomotor tone during THA in mongrel dogs, while intermittently assessing gas exchange abnormalities. In all 16 dogs the instillation of polymethylmethacrylate (PMMA) into the femoral medullary shaft resulted in tracheal dilation. In ten dogs we demonstrated simultaneous hypotension, hypoxemia and increase in shunt fraction (Qs/Qt) after THA. In six dogs the medullary canal was thoroughly lavaged prior to PMMA injection, and no hypoxemia, hypotension or increased Qs/Qt was found in spite of persisting tracheomotor relaxation. We conclude that these well-documented gas-exchange abnormalities are not mediated by changes in large airway tone. Since the hypoxemia was abolished by medullary lavage in our animal model, we suggest that this syndrome is mediated by alterations in lung perfusion and associated small airway constriction and not by changes in large airway smooth muscle tone.