Inspiratory impedance threshold device effects on hypotension in heat-stroked swine

Introduction: Heat stroke is still an epidemiologically relevant disease with overall mortality rates as high as 15-33%. In heat stroke, hypotension is caused by relative central hypovolemia. We performed a feasibility study to determine if an inspiratory impedance threshold valve (ITD™) set to open at -12 cm H₂O would reverse systemic hypotension by enhancing venous return in hyperthermic, hypotensive, but spontaneously breathing animals. Methods: Seven anesthetized pigs weighing 30 ± 2 kg were warmed with a heating device until a mean rectal temperature of 44°C was reached, and mean arterial blood pressure (MAP) was ≤ 60 mmHg. The animals were then treated with the ITD. An intravenous bolus of 200 cc of 4°C normal saline was delivered 20 min after the ITD was placed, and external cooling was started. Results: Heat stroke criteria were achieved within 105 ± 15 min. MAP had decreased from 105 ±5 to 57 ± 5 mmHg, and respiratory rates had increased from 33 ± 2 to 101 ± 13 breaths/min. Addition of the ITD significantly improved MAP to 85 ± 4 mmHg, and reduced respiratory rate to 54 ± 6 breaths/min within 2 min. The effect was sustained until fluid replacement and external cooling were delivered 20 min later. At that point, MAP returned to baseline within 30 min, and 6/7 animals survived for an additional 30 min. Conclusions: Use of an inspiratory impedance threshold device resulted in an immediate rise in blood pressure in animals in heat stroke and preserved blood pressure for at least 20 min prior to cooling and fluid replacement.