Treatment of hypotension in pigs with an inspiratory impedance threshold device: A feasibility study

Objective: An inspiratory impedance threshold device was evaluated in spontaneously breathing animals with hypotension to determine whether it could help improve systemic arterial pressures when fluid replacement was not immediately available. Design: Prospective, randomized. Setting: Animal laboratory. Subjects: Thirty-nine female farm pigs (weight, 28-33 kg). Interventions: A total of 39 anesthetized spontaneously breathing pigs were treated with an impedance threshold device, with cracking pressures from 0 to -20 cm H ₂O. Four separate experimental protocols were performed: protocol A, in which the hemodynamics of seven pigs were examined during application of an impedance threshold device at various levels of inspiratory impedance (-5, -10, -15, and -20 cm H ₂O), both before and after a severe, controlled hemorrhage to a systolic blood pressure of 50-55 mm Hg; protocol B, in which nine pigs bled to systolic blood pressure of 50-55 mm Hg were treated with an impedance threshold device set at -12 cm H ₂O and were compared with nine others treated with a sham device; protocol C, in which the effects of the impedance threshold device on mixed venous gases were measured in seven hemorrhaged pigs; and protocol D, in which the effects of the impedance threshold device on cardiac output in seven hemorrhaged pigs were measured. Methods and Main Results: During initial studies with both normovolemic and hypovolemic pigs, sequential increases in inspiratory impedance resulted in a significant increase in systolic blood pressure, whereas diastolic left ventricular and right atrial pressures decreased significantly and proportionally to the level of impedance. When comparing the sham vs. active impedance threshold device (-12 cm H ₂O) in hypotensive pigs, systolic blood pressure (mean ± SEM) with active impedance threshold device treatment increased from 70 ± 2 mm Hg to 105 ± 4 mm Hg (p < .01). Pressures in the control group remained at 70 ± 4 mm Hg (p < .01). Cardiac output increased by nearly 25% (p < .01) with the active impedance threshold device when calculated using the mixed gas equation and when determined by thermodilution. Conclusions: These studies demonstrate that it is feasible to use a device that creates inspiratory impedance in spontaneously breathing normotensive and hypotensive pigs to increase blood pressure and enhance cardiopulmonary circulation in the absence of immediate fluid resuscitation. Further studies are needed to evaluate the potential long-term effects and limitations of this new approach to treat hypovolemic hypotension.