Effects of an inspiratory impedance threshold device on blood pressure and short term survival in spontaneously breathing hypovolemic pigs

Background: The inspiratory impedance threshold device (ITD) has been shown to improve hemodynamic variables and survival outcomes during cardiopulmonary resuscitation in animals and humans. We hypothesized that use of an ITD, with a resistance of -10 cm H₂O, will improve hemodynamics and short-term survival rates during hypovolemic hypotension in spontaneously breathing pigs. Methods: Female farm pigs (∼26 kg) were intubated and anesthetized with propofol with the dose adjusted to permit spontaneous respirations. They were bled to 50% of calculated blood volume through an arterial catheter and then prospectively randomized to either treatment with an ITD or observation alone. Arterial and intratracheal pressures as well as arterial blood gases were measured. After 90 min the ITD was removed, normal saline was administered to all surviving animals, the anesthetic was discontinued, and animals were allowed to recover. Statistical analysis was performed with one-way repeated ANOVA and survival rates were calculated with Kaplan-Meier analysis. Results: Treatment with the ITD resulted in lower intratracheal inspiratory pressure in the treatment group (-11 ± 0.4 mmHg versus -4 ± 0.7 mmHg, respectively, P < 0.005). Mean arterial pressure after 30 min of treatment with the ITD was higher in the treatment group (61.1 ± 5.5 mmHg versus 37.4 ± 2.1 mmHg, respectively, P < 0.005). All pigs in the control group died within 65 min of the initial bleed, whereas 7/8 (87%) treated with an ITD survived for >90 min (P < 0.001). During the recovery phase, 6/8 (75%) in the ITD group survived for >3 h and awoke without neurological deficit; one surviving animal in the ITD group never woke up. Arterial oxygenation was not compromised in the ITD group. Conclusions: Use of an ITD improved blood pressure and short-term survival rates in a spontaneously breathing porcine model of hypovolemic hypotension.