Feasibility and effects of transcutaneous phrenic nerve stimulation combined with an inspiratory impedance threshold in a pig model of hemorrhagic shock

Objective: Intrathoracic pressure changes are of particular importance under hypovolemic conditions, especially when central venous blood pressure is critically low. Accordingly, the purpose of this study was to assess the feasibility of transcutaneous phrenic nerve stimulation, used in conjunction with an inspiratory impedance threshold, on hemodynamic variables during hemorrhagic shock. Design: Prospective, randomized laboratory investigation using a porcine model for measurement of hemodynamic variables, left and right ventricular diameter, and transmitral, transpulmonary, and transaortic blood flow employing transesophageal echo-Doppler technique. Setting: University hospital laboratory. Subjects: Thirteen female pigs weighing 28-36 kg. Interventions: The anesthetized pigs were subjected to profound hemorrhagic shock by withdrawal of 55% of estimated blood volume over 20 mins. After a 10-min recovery period, the diaphragm was stimulated with a prototype transcutaneous phrenic nerve stimulator at a rate of ten per minute while the airway was intermittently occluded with an inspiratory threshold valve between positive pressure ventilations. Hemodynamic variables were monitored for 30 mins. Measurements and Main Results: Phrenic nerve stimulation in combination with the inspiratory threshold valve significantly (p < .001) improved right and left ventricular diameter compared with hypovolemic shock values by 34 ± 2.5% and 20 ± 2.5%, respectively. Moreover, phrenic nerve stimulation together with the inspiratory threshold valve also increased transaortic, transpulmonary, and transmitral valve blood flow by 48 ± 6.6%, 67 ± 13.3, and 43 ± 8.2%, respectively (p < .001 for comparisons within group). Mean ± SEM coronary perfusion and systolic aortic blood pressures were also significantly (p < .001) higher compared with values before stimulation (30 ± 2 vs. 20 ± 2 mm Hg, and 37 ± 2 vs. 32 ± 3 mm Hg, respectively). Conclusions: This feasibility study suggests that phrenic nerve stimulation with the inspiratory threshold valve may improve cardiac preload and, subsequently, key hemodynamic variables in porcine model of severe hemorrhagic shock.