Effects of Intrathoracic Pressure changes on Diaphragmatic Blood Flow during Mechanical Ventilation

Abstract

IntroductionAlthough mechanical ventilation (MV) is a life‐saving intervention, prolonged MV can lead to deleterious effects on diaphragm function, such as ventilator‐induced diaphragmatic dysfunction (VIDD) and vascular incompetence. Specifically, prolonged MV elicits time‐dependent reductions in diaphragm blood flow, induces vascular dysfunction, and limits the hyperemic response to contractions associated with weaning. During MV, positive‐end expiratory pressure (PEEP) is used to maintain small airway patency and mitigate alveolar damage; yet increased intrathoracic pressure with high levels of PEEP may impair diaphragm perfusion.PurposeTo test the following hypotheses: 1) Diaphragm blood flow during MV will be less with high vs. low‐peep, and 2) ablation of intrathoracic pressures (via pneumothorax (PTX)) will not alter diaphragm blood flow during MV vs. the low‐peep condition.MethodsFemale Sprague‐Dawley rats (~5 mo) were randomly divided into low PEEP (LP; 1cmH2O, n=7) or high PEEP (HP; 9 cmH2O, n=6) groups during mechanical ventilation. Blood flow, via the fluorescent microsphere technique, was determined during spontaneous breathing (SB) and 10 minutes after intubation and the onset of MV with LP or HP. Thereafter, for a third determination of diaphragm blood flow, the LP group underwent a surgical laparotomy (LAP; to determine effects of intrabdominal pressure on perfusion) and the HP group was subjected to a pneumothorax (PTX), with microsphere infusions occurring 10 minutes after the completion of surgery. Following the final fluorescent microsphere infusion, tissues were harvested for blood flow analysis. The diaphragm was sectioned into costal (ventral, medial, and dorsal) and crural portions to determine diaphragmatic blood flow distribution.ResultsCompared to SB, both LP and HP MV significantly reduced diaphragm medial costal blood flow (LP, 61.9 ± 7.5 vs 43.8 ± 4.9; HP, 46.2 ± 7.7 vs 21.8 ± 3.7 mL/100g/min; P≤0.05). HP MV significantly reduced blood flow to the ventral and medial costal diaphragm compared to LP MV (P≤0.05). In the HP MV+PTX condition, medial and dorsal costal blood flow was significantly increased versus HP MV (P≤0.05). LP MV + LAP did not significantly increase diaphragm blood flow compared to LP MV. Diaphragm perfusion during HP MV+PTX and LP MV were not different (P>0.05).ConclusionThis study demonstrates that HP MV reduces diaphragmatic blood flow to a greater extent than LP MV. Following a PTX in the HP MV condition, diaphragmatic blood flow is not different compared to the LP MV condition, suggesting that the reductions in diaphragmatic blood flow with LP MV are independent of intrathoracic pressure changes, and due likely to diaphragm quiescence.Support or Funding InformationSupported by: NIH HL137156‐01A1