Abdominal muscle tone provides a fulcrum for the diaphragm to contract to displace the lower thorax and decrease intrathoracic pressure (1,2,3,4,5). Inability to generate intrabdominal pressure during diaphragm contraction is observed in patients with abdominal hernias, prune belly, and quadriplegia (6,7,8,9,10).
A 66-year-old man was admitted after a fall. MRI revealed cervical stenosis and edema from C3-6. Decompression was performed. Postoperative course was complicated by intractable respiratory failure. Two years prior, the patient sustained injuries that required an exploratory laparotomy and multiple reexplorations resulting in a large ventral hernia. On physical exam the contour of the small intestines could be appreciated. Ventral displacement of abdominal contents occurred during inspiration. Manual reduction improved chest wall expansion.We applied an elastic abdominal binder to improve respiratory mechanics, but cephalad migration limited chest wall expansion. We constructed a plaster abdominal cast kept in place by Velcro straps.Esophageal (Pes), gastric (Pga), and transdiaphragmatic pressures (Pdi), respiratory rate, flow, tidal volume, minute ventilation, oxygen consumption (VO2), carbon dioxide production (VCO2), and arterial blood gas were measured during tidal breathing for 5 minutes with and without the cast.
Measured variables are displayed in Table 1
Cast Off versus OnParametersCast Off (Mean±SD)Cast On (Mean± SD)Pes, cm H2O-14.3 ± 1.1-15.6 ± 1.0Pga, cm H2013.9 ± 0.921.4 ± 0.9Pdi, cm H2O23.8 ± 1.535.9 ± 1.0Respiratory Rate, bpm34.3 ± 3.734.6 ± 5.2Tidal Volume, ml382.6 ± 65.5404.8 ± 70.6PaCO2, mmHg4641VO2, L/min0.239 ± .02.240 ± .01VCO2, L/min0.178 ± .010.194 ± .01. Cast application resulted in a 54.5% increase in end-inspiratory Pga, and a 50.8% increase in end-inspiratory Pdi. Analysis of esophageal and gastric pressure swings during spontaneous tidal breathing revealed dysynchrony between end-inspiratory Pes (PesI) and Pga (PgaI). Application of the cast resulted in improved synchrony.PesI, PgaI, and end-expiratory Pes (PesE) and Pga (PgaE) are plotted in Figure 1$$. Pressure plots with the cast off demonstrate a negative Pes:Pga slope as gastric pressure decreases during inspiration. With the cast, the plots demonstrate a clockwise rotation with a more positive Pes:Pga slope as gastric pressure increases during inspiration.Prior to cast application, the patient weaned 3.9 ± 3.2 hours per day for 13 days. After cast application, the patient weaned for 12.3 ± 5.8 hours per day for 3 days and was ventilator-independent on day 4.DISCUSSION: During inspiration the patient’s abdominal contents protruded into the hernia due to lack of an effective abdominal wall. Force that should have contributed to Pdi generation and elevation of the lower rib cage was spent displacing abdominal contents. The cast recreated an effective abdominal wall preventing the displacement of abdominal content as demonstrated a 54.5% increase in Pga.Improvements in Pdi were also due to more synchronous thoracic and abdominal movements. Without cast the most negative intrapleural pressures during inspiration (PesI) were not synchronous with the most positive gastric pressures, as should occur in normals.Figure 1 compares the Pes:Pga slopes which demonstrate a negative slope while the cast is off. A negative slope is associated with decreased exercise endurance (13) and suggestive of dysynchronous contraction of the diaphragm and rib cage muscles. The cast made the slope positive indicating more effective contraction of the diaphragm and decreased accessory muscle use.An increased VCO2 with cast application was not accompanied by a decrease in PaCO2. We believe the increase in elimination of carbon dioxide in conjunction with a decrease in PaCO2 is reflective of increased alveolar ventilation. The increase in CO2 production was not reflective of increased metabolic demands (O2 consumption was unchanged), suggesting a more efficient ventilation.
Application of an abdominal cast in a patient with an ineffective abdominal wall contributed to increased abdominal pressure, a more favorable fulcrum for diaphragmatic contraction, improved synchrony of intrapleural and gastric pressure swings, increased Pdi generation, and increased alveolar ventilation without an increased metabolic cost. These improvements translated to improved periods of spontaneous breathing and weaning from prolonged mechanical ventilation.
C. Petersen, None.