Monitoring of end-tidal carbon dioxide partial pressure changes during infrarenal aortic cross-clamping: a non-invasive method to predict unclamping hypotension

Background: To assess the variations in end-tidal CO2 in response to aortic cross-clamping and the relationship with systolic arterial pressure (SAP) changes induced by unclamping. Methods: Thirty-three patients undergoing infrarenal aortic abdominal aneur ysm repair by aorto-aortic prothetic bypass were prospectively studied. All patients were anesthetized with iv midazolam (0.05 mg . kg(-1)), thiopento ne (3-5 mg . kg(-1)), fentanyl (5 mug . kg(-1)), pancuronium (0.1 mg . kg(- 1)) and the maintainance of anesthesia used was 1-1.5% end-tidal isoflurane and iv fentanyl. The perioperative management was standardized. End-tidal CO2 and SAP were measured 5 min before (Pre-XAA), 15 min after infrarenal a ortic cross-clamping (XAA), 5 min before (Pre-UXAA) and immediately after u nclamping (UXAA). Results: A total of 16 (48.5%) from 33 patients presented decrease in SAP f ollowing aortic unclamping, and 13 out of these patients had arterial hypot ension defined as SAP <90 mmHg. End-tidal CO2 variation (PreXAA-PreUXAA) in duced by aortic clamping was correlated with SAP variation (PreUXAA-UXAA) i nduced by unclamping (r=0.763; P=0.0001). An end-tidal CO2 reduction above 15% after aortic cross-clamping was found to have a 100% sensitivity to det ect a SAP decrease greater than 20% after unclamping, with a 100% specifici ty and a negative predictive value of 1.0. Complete aortic occlusion durati on was not correlated to SAP unclamping variation (<Delta>SAP). Intraoperat ive characteristics (fluid loading, hematocrits, urinary output) were compa rable, although blood loss was higher in patients experiencing Delta SAP>20 %. Conclusions: End-tidal CO2 variation monitoring during aortic cross-clampin g may provide a reliable and non-invasive method to predict unclamping hypo tension. When the aortic clamp was released, systolic hypotension (>20%) oc curred in those subjects who had a decrease in end-tidal CO2 greater than 1 5% during aortic cross-clamping.