Selective cerebral perfusion (SCP) with hypothermia has been reported as an effective method of brain protection in ascending aorta and aortic arch repair. 1–5 SCP is usually combined with open distal anastomosis (OD) in which distal anastomosis is carried out under circulatory arrest in the lower body. 1–4 Although satisfactory brain protection by SCP has been reported, 1–5 the risk of body ischemia during OD has remained unknown and serious complications, such as vital organ dysfunction or spinal cord injury cannot be predicted. Therefore, a reliable monitoring system to evaluate visceral ischemia during OD is needed to establish safe circulatory assistance in aortic arch surgery. Gastric tonometry, which reflects the gastric intramucosal pH (pHi), has been shown to be a reliable index of gastrointestinal tissue perfusion or, more precisely, tissue oxygenation, 6,7 and recently, it has been used for evaluation of visceral tissue perfusion during cardiopulmonary bypass. 8,9 In this study, we measured the pHi during cardiopulmonary bypass in patients undergoing arch aneurysm repair and evaluated the efficacy of gastric tonometry on monitoring visceral ischemia during lower body circulatory arrest in OD.
Between March and August of 1999, five patients underwent repair for aneurysms involving the aortic arch and/or ascending aorta using SCP and OD techniques with moderate hypothermia (group OD). Four patients were men and one was a woman with a mean age of 71 years, ranging from 65 to 78 years. Eight patients (mean age, 66 years; range, 52 to 78 years), who underwent cardiac surgery (coronary artery bypass surgery in six and valve surgery in two) using standard cardiopulmonary bypass (CPB) with mild hypothermia, served as controls (group C). In all five patients of group OD and four patients of group C, gastric pHi was measured before CPB (pre-CPB), at 30 min of CPB (CPB 30), 10 min after OD (OD10) or the release of aortic cross-clamping (AC10), the end of CPB, and 30 min after termination of CPB (post-CPB); findings from the groups were compared. Aspartate aminotransferase (AST), alanine aminotransferase (ALT), serum total bilirubin (T Bil), blood urea nitrogen (BUN), serum creatinine (Cr) and creatinine clearance (CCr) were measured as indicators of hepatorenal function before surgery and on the 1st and 3rd postoperative days. The relationship between OD10 pHi and perioperative variables were examined in group OD.
Gastric pHi was measured using a nasogastric tonometer (Trip; Tonometrics Division, Instrumentarium Corp., Helsinki, Finland). The position of the tonometer tip was confirmed by auscultation and aspirating stomach contents. Gastric air and juice were completely aspirated to ensure good mucosa/balloon coaptation. The balloon was instilled with saline at room temperature. After an equilibration period of 30 min, the PCO2 of saline was determined simultaneously with arterial bicarbonate concentration and pHi was calculated by the Henderson-Hasselbach equation. CPB was conducted using a membrane oxygenator and a centrifugal pump. Pump flow was maintained between 2.0 and 2.6 L/min per m2, and mean arterial pressure was adjusted to between 50 and 80 mm Hg. Arterial blood gas was regulated by pH-stat methods during SCP and by alpha-stat at other times. Body temperature was assessed by urinary bladder temperature and kept at 25°C in group OD and 33°C in controls.
Data are presented as mean ± SD. Statistical analysis was performed with StatView version J 4.02 (Abacus Concepts Inc., Berkeley, CA). Analysis of statistical differences between two means was performed using one way analysis of variance or a nonparametric Mann-Whitney U test. Correlations between OD10 pHi in group OD and perioperative variables were analyzed by univariate linear regression and Pearson correlation coefficients and corresponding p values were calculated. Differences were considered statistically significant if the p value was <0.05.
There was no significant difference in age, gender, CPB time, aortic cross-clamping time, or postoperative cardiac index between the two groups. The mean duration of open distal anastomosis in group OD was 38 ± 7.4 min (range, 30 to 47 min). One patient with preoperative renal dysfunction in group OD developed paraplegia and renal failure postoperatively, necessitating dialysis (Table 1).
In group OD, pHi was significantly decreased at 10 min after OD (OD10) (7.35 ± 0.03 at CPB30 vs. 7.23 ± 0.07 at OD10, p < 0.05) and recovered at the end of CPB (7.32 ± 0.02), whereas no significant change was observed throughout CPB in group C. The pHi at OD10 in group OD was significantly lower than the pHi at the same period in group C (7.23 ± 0.07 in group OD vs. 7.37 ± 0.08 in group C, p < 0.05) (Figure 1).
There was no significant difference in postoperative values of hepatic function between the groups. CCr level on the first postoperative day was significantly lower in group OD than in group C (p < 0.05). No significant difference was observed in postoperative BUN and Cr levels (Table 2).
In group OD, significant correlation to pHi value (OD10) was observed with BUN (r = −0.973, p = 0.0054), Cr (r = −0.977, p = 0.0043), and CCr (r = 0.908, p = 0.0328) levels on the 1st postoperative day and Cr (r = −0.979, p = 0.0036) on the 3rd postoperative day (Table 3 and Figure 2).
A patient in group OD developed paraplegia postoperatively; a critical decrease in pHi (pHi = 7.11) was observed at OD10 (Figure 3).
Although deep hypothermic circulatory arrest has been a popular technique for brain protection in aortic arch surgery, selective cerebral perfusion (SCP) is also advocated and widely used because it allows a longer safe period for cerebral protection during arch reconstruction. 1–5 During SCP, the distal anastomosis is usually performed with an open anastomosis technique in which the lower part of body is exposed to circulatory arrest. Despite the prolonged safety period for brain protection in SCP, the risk of ischemic complications in the lower body such as visceral organ dysfunction and spinal cord injury, has remained an unsolved problem. In recent years, SCP has been conducted with moderate hypothermia rather than deep hypothermia so as to attenuate the adverse effects such as coagulopathy, vital organ dysfunction, and prolongation of cardiopulmonary bypass (CPB). 3–5 This method may result in the increased risk for splanchnic hypoxia during open distal anastomosis, and a reliable means of monitoring for evaluation of lower body ischemia is needed to establish safe circulatory assistance in aortic arch surgery.
Gastric intramucosal pH (pHi), which can be measured by gastric tonometry, has been known as a reliable and minimally invasive index of gastrointestinal tissue oxygenation. 6–10 It has been reported that an abnormally low pHi (below 7.25–7.35) is associated with increased morbidity and mortality in patients after cardiac or general surgery and in intensive care unit patients. 6–8 Recently, pHi has also been used as a monitor of splanchnic tissue perfusion, or more precisely tissue oxygenation, during CPB. 8–10 In standard CPB, an increase in blood lactate concentration and decrease in pHi are usually observed, and such changes cannot be explained by global hemodynamic indices such as cardiac output, blood pressure, and venous oxygen saturation alone. 9,10 Several factors, such as an imbalance between gastric mucosal oxygen supply and demand, which may be caused by maldistribution of regional tissue blood flow, inflammatory response, and endotoxemia from bacterial translocation through the disrupted integrity of the gut mucosal barrier, are currently suggested to influence pHi during CPB. 8–10 Thus gastric tonometry, which can assess the gastric mucosal oxygenation directly, may enable earlier detection of changes in regional splanchnic perfusion than can conventional global hemodynamic indices as long as many factors are intricately involved to determine visceral tissue blood flow. In this study, we evaluated the possibility of estimating splanchnic ischemia by gastric tonometry during open distal aortic anastomosis. Immediately after the procedure, pHi was significantly decreased but had recovered by the end of CPB. This reduction in pHi was not observed in patients undergoing cardiac surgery with standard CPB. Moreover, the severity of decrease in pHi was correlated with postoperative renal dysfunction. These results suggest that gastric tonometry may be useful to estimate visceral organ damage during open distal anastomosis and predict the occurrence of postoperative organ dysfunction in aortic arch surgery. A recent advance in technology proposes the automated detection of tonometry, and this method may allow a further increase in reliability when estimating visceral oxygenation during CPB. 11
There are limitations to the interpretation of our results that should be discussed. Tonometry has been known as a good indicator of gastric mucosal oxygenation; however, it is unclear whether one can extrapolate the viability or oxygenation of other organs from the tonometry data. 9,10 In our results for example, hepatic function did not correlate with pHi. Similarly, further studies, including animal experiments are required to examine the relationship between pHi and spinal ischemia during OD, although one of our patients with severely reduced pHi during OD developed paraplegia postoperatively. In conclusion, monitoring of intramucosal pHi by gastric tonometry may be useful for the evaluation of splanchnic ischemic insult during OD in arch aneurysm repair and may contribute to improvements in extracorporeal circulatory techniques during aortic surgery.
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Copyright © 2001 by the American Society for Artificial Internal Organs
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