NOMI is a rare but life-threatening complication after cardiovascular surgery. Although it is assumed to be the result of microcirculatory alterations initiated during CPB and the vasospasm of mesenteric artery, its pathomechanism is as yet unclear. It is considered that it is caused by not only the invasion with cardiovascular surgery itself but also various factors such as the exacerbation of general conditions and using each therapeutic medication for it after surgery [8, 9]. In previous studies, its incidence rate was reported to be about 0.4 to 9.0%, and mortality was 30 to 93% [1,2,3,4,5]. In the present study, the incidence rate was 1.91% and in-hospital mortality was 75.0%, which was a similar result.
We have reviewed the clinical data of 12 NOMI cases and assumed the mechanism and the cause at the onset. Almost all the patients used high-dose NOE after surgery because of their condition of LOS, sepsis, or need for hemodialysis. When we investigated their clinical course in detail, it turned out that there were several factors that developed in combination at the onset of NOMI. In some cases, NOE was dose up just before the onset of NOMI due to the deterioration of LOS and sepsis or at the beginning of dehydration by hemodialysis. The effect of NOE, which induces vasoconstriction, increases resistance in peripheral splanchnic vessels, and stimulates β receptors in a dose-dependent manner to increase intestinal oxygen consumption, is likely to cause mesenteric ischemia [2, 5, 10]. As the result of the multivariate analysis of this study, the use of NOE > 0.10 μg/kg/min has shown significantly high OR and may have a strong association with the development of NOMI. It is difficult to detect the definite cause of NOMI because this cannot be explained by only the use of NOE and several factors may be related in complicated. However, the status of NOE and the presence of LOS, sepsis, and hemodialysis, which may be the reason for the use of NOE, can be the important factor in the pathogenic mechanism of NOMI.
Selective catheter angiography for mesenteric artery remains the gold standard for the diagnosis of NOMI. In previous studies, angiography was performed for all patients with suspected mesenteric ischemia because of the decreased intestinal peristalsis after cardiovascular surgery [2, 3]. In addition to the detection of the mesenteric arterial vasospasm, the inserted catheter allows the selective mesenteric intraarterial infusion of vasodilatative drugs. Using this diagnosis and treatment method, the result of low mortality was reported . However, there were also reported cases that required surgical exploration and intestinal resection eventually, so its therapeutic effects are not clear yet [3, 11]. Angiography is an invasive procedure that cannot be applied for all patients with suspected symptoms.
In CT scan findings, pneumatosis intestinalis and portal venous gas are specific radiological signs, but both may be seen as nonischemic conditions . Hasan et al. have reported that, among 26 patients with suspected NOMI from CT scan findings, 13 (50%) patients have confirmed bowel ischemia by surgical exploration and definite diagnosis of NOMI . They have indicated the inaccuracy of the diagnosis by CT scan findings and necessity of surgical exploration without delay. A surgical exploration is only reliable way to provide an accurate assessment of bowel viability and necrotic sections requiring segmental intestinal resection [7, 10, 12]. Therefore, when we suspected the presence of intestinal ischemia from the comprehensive evaluation of the clinical course and laboratory data, we make it a rule to perform the CT scan and surgical exploration without hesitation.
In previous studies, the analysis for each laboratory data to predict and detect NOMI has been reported. In particular, there are several studies that indicate the elevation of lactate at the onset of NOMI [2, 3, 5, 13]. However, because hyperlactatemia can be found in various conditions, it is difficult to distinguish whether the cause of hyperlactatemia is NOMI or other factors [6, 14]. Simon et al. provided the results of analyzed laboratory data including lactate, creatine kinase, and lactate dehydrogenase isozyme. There were no significant differences between confirmed and negative diagnoses among patients with suspected NOMI .
In the present study, 7 of 12 cases were diagnosed with NOMI with hyperlactenemia as initial findings. However, because the elevation of serum lactate level was thought to be reflected irreversible and lethal mesenteric ischemia, all of 7 cases having shown hyperlactatemia could not be saved. When the serum lactate level has been elevated, it is highly likely to be too late for cure. Thus, lactate has lower usefulness for early diagnosis, and rather nonspecific gastrointestinal symptoms such as abdominal pain or vomiting may be more useful. Also, among the 12 cases in this study, 3 cases who survived were diagnosed early from subjective symptoms such as abdominal pain, not objective laboratory data. However, it was difficult to find subjective symptoms early because almost all NOMI patients were under sedation for mechanical ventilation. Such a situation is often seen at the onset of NOMI and may impede the early diagnosis.
Although there have been several studies that reported predictive factors for occlusive mesenteric ischemia after cardiovascular surgery or NOMI during intensive care, only a few focused on NOMI after cardiovascular surgery [2, 5, 13,14,15,16,17,18,19]. As the result of this analysis, NOE > 0.10 μg/kg/min and prolonged ventilation were identified as isolate risk factors for the onset of NOMI. The prolonged ventilation may be the cause of mesenteric ischemia because of peripheral hypoperfusion under sustained sedation and long-term bedridden condition [2, 15,16,17,18]. It is also suggested that PEEP decreases mesenteric blood flow and lung injury by mechanical ventilation can spread the pulmonary inflammation to distant organs [20, 21].
In this present study, we have further analyzed the use of NOE and prolonged ventilation as independent risk factors for NOMI, focusing on corresponding cases. This is because we considered that the definite cutoff value of NOE and ventilation factor can be further effective to predict the incidence of NOMI. For the NOE factor, total quantity, duration, and maximum dose of NOE were analyzed. For the prolonged ventilation factor, maximum PEEP, total ventilation time, and index calculated from PEEP * ventilation time/ BW were analyzed. However, between NOMI and non-NOMI groups corresponding to each factor, there were no significant differences and definite cutoff value could not be calculated in both factors (Table 5). This is because each patient status after cardiovascular surgery was individually different, and the threshold of the onset of mesenteric ischemia was varied depending on the patient’s general conditions. For example, even between patients who used the same high dose of NOE, there are several factors related to intestinal blood flow. Consequently, the actual dose that can be the cause of mesenteric ischemia for each case is not expected to be the same. If insufficient circulatory dynamics or severe arteriosclerotic change is present, mesenteric ischemia can be caused by an even lower dose of NOE. The same applies to the ventilation factor. Hence, the use of high dose of NOE and prolonged ventilation are important for the onset of NOMI; however, it is very difficult to detect the definite and detailed cutoff value.
There are several limitations to our study. Because this is a retrospective and single-center database study, the reliability of each data is insufficient. In particular, suspecting the onset of NOMI and judging to perform the CT scan and laparotomy are dependent on the surgeon’s judgment. Furthermore, we have detected the independent risk factors from the multivariate regression model, but statistical reliability does not seem to be high because of a small number of NOMI cases.
There may be patients with mesenteric ischemia who were excluded from the NOMI group as we did not confirm mesenteric ischemia with surgical exploration. We confirmed no occlusion of mesenteric vessels from the CT scan findings instead of the selective mesenteric angiography. The cause of mesenteric ischemia may have been the occlusion or thrombosis.