Acid response to sham feeding as a test for completeness of vagotomy

The effect of sham feeding on neurocardiac regulation in healthy human volunteers

BACKGROUND

Distension and electrical stimuli in the esophagus alter heart rate variability (HRV) consistent with activation of vagal afferent and efferent pathways. Sham feeding stimulates gastric acid secretion by means of vagal efferent pathways. It is not known, however, whether activation of vagal efferent pathways is organ- or stimulus-specific.

OBJECTIVE

To test the hypothesis that sham feeding increases the high frequency (HF) component of HRV, indicating increased neurocardiac vagal activity in association with the known, vagally mediated, increase in gastric acid secretion.

METHODS

Continuous electrocardiography recordings were obtained in 12 healthy, semirecumbent subjects during consecutive 45 min baseline, 20 min sham feeding (standard hamburger meal) and 45 min recovery periods. The R-R intervals and beat-to-beat heart rate signal were determined from digitized electrocardiography recordings; power spectra were computed from the heart rate signal to determine sympathetic (low frequency [LF]) and vagal (HF) components of HRV.

RESULTS

Heart rate increased during sham feeding (median 70.8 beats/min, 95% CI 66.0 to 77.6; P<0.001), compared with baseline (63.6, 95% CI 60.8 to 70.0) and returned to baseline levels within 45 min. Sham feeding increased the LF to HF area ratio (median: 1.55, 95% C.I 1.28 to 1.77; P<0.021, compared with baseline (1.29, 95% CI 1.05 to 1.46); this increase in LF to HF area ratio was associated with a decrease in the HF component of HRV.

CONCLUSIONS

Sham feeding produces a reversible increase in heart rate that is attributable to a decrease in neurocardiac parasympathetic activity despite its known ability to increase vagally mediated gastric acid secretion. These findings suggest that concurrent changes in cardiac and gastric function are modulated independently by vagal efferent fibres and that vagally mediated changes in organ function are stimulus- and organ-specific.

Vagal-sparing esophagectomy: a more physiologic alternative

OBJECTIVE

To evaluate the function of the vagal nerves and the gastric reservoir after vagal-sparing esophagectomy.

SUMMARY BACKGROUND DATA

Esophagectomy as currently performed includes division of the vagal nerves and surgical alteration of the stomach, with attendant postoperative dumping, diarrhea, reduced meal capacity, and weight loss. Vagal-sparing esophagectomy has been introduced as a technique for removal of the esophagus while preserving the vagal nerves and gastric reservoir. The procedure is touted as having a low morbidity and is applicable to patients with end-stage benign or early malignant disease.

METHODS

A random sample of 15 patients at a median of 20 months after a vagal-sparing esophagectomy was compared to 23 asymptomatic normal subjects; 10 randomly selected patients, 29 months after esophagogastrectomy with colon interposition; and 10 randomly selected patients, 47 months after standard esophagectomy with gastric pull-up. Gastric mucosal acidification was tested with Congo red staining. Vagal secretory function was measured by gastric acid output and pancreatic polypeptide response to sham feeding. Vagal motor function was assessed by a technetium gastric emptying scan and a questionnaire to evaluate dumping and diarrhea. Gastric reservoir function was evaluated by measuring meal capacity and postoperative changes in body mass index.

RESULTS

Vagal-sparing esophagectomy preserved the function of the vagi, as evident by an increase in gastric acid output, a rise in serum pancreatic polypeptide following sham feeding, and preservation of normal postoperative gastric emptying in 70% of the patients. After vagal-sparing esophagectomy, patients were free of dumping and diarrhea and were analogous to normal subjects in meal capacity but had a slight reduction in the speed of eating.

CONCLUSIONS

Vagal-sparing esophagectomy preserves gastric secretory, motor, and reservoir function. Postoperatively, patients have normal alimentation, bowel regulation, and no weight loss. It is an ideal procedure for patients with end-stage benign disease, Barrett's esophagus with high-grade dysplasia, or esophageal carcinoma limited to the lamina propria.

Nutritional implications of cephalic phase gastrointestinal responses

Palatable cephalic stimuli induce a simultaneous activation of gastrointestinal motility, gastric acid and pancreatic enzyme secretion, as well as, release of the gastrointestinal hormones gastrin and pancreatic polypeptide. Cholinergic neural input is the dominant mediator of these responses with cholecystokinin and gastrin acting as additional stimulatory modulators. Central cholinergic circuits, neuropeptide Y, and thyrotropin releasing hormone are candidate central stimulators of the cephalic phase. There are good arguments for glucagon-like peptide-1 and peptide YY to be physiological inhibitors of cephalic-phase responses with these peptides being released in the intestinal phase of digestion and putatively contributing to termination of the cephalically stimulated pattern. Cephalic-phase responses are used clinically as diagnostic tests to assess completeness of selective proximal vagotomy and to explore autonomic neuropathy. Pancreatic polypeptide secretion with sham feeding is an appropriate test of abdominal vagal function. Cephalically stimulated motor and secretory activity contribute greater than 50% of overall postprandial responses. Pharmacological inhibition of cephalic-vagal stimulation, resulting in reduced food intake, may be a novel approach to obesity management. Glucagon-like peptide-1 is a particular candidate because it inhibits the cephalic phase of digestion, diminishes food intake, and reduces the glycemic excursion after a meal by retarding gastric emptying, stimulating insulin and lowering glucagon release.

A retrospective study of the usefulness of acid secretory testing

BACKGROUND

Gastric analysis is useful for diagnosing and monitoring the control of hypersecretory conditions and to distinguish appropriate from inappropriate causes of hypergastrinaemia. Pentagastrin, used to measure maximal acid output (MAO), is no longer available in the USA.

METHODS

We examined the University of Pennsylvania Health System gastric analysis database, which includes demographic data, study indications, gastric analysis, and serum gastrin and secretin testing results according to referral indications, paying specific attention to discordant basal acid output (BAO) and MAO measurements.

RESULTS

One hundred and twenty-four gastric analyses were performed in 103 patients (42 males, mean age 47.5 years, 14 with prior acid-decreasing surgery). Recurrent ulceration or pain unresponsive to antisecretory therapy was the indication in 42 patients. Twelve were hypersecretory, including three each with isolated elevations of BAO or MAO. Hypergastrinaemia was the indication in 35 patients. Five were hypersecretory (four with Zollinger-Ellison syndrome), three had isolated MAO elevations and 16 were hypo- or achlorhydric, indicating appropriate hypergastrinaemia. Of the seven patients with isolated MAO elevations, two had clear benefit from the stimulated portion of the study (four additional patients had equivocal benefit).

CONCLUSIONS

Gastrin concentrations cannot be interpreted without knowledge of acid secretory capacity. MAO measurement has a small but significant benefit over measuring BAO alone.

Gastric acid secretion and intragastric acidity: measurement in health and disease

Gastric acid secretion tests have limited use in clinical practice. For practical purposes, a pH measurement on a fasting gastric aspirate will provide strong evidence of the presence or absence of achlorhydria. Tests of gastric acidity, in particular 24-h acidity studies, have provided considerable insight into normal and abnormal gastric physiology, and have largely determined the dosing regimens for the management of acid-peptic diseases. Acid tests may be simple to perform, so much so that they have been suggested as 'practicals' for student teaching (Nicol et al, 1991). However, reproducible and meaningful results require careful attention to detail, and the appropriate mathematical analysis is still subject to some debate. It is important that the presentation of the data should allow the reader to assess the response over the 24-h period, and also the range of individual responses. Despite the many years of research into gastric acid secretion, only recently have the effects of age, sex, diet, smoking and mental stress been identified. In addition, many data need to be reviewed in the light of the effects of H. pylori infection on gastrin release. H2-receptor antagonists had been studied extensively before and since their first clinical use in 1974, but surprisingly only recently have the issues of tolerance and rebound been defined. The 24-h intragastric acidity profile remains an essential study before the start of clinical trials on any new drug to be used for the treatment of acid-peptic diseases.

Urine acid output as a test of completeness of vagotomy

Peak acid output in response to sham feeding and changes in urine acid output 2 and 3 hours after a test meal have been measured in 20 normal volunteers, 17 asymptomatic patients after vagotomy, six patients with recurrent duodenal ulcer after vagotomy and ten normal subjects given a 48-h course of ranitidine, 150 mg 12-hourly. Gastric peak acid output in normal volunteers ranged from 6.9 to 22.1 mmol/h. All asymptomatic patients after vagotomy had a peak acid output less than 7 mmol/h, consistent with complete vagotomy. Five patients with recurrent ulcer had a peak acid output greater than 8 mmol/h, suggesting an incomplete vagotomy. Urine acid output after a test meal, expressed as the change from the basal rate of acid output, was always in a negative direction in normal subjects (fall in acid output = postprandial alkaline tide). This change was abolished in patients with complete vagotomy, in whom urine acid output increased after a meal. In five patients with incomplete vagotomy (and one other with recurrent ulcer and unknown vagal status) the urine acid output changed in a negative direction after a test meal. The relationship of urine acid output to gastric secretion was confirmed by the abolition of the postprandial alkaline tide in normal subjects given ranitidine. The results in patients with incomplete vagotomy did not overlap with those from patients with complete vagotomy. This suggests that this test could be used for the routine postoperative assessment of completeness of vagotomy.

Sham feeding for testing gastric secretory capacities before and after parietal cell vagotomy

The usefulness of acid secretory data collected during a modified sham-feeding (MSF) test was examined in 50 patients suffering from duodenal ulcer before and after parietal cell vagotomy. In these patients basal acid output pentagastrin-stimulated acid output and MSF-stimulated acid output were measured. Preoperative MSF-stimulated acid output was found to be correlated with preoperative basal acid output, and postoperative MSF-stimulated acid output was correlated with both preoperative basal acid output and postoperative basal and pentagastrin-stimulated acid outputs. The addition of preoperative MSF-stimulated acid output to preoperative pentagastrin-stimulated acid output did not noticeably increase the close relationship between the latter and postoperative pentagastrin-stimulated acid output. It is concluded that basal and MSF-stimulated secretory rates overlap. Therefore, it is suggested that in both preoperative and postoperative conditions the addition of the sham-feeding test to the more simple basal and pentagastrin-stimulated studies could be discarded.

A criterion for completeness of vagotomy based on basal and vagally stimulated gastric acid secretion after esophagectomy or proximal gastric vagotomy

The variation in basal acid secretion was determined in 10 patients after resection of the esophagus, an operation resulting in a total transection of all vagal nerves to the abdomen. After recording basal acid secretion over a 3-h period, a 15-min modified sham feeding procedure was performed, and the acid output was studied for an additional hour. The mean basal acid output +2 SD was 0.27 mmol/15 min. The difference between the highest and lowest recorded 15-min output--that is, the oscillation of basal acid output--was calculated for each patient. The mean oscillation of basal acid output +2 SD was 0.58 mmol/15 min. Vagal stimulation accomplished by sham feeding produced no significant increase in acid output above this level. The variation in basal acid secretion was also investigated in 20 duodenal ulcer patients after proximal gastric vagotomy. These patients were insulin-negative and remained asymptomatic during a 7- to 10-year follow-up study. Shortly after the vagotomy, measurement of basal acid secretion over 3 h showed a mean basal acid output +2 SD of 0.58 mmol/15 min. The mean oscillation of basal acid output +2 SD was 0.66 mmol/15 min. On the basis of the oscillation in basal acid secretion after complete vagotomy we propose a new criterion for completeness of vagotomy, namely a response to physiologic vagal stimulation which does not exceed the lowest basal level by more than 0.6 mmol/15 min.

Modified sham feeding test after parietal cell vagotomy for juxtapyloric ulcer disease in patients with and without recurrent ulcers

The mean of individual coefficients of variation of acid output after modified sham feeding was 39% in 22 patients operated on with parietal cell vagotomy. The reproducibility of the interpretation of the sham feeding test as 'positive' or 'negative' was good. An intragastric infusion of a marker to correct for pyloric loss did not increase the accuracy of the test. The prognostic value of the qualitative estimation of the sham feeding test 2 months after operation to predict recurrent ulcer after parietal cell vagotomy was poor in 39 patients studied prospectively over 3 years. With the criterion sham feeding minus basal acid output over 1.0 mmol/30 min as a positive test, 63% of patients with a positive and 24% with a negative test later had recurrent ulcers. The consistency of the interpretation as either positive or negative was low in annual tests during the 3 years of follow-up study. After parietal cell vagotomy the sham-feeding-stimulated acid output was higher in patients with duodenal than in those with prepyloric recurrent ulcers and also in those without recurrences. This indicates that the amount of vagal innervation left after parietal cell vagotomy is of special importance in the occurrence of duodenal ulcer relapse.

Abolition of postprandial alkaline tide after vagotomy and its use as a screening test in the assessment of vagotomy

Assessment of completeness of vagotomy has always been an irksome and time-consuming affair. The commonly employed test of insulin-induced hypoglycaemia (Hollander) is tedious, risky and associated with practical limitations and theoretical objections, which have resulted in its being replaced by the modified sham feed technique in some centres. The postprandial alkaline tide is attributed to HCO3- release consequent to H+ secretion. Any procedure which reduces acid secretion is expected to affect the postprandial urinary pH. This study was performed to evaluate the effect of vagotomy on the postprandial alkaline tide by measuring prevagotomy and postvagotomy fasting and postprandial urinary pH. The results show that postprandial urinary pH falls after vagotomy and this method may be used to assess the completeness of vagotomy. The proposed test is convenient and may be used as a screening method in the assessment of vagotomy and other acid reducing operative procedures, if further validated, by comparison with already established tests.

Gastric secretory responses to modified sham feeding (MSF) and insulin after vagotomy

Modified sham feeding (MSF) and insulin tests were carried out in 28 patients after vagotomy. Basal and pentagastrin collections were also performed. Using the secretory data from 9 of the patients with endoscopically proven recurrent ulcer as a reference, 20 gastric secretory indices were studied and critical levels were chosen to provide the maximum sensitivity and specificity. The eleven most discriminating indices were then used to evaluate the remaining 19 patients. There was agreement between the responses to MSF and insulin as to the adequacy of vagotomy in 16 of the patients (84 per cent) and contradictory responses in 3 patients (16 per cent). Overall, MSF responses were as discriminating as the responses to insulin. A simplified ward test, based on crude volume measurements in response to MSF is proposed.

Pitfalls in postoperative testing of the completeness of vagotomy

The gastric acid response to insulin hypoglycemia represents an effect of several stimulatory and inhibitory mechanisms. In the intact stomach the direct vagal excitation of the acid secreting glands is the predominant mechanism. After vagotomy, however, the balance between the stimulatory and inhibitory mechanisms is unpredictable. Some stimulatory and inhibitory mechanisms are non-vagal, and may after vagotomy result in a false conception of remaining vagal fibers and complete vagotomy, respectively. Sham feeding may be a safer and more reliable test of completeness of vagotomy. A study of the spontaneous variation of basal acid secretion over several hours after vagotomy in 22 patients showed in 15-min samples a maximal range of 0.49 and 0.65 mmol with a P value of 0.05 and 0.01, respectively. A higher range may thus indicate a true acid response to a given stimulus. In 3 patients with an acid response to sham feeding but no acid response to insulin in a dose of 0.2 IU/kg after vagotomy, a repeated test with a lower dose of insulin resulted in an unequivocal acid response. In 4 patients with no acid response to sham feeding and a substantial acid response to insulin after vagotomy, the acid response to insulin was abolished after pretreatment with an adrenergic blocker. The insulin test may thus give false negative and positive information about the completeness of vagotomy.

Plasma catecholamine and serum gastrin concentrations during sham feeding

Plasma adrenaline, plasma noradrenaline and serum gastrin concentrations were measured before and after sham feeding in eight patients with duodenal ulcer and in four normal subjects. No significant change in the concentrations was observed after sham feeding. In three patients with duodenal ulcer an insulin test resulted in a 25-fold rise in plasma adrenaline. The ulcer patients showed significantly higher levels of plasma adrenaline and plasma noradrenaline than the normal subjects both before and after sham feeding, and this difference was probably not caused only by age difference in the two groups. It is concluded that sympathetic nervous activity and serum gastrin concentrations are not influenced by sham feeding in contrast to the influence of insulin hypoglycemia.

Gastric acid response to sham feeding and pentagastrin before and after parietal cell vagotomy in patients with duodenal ulcer

The influence of sham feeding (PAOSh) preceding pentagastrin-stimulated gastric acid secretion (PAOPg) was investigated in 28 patients with duodenal ulcer (DU) before vagotomy and in 36 after parietal cell vagotomy (PCV). Sham feeding had little influence on PAOPg, and it is concluded that the two secretion tests may be combined. The ratio PAOSh/PAOPg was significantly reduced by PCV. Fourty-four patients with DU were studied for 1 year after PCV, and their PAOPg was measured preoperatively and their PAOPg and PAOSh postoperatively. Seven of the 44 patients had recurrent ulcer within 1 year. PAOPg had no predictive value pre- and post-operatively, but postoperative PAOSh and PAOSh/PAOPg were both significantly higher in patients with recurrent ulcer. It is concluded that PAOSh and PAOSh/PAOPg after PCV may assess completeness of vagotomy, but the relationship between PAOSh and risk of recurrent ulcer may be stronger than that between PAOSh/PAOPg and recurrence.