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Determinants of esophageal acid clearance in normal subjects
GASTROENTEROLOGY
1983:85:607-l
2
Determinants of Esophageal Acid Clearance in Normal Subjects JAMES F. HELM, WYLIE J. DODD& DAVID R. RIEDEL, BRUCE C. TEETER, WALTER J. HOGAN, and RONALD C. ARNDORFER Departments Wisconsin
of Medicine
and Radiolonv..__ The Medical
In this study, we evaluated factors that ajfect esophageal acid clearance in normal subjects. A 15-ml bolus of 0.1 N HCJ (pH 1.2) was injected into the esophagus, and the subject then swallowed every 30 s. Manometric and pH monitoring demonstrated that esophageal acid clearance occurred by a series of step increases in pH, each associated with a swallow-induced peristaltic sequence. Between peristaltic sequences, pH increase was minimal. Saliva stimulation by oral lozenge greatly improved acid clearance, while oral aspiration of saliva abolished the step increases in esophageal pH and markedly delayed acid clearance. Replacement of aspirated saliva with a bicarbonate solution reproduced the step increases in esophageal pH and restored acid clearance toward normal, while replacement with water alone failed to improve acid clearance. SimiJar to the effect of‘ the oral lozenge, bethanechol (5 mg subcutaneously) improved esophageal acid clearance, but this improvement was reversed by oral aspiration of saliva, which markedly delayed acid clearance. A change from the recumbent to the sitting position tended to improve acid clearance slightl!~, but this improvement was not statistically significant. We concluded that in normal subjects (a) swallowing carries saliva into the esophagus and peristalsis empties intraesophageal fluid into the stomach, (bJ the neutralization of acid by saliva carried into the esophagus with each swallow ac-
Keceived July 26. 1982. Accepted March 21. 1983. Address requests for reprints to: Wylie J, Dodds. M.D.. Department of Kadiology. Milwaukee County General Hospital, 8700 West Wisconsin A\renue, Milwaukee, M’isconsin 53226. This work was supported in part by the National Institutes of Health Grants AM 15540 and AM 25731, and by a grant from the Clinic.al Center’s Program No. 0005A. This Lvork was abstracted, in part, in CASTROENTEKOLOGY (11. ‘( 1983 hv the American Gastroenterological fJO1 (i-508.7 ‘8~~Y$3.00
Association
College
of Wisconsin,
Milwaukee
counts for the occurrence of acid clearance by step increases in pH, (c) the improvement in acid clearance with bethanechol is due to saliva stimulation, and [d) gravity contributes little to esophageal acid clearance in the presence of normal peristaltic stripping waves. Esophageal acid clearance is an important mechanism protective against the development of reflux esophagitis (2). The determinants of esophageal acid clearance, however, have been incompletely studied. Our aim in this study was to [a) characterize esophageal acid clearance in normal subjects, (b) evaluate the contributions of esophageal peristalsis, salivation, and gravity to acid clearance in normal subjects, and (c) determine the mechanism by which cholinergic drugs influence esophageal acid clearance.
Methods We studied 23 men and 4 women, aged 20-30 yr. All volunteers were healthy and were without a history of heartburn. Esophageal manometry demonstrated normal primary peristalsis in each subject. The study protocol was approved by the Human Research Review Committee of the Medical College of Wisconsin. The subjects were studied in the supine position after an overnight fast. We monitored esophageal motor activity using an intraluminal transducer assembly consisting of a s-mm diameter tube with three pressure transducers spaced at s-cm intervals (esophageal transducer model MP-3, Honeywell, Denver, Col.). After nasoesophageal intubation, the lower esophageal sphincter (LES) was located by a pull-through and the distal transducer was positioned 5 cm above the sphincter. A Z-mm diameter polyvinyl catheter attached to the transducer assembly allowed subsequent injection of acid into the distal esophagus. To monitor esophageal pH, a pH electrode (Beckman Instruments, Inc., Schiller Park, Ill.) was passed transnasally and stationed 5 cm above the LES. Esophageal motor
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1. Esophageal acid clearance in 27 normal subjects. Val-
ues are shown for acid clearance time to pH 4 and the corresponding number of swallows taken at 30-s intervals. Each data point represents the mean value obtained for three observations in a single subject. The bar and vertical line indicate the group mean c 1 SE.
pH were recorded simultaneously on a polygraph (Beckman). To study esophageal acid clearance, a Is-ml bolus of 0.1 N HCl (pH 1.2) was injected into the distal esophagus within a 3-4-s interval. Swallowing was standardized by instructing the subject to take a dry swallow every 30 s after injection of the acid bolus until the esophageal pH rose above 5. Three control acid clearance sequences were obtained for each subject. The contribution of saliva to esophageal acid clearance was studied by saliva stimulation and saliva elimination. An oral peppermint lozenge was used to stimulate saliva flow and increase the ability of saliva to neutralize acid (3). To minimize the volume of saliva carried into the esophagus with each swallow, saliva was aspirated from the mouth. To evaluate the mechanism by which saliva influences esophageal acid clearance, aspirated saliva was replaced with water or a bicarbonate solution swallowed every 30 s to approximate each subject’s saliva flow. The bicarbonate solution was individualized to simulate the capacity of each subject’s saliva to neutralize acid. Before studying acid clearance, saliva was collected from the intubated subject by expectoration, and its flow, pH, and ability to neutralize acid were assessed by methods previously described (3). The 5 subjects in whom saliva replacement was studied had a saliva flow of 0.88 + 0.20 mlimin with a pH of 7.31 * 0.03. The ability of each subject’s saliva to neutralize acid was demonstrated by a titration curve plotted as the volume of saliva required to titrate 1 ml of 0.1 N HCl to pH 6. The acid titration curve obtained for saliva from each subject was compared with the previously described family of curves (3) resulting from the titration of 1 ml of 0.1 N HCl with NaHC03 solutions of various concentrations. Aspirated saliva from a given subject was replaced by a 15-22-mM NaHC03 solution with an acid activity
and
titration curve similar to the titration curve for saliva from that subject. The pH of the bicarbonate solution was adjusted to that of the subject’s saliva. The following additional variables were studied for their effect on esophageal acid clearance: [a) interval between dry swallows (30 and 60 s) after acid injection, (b) bethanechol (5 mg s.c.), (c) atropine (12 pglkg i.v.), and (d) the sitting position. For data analysis, acid clearance time (ACT) was scored as the time required for esophageal pH to rise to 4 after acid injection. The amplitude of esophageal peristaltic pressure complexes was measured as the difference between peak pressure and mean basal intraesophageal pressure. Group values given in the text are shown as % i SE. Unless otherwise noted, a value for an individual subject represents the mean for three acid clearance sequences. Thus, a group mean value represents the mean of individual subject means. Variations in control values of ACT were assessed by analysis of variance. To test experimental results for statistical significance, we used the Student’s
paired t-test.
Results For all 27 subjects, esophageal ACT to pH 4 was 267 * 22 (SE) s, a value corresponding to an average of nine swallows taken at 30-s intervals (Figure 1). Acid clearance time varied among subjects from 111 to 545 s. Between-sample variation of ACT in the same subject, however, was significantly less than the variation of ACT among subjects (p < 0.005). Close correlation was found for ACT between studies in the same subject (r = 0.8, p < 0.01). For three successive determinations of ACT, the group mean values did not differ significantly. A representative example of esophageal acid clearance recorded during standardized control conditions with dry swallows every 30 s is shown in Figure 2. Injection of a 15-ml acid bolus into the
6
Esophageal 3
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PH 140
Pressure 70
mm Hg 0 11
f
Acid Bolus
t
DS
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0
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DS
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DS
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0
2
I
Time
t
DS
in
t
DS
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Minutes
Figure 2. Relation of esophageal acid clearance to peristalsis. Only pressure complexes from the distal esophagus are shown. The initial pressure rise was due to injection of the acid bolus. Dry swallows are indicated by DS. Esophageal pH swallow-induced injection.
did not begin to rise until the first peristaltic sequence 30 s after acid
September
1983
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Figure
.
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Clsarancs
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Amplitude
of esophageal acid clearance time to ampli3. Kelatioo complexes in the distal tude of peristaltic pressure esophagus. Three observations are shown for each of 18 subjects. Y ~2 1.41X + 173. r = 0.336 (p < 0.05).
esophagus invariably caused an immediate pressure rise of 10-30 mmHg that was generally followed by a secondary peristaltic sequence within 5-15 s. A precise relationship existed between esophageal acid clearance and peristalsis. After injection of an acid bolus, esophageal pH returned to its original value in a series of steplike increases, each associated with a swallow-induced peristaltic sequence. The step increase in esophageal pH did not occur when swallowing occasionally failed to produce a peristaltic sequence. Between peristaltic sequences, negligible change occurred in pH. In 6 subjects, increasing the interval between swallows from 30 to 60 s increased ACT significantly from 260 ? 34 s to 682 t 121 s (p < 0.05). Unexpectedly, linear regression analysis suggested that an increase in the amplitude of peristaltic contractions was associated with delay in esophageal acid clearance (Figure 3). This apparent association, however, was extremely weak, as indicated by the coefficient of determination (r’ = 0.11). When three outlying data points from a subject with peristaltic amplitudes >120 mmHg were excluded, and a regression analysis was done on the 51 remaining data points, then ACT no longer correlated with the amplitude of peristaltic contractions (r = 0.06, p > 0.25). Saliva stimulation by oral lozenge reduced ACT by -50%. while the amplitude of esophageal peristalsis
remained unchanged (Figure 4). In contrast, diversion of saliva from the esophagus by oral aspiration abolished the discrete step increases in esophageal pH and markedly prolonged ACT [Figure 5). During saliva aspiration, the esophageal pH demonstrated a continuous, barely perceptible rise. Replacement of aspirated saliva by swallows of water approximating the volume of each subject’s saliva flow did not improve esophageal acid clearance. On the other hand, ACT was restored toward normal by replacing aspirated saliva with an equal volume of a bicarbonate solution that approximated the acid neutralizing ability of saliva for the individual. Esophageal acid clearance again occurred by a series of step increases in pH, each associated with a peristaltic sequence that carried swallowed bicarbonate solution into the esophagus. The amplitude of esophageal peristalsis remained unchanged by saliva elimination or replacement. Bethanechol, a drug that stimulates salivation (3), reduced ACT by -50% (Figure 6), an effect similar to that of the oral lozenge (Figure 4). The amplitude of esophageal peristalsis tended to increase after bethanechol, but the difference from the control value did not reach statistical significance. Oral aspiration of saliva reversed the accelerating effect of bethanechol on acid clearance, resulting in an ACT identical to that for aspiration alone. Atropine, in a 12-pgikg iv. dose that eliminates salivation (3), abolished both esophageal peristalsis and acid clearance. The effect of gravity on esophageal acid clearance was studied in 10 subjects. A change from the recumbent to the sitting position was accompanied by a decrease in ACT from 261 ? 27 s to 216 -r- 43 s,
1
ssc
400 300 Acid Clswonca 200 Time 100
0
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* n__ -
I-
mmHO1
C.I”,VJ Figure
4. Effect of time and represent SE. The clearance effect on
L0Wlpe
the oral lozenge oo esophageal acid c1earanc.r: peristaltic amplitude io 5 subjecis. The hors mean values and the vertical T’s indicate 1 lozenge caused a significant redur_tion in acid time (*p i 0.02) whilrx ha\ ing no significant peristaltic amlIlitudr.
610
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ET AL.
CASTROENTEROLOGY
600
Acid Clearance Time
mmHg 100 1 Peristaltic Amplitude
Control I
Figure
5
Oral H20 Suction
Effect
of saliva on esophageal acid clearance time (ACT) and peristaltic amplitude in 5 subjects. Control values are shown by open bars. Diagonal lines indicate oral aspiration of saliva. Dry swallows are noted by DS. Shading represents replacement of aspirated saliva with water (light shading) or a bicarbonate solution [darker shading). The bars represent mean values and the vertical T’s indicate 1 SE. Unless the diverted saliva was replaced with a bicarbonate solution, aspiration so effectively impaired esophageal acid clearance that test sequences had to be terminated before the return of esophageal pH to above 4. In these instances, mean values for ACT represent only a lower limit, and the SE cannot be calculated. The following designations were used for statistical comparisons: *p < 0.01. greater than control: **p c 0.05. greater than control. but less than saliva aspiration alone.
but this change 0.20).
was not statistically
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taken at 30-s intervals. An earlier report suggested or that acid clearance to pH 6 occurs in 10 swallows less in normal subjects (9). In contrast, a third of our subjects required 10-20 swallows for acid clearance to pH 4. Our findings in normal subjects are in general agreement with those reported by Kjellen and Tibling (11). In these later studies, acid clearance to pH 5 required 14 ? 8 (SD] swallows in 21 volunteers. In our study, ACT varied widely from subject to subject, but values obtained for a single subject or group of subjects were quite reproducible. Our observations support the general belief that peristalsis is an important mechanism for esophageal acid clearance. During acid clearance, esophageal pH increased in a series of steps, each associated with a swallow-induced peristaltic sequence. When swallowing occasionally failed to generate an esophageal peristaltic sequence, the step increase in esophageal pH did not occur. Further, doubling the interval between swallows prolonged ACT more than twofold. Statistical analysis suggested an apparent association between ACT and the amplitude of peristaltic contractions. Contrary to our expectation, however, an increase in the contraction amplitude
>1163*
*
Acid Clearance
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Discussion Esophageal acid clearance is believed to be an important factor protective against the development of reflux esophagitis. After gastroesophageal reflux, the efficacy of esophageal acid clearance determines the exposure of esophageal mucosa to refluxed acid. The severity of esophagitis is related to the potency of refluxed material and the duration of its contact with the esophageal mucosa (4-8). In this study, we evaluated factors of potential importance for esophageal acid clearance in normal subjects. The study findings indicate that both esophageal peristalsis and the ability of swallowed saliva to neutralize acid are major determinants of esophageal acid clearance in normal subjects. In this study, we evaluated esophageal acid clearance by a standardized method similar to that used by others (9-11). We found that the mean ACT to pH 4 was 267 s, a value corresponding to nine swallows
mmHg
t
Peristaltic Amplitude
Control
Saliva
Suction L
Figure
Suction
Bsthanechol
1
6. Effect of bethanechol (5 mg s.c.) on esophageal acid clearance time and peristaltic amplitude in !? subjects. Open bars indicate control values, Diagonal fines indicate oral aspiration of saliva. Values obtained after bethanechol are shown by stippled bars. The bars represent mean values and the vertical T‘s indicate 1 SE Saliva aspiration so effectively impaired esophageal acid clearance that test sequences had to be terminated before the return of esophageal pH to above 4. Thus, mean values for ACT obtained during saliva aspiration represent only a lower limit, and the SE cannot be calculated. The following designations were used for statistical comparisons: *p < 0.02, less than control: **p < 0.02. greater than control or bethanechol alone.
within the range of normal was associated with delay in esophageal acid clearance. This statistical association was quite weak, as the coefficient of determination (r”) indicates that only ll(% of the variation in ACT could be explained by the amplitude of peristaltic contractions. Furthermore, with the exclusion of three outlying data points from the regression analysis, ACT no longer correlated with the amplitude of peristaltic contractions. Knowledge of the amplitude of peristaltic contractions is of little value in predicting ACT in normal subjects. LYe believe that the specific amplitude of normal peristaltic contractions is not a critical factor in acid clearance because a wide range of contraction amplitudes within the range of normal may effectively obliterate the esophageal lumen and strip fluid from the esophagus. Although closely associated with swallow-induced peristaltic sequences, esophageal acid clearance cannot be the result of peristalsis alone. Because a pH electrode measures acid concentration rather than volume. it cannot distinguish between a drop of acid and a liter of acid. For this reason. a progressive reduction in acid volume by peristaltic sequences cannot account for the step increases in esophageal pH by itself. Our findings demonstrate the important contribution of saliva to esophageal in acid clearance, as shown by the 509, reduction ACT by stimulation of saliva with the oral lozenge, and the marked prolongation of acid clearance by saliva aspiration. A wet swallow is associated with a greater amplitude of peristaltic pressure than is a dry swallow (12). This phenomenon, however, cannot account for the influence of saliva on acid clearance because replacement of aspirated saliva with water failed to impro\re acid clearance. In the past. investigators proposed that if saliva had any role in esophageal acid clearance, its role was primarily one of lubrication or rinsing, rather than acid neutralization (11). If, however, a progressive reduction in esophageal acid volume alone cannot ac.c:ount for acid clearance, then dilution or neutralization of acid must occur if esophageal pH is to return to normal. M’e have recently shown (3) that physiologic rates of saliva flow are capable of neutralizing small amounts of acid, and that the ability of saliva to neutralize acid is primarily due to bicarbonate. The findings from this study suggest that the ability of slvallowed saliva to neutralize acid is responsible for the typical pattern of acid clcarance by a series of step increases in esophageal pH. In support of this conclusion. we found that replacement of aspirated saliva \zith a bicarbonate solution reproduced the step increases in esophageal pH and restored ;I
During saliva aspiration without replacement, or with replacement by water alone. the esophageal pH increased minimally as a slow. barely perceptible rise. This gradual rise in esophageal pH may be due to either leakage of hydrogen ions across the esophageal mucosa, or acid neutralization by esophageal secretions. We found that while bethanechol decreased ACT by -50%,, the associated increase in the amplitude of peristaltic contraction was small. Further. the amplitude of peristaltic contractions was not a critical factor in acid clearance when the amplitude varied within the normal range. For these reasons, any stimulation of peristalsis in normal subjects by bethanechol would not appear to explain a significant reduction in ACT. Bethanechol has been shown to increase saliva flow and the abilitv of saliva to neutralize acid similar to the effect of the oral lozenge (31, an agent that also ;mpro\res esophageal acid clearance. Because aspiration of saliI,a reversed the effect of bethanechol and markedly prolonged ACT, we believe that in normal subjects the improvement of acid clearance by bethanechol is due: to stirnulation of saliva. In patients w:ith reflux esophagitis. previous studies have sholvn that bethanechol improves esophageal acid clearance (1X,14). This effect has been attributed to enhanced peristalsis (15,16). Nevertheless, in some patients \vith gastrocsophageal reflux who are treated with bcthancchol. saliva stimulation may be a major reason for improvement of acid clearance and reduction of heartburn symptoms (17). In contrast to the beneficial effect of bethanechol on ACT, atropino inhibited salivation, depressed peristalsis, and virtually abolished esophageal clearance of acid. This fintling adds support to the notion that anticholinergic: agents are wntraindicated in patients with reflux esophagitis. Gravity is known to be important for esophageal emptying in patients with abnormal rwphageal motor function. For example, \vhr:n esophageal peristalsis is impaired, as in patients with scleroderma, barium is retained in the esophagus while the patient is recumbent, but empties rapidly into the stomach when the patient assumes the upright position (18). Some patients with rellux c>;ophagitis have abnormal esophageal peristalsis that ma\’ compromise esophageal emptying in the> recumbent position, and thereby prolong acid clearance (I 0). Elevation of the head of the b:ti oi;tc:n improves esophageal acid clearance in patients with reflux esophagitis (10,14.19). Thus. gra\,ity’ is important foi acid clearance in the patient l,vith int:tfecti\e esophageal emptying due to abnormal esophageal motor function. In contrast, when normal subjects assumed the sitting position. we found only a small improvement in esophageal acid clearanc:c:. A similar finding
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has been noted by Kjellen and Tibling (11).These investigators also reported that the 15” head-down position prolonged acid clearance as compared with the supine position. Studies monitoring spontaneous reflux episodes showed a slight tendency for the acid clearance time to decrease when normal subjects assumed the upright position (20).Thus, gravity would appear to influence esophageal acid clearance in the normal subject, but this influence is small. Perhaps the contribution of gravity to acid clearance in a healthy, upright subject is minimal because normal peristalsis alone efficiently strips fluid from the esophagus. Both esophageal peristalsis and the neutralization of acid by swallowed saliva are major determinants of esophageal acid clearance in normal subjects. Swallowing carries saliva into the esophagus and peristalsis empties intraesophageal fluid into the stomach. During conditions of minimal stimulation in awake subjects, a resting saliva flow of about 0.5 mlimin (3) is transported into the esophagus by spontaneous swallows that occur about once a minute (12,21,22). The neutralization of acid by saliva carried into the esophagus with each swallow accounts for the phenomenon of esophageal acid clearance by a series of step increases in pH. At night, gastroesophageal reflux and esophageal clearance of acid occur only during arousals from sleep (22). If esophageal acid clearance is not completed before a subject resumes sleep, acid clearance is delayed until another arousal from sleep, because during sleep peristalsis seldom occurs (22,23)and saliva flow virtually ceases (24).In theory, impaired salivation as well as abnormal peristalsis may prolong esophageal acid clearance.
References Helm JF, Riedel DR, Dodds WJ, Hogan WJ, Pate1 GW, Amdorfer RC. Determinants of esophageal acid clearance in normal subjects (abstr). Gastroenterology 1980;78:1181. Dodds WJ, Hogan WJ, Helm JF, Dent J. Pathogenesis of reflux esophagitis. Gastroenterology 1981;81:376-94. Helm JF, Dodds WJ, Hogan WJ, Soergel KH, Egide MS, Wood CM. Acid neutralizing capacity of human saliva. Gastroenterology 1982;83:69-74. Goldberg HI, Dodds WJ, Gee S, Montgomery C. Zboralske FF.
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Role of acid and pepsin in acute experimental esophagitis. Gastroenterology 1969;56:223-30. 5. Goldberg HI, Dodds WJ, Montgomery C, Baskin SA, Zboralske FF. Controlled production of acute esophagitis. Invest Radio1 1970;5:254-6. 6. Dodds WJ, Goldberg HI, Montgomery C, Ludemann WB. Zboralske FF. Sequential gross, microscopic, and roentgenographic features of acute feline esophagitis. Invest Radio1 1970;5:209-19. 7. Johnson LF, DeMeester TR, Haggitt RC. Esophageal epithelial response to gastroesophageal reflux. A quantitative study. Am J Dig Dis 1978;23:498-509. 8. Harmon JW, Johnson LF, Maydonovitch CL. Effects of acid and bile salts on the rabbit esophageal mucosa. Dig Dis Sci 1981;26:65-72. 9. Booth DJ, Kemmerer WT, Skinner DB. Acid clearing from the distal esophagus. Arch Surg 1968;96:731-4. 10. Stanciu C, Bennett JR. Oesophageal acid clearing: one factor in the production of reflux oesophagitis. Gut 1974;15:852-7. 11. Kjellen G, Tibling L. Influence of body position, dry and water swallows, smoking, and alcohol on esophageal acid clearing. Stand J Gastroenterol 1978;13:283-8. 12. Dodds WJ, Hogan WJ, Reid DP, Stewart ET, Arndorfer RC. A comparison between primary esophageal peristalsis following wet and dry swallows. J Appl Physiol 1973;35:851-7. KP, Dodds WJ, Hogan WJ, Barreras 13. Miller WN, Ganeshappa RF, Arndorfer RC. Effect of bethanechol on gastroesophageal reflux. Am J Dig Dis 1977:22:230-4. 14. Johnson LF, DeMeester TR. Evaluation of elevation of the head of the bed, bethanechol, and antacid foam tablets on gastroesophageal reflux. Dig Dis Sci 1981;26:673-80. 15. Humphries TJ, Caste11 DO. Effect of oral bethanechol on parameters of esophageal peristalsis. Dig Dis Sci 1981: 26:129-32. 16. Phaosawasdi K, Malmud LS, Tolin RD. Stelzer F. Applegate G, Fisher RS. Cholinergic effects on esophageal transit and clearance. Gastroenterology 1981;81:915-20. therapy and 17. Farrell RL, Roling GT, Caste11 DO. Cholinergic chronic heartburn. A controlled trial. Ann Intern Med 1974;80:573-6. disorders. In: Margulis A, 18. Dodds WJ. Hare11 GS. Motility Burhenne H, eds. Alimentary tract roentgenology. Vol 1, 2nd ed.. St. Louis: CV Mosby Co., 1973:461-83. on gastro-oesopha19. Stanciu C, Bennett JR. Effects of posture geal reflux. Digestion 1977;15:104-9. 20. DeMeester TK, Johnson LF, Joseph GJ. Toscano MS. Hall AW, Skinner DB. Patterns of gastroesophageal reflux in health and disease. Ann Surg 1976;184:459970. JB Jr, Moorrees CFA. The frequency of 21. Lear CSC, Flanagan deglutition in man. Arch Oral Biol 1965;10:83-100. RH. et al. Mechanism of gastro22. Dent J, Dodds WJ, Friedman esophageal reflux in recumbent asymptomatic subjects. J Clin Invest 1980;65:256-67. during sleep. 23. Lichter I, Muir RC. The pattern of swallowing Electroencephalogr Clin Neurophysiol 1975;38:427-32. LH, Pigman W, Hanahan L, Gilmore RW. Rate of 24. Schneyer flow of human parotid, sublingual, and submaxillary secretions during sleep. J Dent Res 1956;35:109-14.
1983:85:607-l
2
Determinants of Esophageal Acid Clearance in Normal Subjects JAMES F. HELM, WYLIE J. DODD& DAVID R. RIEDEL, BRUCE C. TEETER, WALTER J. HOGAN, and RONALD C. ARNDORFER Departments Wisconsin
of Medicine
and Radiolonv..__ The Medical
In this study, we evaluated factors that ajfect esophageal acid clearance in normal subjects. A 15-ml bolus of 0.1 N HCJ (pH 1.2) was injected into the esophagus, and the subject then swallowed every 30 s. Manometric and pH monitoring demonstrated that esophageal acid clearance occurred by a series of step increases in pH, each associated with a swallow-induced peristaltic sequence. Between peristaltic sequences, pH increase was minimal. Saliva stimulation by oral lozenge greatly improved acid clearance, while oral aspiration of saliva abolished the step increases in esophageal pH and markedly delayed acid clearance. Replacement of aspirated saliva with a bicarbonate solution reproduced the step increases in esophageal pH and restored acid clearance toward normal, while replacement with water alone failed to improve acid clearance. SimiJar to the effect of‘ the oral lozenge, bethanechol (5 mg subcutaneously) improved esophageal acid clearance, but this improvement was reversed by oral aspiration of saliva, which markedly delayed acid clearance. A change from the recumbent to the sitting position tended to improve acid clearance slightl!~, but this improvement was not statistically significant. We concluded that in normal subjects (a) swallowing carries saliva into the esophagus and peristalsis empties intraesophageal fluid into the stomach, (bJ the neutralization of acid by saliva carried into the esophagus with each swallow ac-
Keceived July 26. 1982. Accepted March 21. 1983. Address requests for reprints to: Wylie J, Dodds. M.D.. Department of Kadiology. Milwaukee County General Hospital, 8700 West Wisconsin A\renue, Milwaukee, M’isconsin 53226. This work was supported in part by the National Institutes of Health Grants AM 15540 and AM 25731, and by a grant from the Clinic.al Center’s Program No. 0005A. This Lvork was abstracted, in part, in CASTROENTEKOLOGY (11. ‘( 1983 hv the American Gastroenterological fJO1 (i-508.7 ‘8~~Y$3.00
Association
College
of Wisconsin,
Milwaukee
counts for the occurrence of acid clearance by step increases in pH, (c) the improvement in acid clearance with bethanechol is due to saliva stimulation, and [d) gravity contributes little to esophageal acid clearance in the presence of normal peristaltic stripping waves. Esophageal acid clearance is an important mechanism protective against the development of reflux esophagitis (2). The determinants of esophageal acid clearance, however, have been incompletely studied. Our aim in this study was to [a) characterize esophageal acid clearance in normal subjects, (b) evaluate the contributions of esophageal peristalsis, salivation, and gravity to acid clearance in normal subjects, and (c) determine the mechanism by which cholinergic drugs influence esophageal acid clearance.
Methods We studied 23 men and 4 women, aged 20-30 yr. All volunteers were healthy and were without a history of heartburn. Esophageal manometry demonstrated normal primary peristalsis in each subject. The study protocol was approved by the Human Research Review Committee of the Medical College of Wisconsin. The subjects were studied in the supine position after an overnight fast. We monitored esophageal motor activity using an intraluminal transducer assembly consisting of a s-mm diameter tube with three pressure transducers spaced at s-cm intervals (esophageal transducer model MP-3, Honeywell, Denver, Col.). After nasoesophageal intubation, the lower esophageal sphincter (LES) was located by a pull-through and the distal transducer was positioned 5 cm above the sphincter. A Z-mm diameter polyvinyl catheter attached to the transducer assembly allowed subsequent injection of acid into the distal esophagus. To monitor esophageal pH, a pH electrode (Beckman Instruments, Inc., Schiller Park, Ill.) was passed transnasally and stationed 5 cm above the LES. Esophageal motor
608
HELM ET AL.
GASTROENTEROLOGY Vol. 85. Ku. 3
set
600
20
. . 15
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400
”
Acid Clearance Time
267 3
20t
! .. .
IO
Number
of
Swallows
!f
c
5
0 Figure
1. Esophageal acid clearance in 27 normal subjects. Val-
ues are shown for acid clearance time to pH 4 and the corresponding number of swallows taken at 30-s intervals. Each data point represents the mean value obtained for three observations in a single subject. The bar and vertical line indicate the group mean c 1 SE.
pH were recorded simultaneously on a polygraph (Beckman). To study esophageal acid clearance, a Is-ml bolus of 0.1 N HCl (pH 1.2) was injected into the distal esophagus within a 3-4-s interval. Swallowing was standardized by instructing the subject to take a dry swallow every 30 s after injection of the acid bolus until the esophageal pH rose above 5. Three control acid clearance sequences were obtained for each subject. The contribution of saliva to esophageal acid clearance was studied by saliva stimulation and saliva elimination. An oral peppermint lozenge was used to stimulate saliva flow and increase the ability of saliva to neutralize acid (3). To minimize the volume of saliva carried into the esophagus with each swallow, saliva was aspirated from the mouth. To evaluate the mechanism by which saliva influences esophageal acid clearance, aspirated saliva was replaced with water or a bicarbonate solution swallowed every 30 s to approximate each subject’s saliva flow. The bicarbonate solution was individualized to simulate the capacity of each subject’s saliva to neutralize acid. Before studying acid clearance, saliva was collected from the intubated subject by expectoration, and its flow, pH, and ability to neutralize acid were assessed by methods previously described (3). The 5 subjects in whom saliva replacement was studied had a saliva flow of 0.88 + 0.20 mlimin with a pH of 7.31 * 0.03. The ability of each subject’s saliva to neutralize acid was demonstrated by a titration curve plotted as the volume of saliva required to titrate 1 ml of 0.1 N HCl to pH 6. The acid titration curve obtained for saliva from each subject was compared with the previously described family of curves (3) resulting from the titration of 1 ml of 0.1 N HCl with NaHC03 solutions of various concentrations. Aspirated saliva from a given subject was replaced by a 15-22-mM NaHC03 solution with an acid activity
and
titration curve similar to the titration curve for saliva from that subject. The pH of the bicarbonate solution was adjusted to that of the subject’s saliva. The following additional variables were studied for their effect on esophageal acid clearance: [a) interval between dry swallows (30 and 60 s) after acid injection, (b) bethanechol (5 mg s.c.), (c) atropine (12 pglkg i.v.), and (d) the sitting position. For data analysis, acid clearance time (ACT) was scored as the time required for esophageal pH to rise to 4 after acid injection. The amplitude of esophageal peristaltic pressure complexes was measured as the difference between peak pressure and mean basal intraesophageal pressure. Group values given in the text are shown as % i SE. Unless otherwise noted, a value for an individual subject represents the mean for three acid clearance sequences. Thus, a group mean value represents the mean of individual subject means. Variations in control values of ACT were assessed by analysis of variance. To test experimental results for statistical significance, we used the Student’s
paired t-test.
Results For all 27 subjects, esophageal ACT to pH 4 was 267 * 22 (SE) s, a value corresponding to an average of nine swallows taken at 30-s intervals (Figure 1). Acid clearance time varied among subjects from 111 to 545 s. Between-sample variation of ACT in the same subject, however, was significantly less than the variation of ACT among subjects (p < 0.005). Close correlation was found for ACT between studies in the same subject (r = 0.8, p < 0.01). For three successive determinations of ACT, the group mean values did not differ significantly. A representative example of esophageal acid clearance recorded during standardized control conditions with dry swallows every 30 s is shown in Figure 2. Injection of a 15-ml acid bolus into the
6
Esophageal 3
11
PH 140
Pressure 70
mm Hg 0 11
f
Acid Bolus
t
DS
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0
t
DS
t
DS
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0
2
I
Time
t
DS
in
t
DS
t
DS ??
3
Minutes
Figure 2. Relation of esophageal acid clearance to peristalsis. Only pressure complexes from the distal esophagus are shown. The initial pressure rise was due to injection of the acid bolus. Dry swallows are indicated by DS. Esophageal pH swallow-induced injection.
did not begin to rise until the first peristaltic sequence 30 s after acid
September
1983
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Figure
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.
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120
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Amplitude
of esophageal acid clearance time to ampli3. Kelatioo complexes in the distal tude of peristaltic pressure esophagus. Three observations are shown for each of 18 subjects. Y ~2 1.41X + 173. r = 0.336 (p < 0.05).
esophagus invariably caused an immediate pressure rise of 10-30 mmHg that was generally followed by a secondary peristaltic sequence within 5-15 s. A precise relationship existed between esophageal acid clearance and peristalsis. After injection of an acid bolus, esophageal pH returned to its original value in a series of steplike increases, each associated with a swallow-induced peristaltic sequence. The step increase in esophageal pH did not occur when swallowing occasionally failed to produce a peristaltic sequence. Between peristaltic sequences, negligible change occurred in pH. In 6 subjects, increasing the interval between swallows from 30 to 60 s increased ACT significantly from 260 ? 34 s to 682 t 121 s (p < 0.05). Unexpectedly, linear regression analysis suggested that an increase in the amplitude of peristaltic contractions was associated with delay in esophageal acid clearance (Figure 3). This apparent association, however, was extremely weak, as indicated by the coefficient of determination (r’ = 0.11). When three outlying data points from a subject with peristaltic amplitudes >120 mmHg were excluded, and a regression analysis was done on the 51 remaining data points, then ACT no longer correlated with the amplitude of peristaltic contractions (r = 0.06, p > 0.25). Saliva stimulation by oral lozenge reduced ACT by -50%. while the amplitude of esophageal peristalsis
remained unchanged (Figure 4). In contrast, diversion of saliva from the esophagus by oral aspiration abolished the discrete step increases in esophageal pH and markedly prolonged ACT [Figure 5). During saliva aspiration, the esophageal pH demonstrated a continuous, barely perceptible rise. Replacement of aspirated saliva by swallows of water approximating the volume of each subject’s saliva flow did not improve esophageal acid clearance. On the other hand, ACT was restored toward normal by replacing aspirated saliva with an equal volume of a bicarbonate solution that approximated the acid neutralizing ability of saliva for the individual. Esophageal acid clearance again occurred by a series of step increases in pH, each associated with a peristaltic sequence that carried swallowed bicarbonate solution into the esophagus. The amplitude of esophageal peristalsis remained unchanged by saliva elimination or replacement. Bethanechol, a drug that stimulates salivation (3), reduced ACT by -50% (Figure 6), an effect similar to that of the oral lozenge (Figure 4). The amplitude of esophageal peristalsis tended to increase after bethanechol, but the difference from the control value did not reach statistical significance. Oral aspiration of saliva reversed the accelerating effect of bethanechol on acid clearance, resulting in an ACT identical to that for aspiration alone. Atropine, in a 12-pgikg iv. dose that eliminates salivation (3), abolished both esophageal peristalsis and acid clearance. The effect of gravity on esophageal acid clearance was studied in 10 subjects. A change from the recumbent to the sitting position was accompanied by a decrease in ACT from 261 ? 27 s to 216 -r- 43 s,
1
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4. Effect of time and represent SE. The clearance effect on
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the oral lozenge oo esophageal acid c1earanc.r: peristaltic amplitude io 5 subjecis. The hors mean values and the vertical T’s indicate 1 lozenge caused a significant redur_tion in acid time (*p i 0.02) whilrx ha\ ing no significant peristaltic amlIlitudr.
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CASTROENTEROLOGY
600
Acid Clearance Time
mmHg 100 1 Peristaltic Amplitude
Control I
Figure
5
Oral H20 Suction
Effect
of saliva on esophageal acid clearance time (ACT) and peristaltic amplitude in 5 subjects. Control values are shown by open bars. Diagonal lines indicate oral aspiration of saliva. Dry swallows are noted by DS. Shading represents replacement of aspirated saliva with water (light shading) or a bicarbonate solution [darker shading). The bars represent mean values and the vertical T’s indicate 1 SE. Unless the diverted saliva was replaced with a bicarbonate solution, aspiration so effectively impaired esophageal acid clearance that test sequences had to be terminated before the return of esophageal pH to above 4. In these instances, mean values for ACT represent only a lower limit, and the SE cannot be calculated. The following designations were used for statistical comparisons: *p < 0.01. greater than control: **p c 0.05. greater than control. but less than saliva aspiration alone.
but this change 0.20).
was not statistically
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taken at 30-s intervals. An earlier report suggested or that acid clearance to pH 6 occurs in 10 swallows less in normal subjects (9). In contrast, a third of our subjects required 10-20 swallows for acid clearance to pH 4. Our findings in normal subjects are in general agreement with those reported by Kjellen and Tibling (11). In these later studies, acid clearance to pH 5 required 14 ? 8 (SD] swallows in 21 volunteers. In our study, ACT varied widely from subject to subject, but values obtained for a single subject or group of subjects were quite reproducible. Our observations support the general belief that peristalsis is an important mechanism for esophageal acid clearance. During acid clearance, esophageal pH increased in a series of steps, each associated with a swallow-induced peristaltic sequence. When swallowing occasionally failed to generate an esophageal peristaltic sequence, the step increase in esophageal pH did not occur. Further, doubling the interval between swallows prolonged ACT more than twofold. Statistical analysis suggested an apparent association between ACT and the amplitude of peristaltic contractions. Contrary to our expectation, however, an increase in the contraction amplitude
>1163*
*
Acid Clearance
600
Time
(p > 0_-!
0
Discussion Esophageal acid clearance is believed to be an important factor protective against the development of reflux esophagitis. After gastroesophageal reflux, the efficacy of esophageal acid clearance determines the exposure of esophageal mucosa to refluxed acid. The severity of esophagitis is related to the potency of refluxed material and the duration of its contact with the esophageal mucosa (4-8). In this study, we evaluated factors of potential importance for esophageal acid clearance in normal subjects. The study findings indicate that both esophageal peristalsis and the ability of swallowed saliva to neutralize acid are major determinants of esophageal acid clearance in normal subjects. In this study, we evaluated esophageal acid clearance by a standardized method similar to that used by others (9-11). We found that the mean ACT to pH 4 was 267 s, a value corresponding to nine swallows
mmHg
t
Peristaltic Amplitude
Control
Saliva
Suction L
Figure
Suction
Bsthanechol
1
6. Effect of bethanechol (5 mg s.c.) on esophageal acid clearance time and peristaltic amplitude in !? subjects. Open bars indicate control values, Diagonal fines indicate oral aspiration of saliva. Values obtained after bethanechol are shown by stippled bars. The bars represent mean values and the vertical T‘s indicate 1 SE Saliva aspiration so effectively impaired esophageal acid clearance that test sequences had to be terminated before the return of esophageal pH to above 4. Thus, mean values for ACT obtained during saliva aspiration represent only a lower limit, and the SE cannot be calculated. The following designations were used for statistical comparisons: *p < 0.02, less than control: **p < 0.02. greater than control or bethanechol alone.
within the range of normal was associated with delay in esophageal acid clearance. This statistical association was quite weak, as the coefficient of determination (r”) indicates that only ll(% of the variation in ACT could be explained by the amplitude of peristaltic contractions. Furthermore, with the exclusion of three outlying data points from the regression analysis, ACT no longer correlated with the amplitude of peristaltic contractions. Knowledge of the amplitude of peristaltic contractions is of little value in predicting ACT in normal subjects. LYe believe that the specific amplitude of normal peristaltic contractions is not a critical factor in acid clearance because a wide range of contraction amplitudes within the range of normal may effectively obliterate the esophageal lumen and strip fluid from the esophagus. Although closely associated with swallow-induced peristaltic sequences, esophageal acid clearance cannot be the result of peristalsis alone. Because a pH electrode measures acid concentration rather than volume. it cannot distinguish between a drop of acid and a liter of acid. For this reason. a progressive reduction in acid volume by peristaltic sequences cannot account for the step increases in esophageal pH by itself. Our findings demonstrate the important contribution of saliva to esophageal in acid clearance, as shown by the 509, reduction ACT by stimulation of saliva with the oral lozenge, and the marked prolongation of acid clearance by saliva aspiration. A wet swallow is associated with a greater amplitude of peristaltic pressure than is a dry swallow (12). This phenomenon, however, cannot account for the influence of saliva on acid clearance because replacement of aspirated saliva with water failed to impro\re acid clearance. In the past. investigators proposed that if saliva had any role in esophageal acid clearance, its role was primarily one of lubrication or rinsing, rather than acid neutralization (11). If, however, a progressive reduction in esophageal acid volume alone cannot ac.c:ount for acid clearance, then dilution or neutralization of acid must occur if esophageal pH is to return to normal. M’e have recently shown (3) that physiologic rates of saliva flow are capable of neutralizing small amounts of acid, and that the ability of saliva to neutralize acid is primarily due to bicarbonate. The findings from this study suggest that the ability of slvallowed saliva to neutralize acid is responsible for the typical pattern of acid clcarance by a series of step increases in esophageal pH. In support of this conclusion. we found that replacement of aspirated saliva \zith a bicarbonate solution reproduced the step increases in esophageal pH and restored ;I
During saliva aspiration without replacement, or with replacement by water alone. the esophageal pH increased minimally as a slow. barely perceptible rise. This gradual rise in esophageal pH may be due to either leakage of hydrogen ions across the esophageal mucosa, or acid neutralization by esophageal secretions. We found that while bethanechol decreased ACT by -50%,, the associated increase in the amplitude of peristaltic contraction was small. Further. the amplitude of peristaltic contractions was not a critical factor in acid clearance when the amplitude varied within the normal range. For these reasons, any stimulation of peristalsis in normal subjects by bethanechol would not appear to explain a significant reduction in ACT. Bethanechol has been shown to increase saliva flow and the abilitv of saliva to neutralize acid similar to the effect of the oral lozenge (31, an agent that also ;mpro\res esophageal acid clearance. Because aspiration of saliI,a reversed the effect of bethanechol and markedly prolonged ACT, we believe that in normal subjects the improvement of acid clearance by bethanechol is due: to stirnulation of saliva. In patients w:ith reflux esophagitis. previous studies have sholvn that bethanechol improves esophageal acid clearance (1X,14). This effect has been attributed to enhanced peristalsis (15,16). Nevertheless, in some patients \vith gastrocsophageal reflux who are treated with bcthancchol. saliva stimulation may be a major reason for improvement of acid clearance and reduction of heartburn symptoms (17). In contrast to the beneficial effect of bethanechol on ACT, atropino inhibited salivation, depressed peristalsis, and virtually abolished esophageal clearance of acid. This fintling adds support to the notion that anticholinergic: agents are wntraindicated in patients with reflux esophagitis. Gravity is known to be important for esophageal emptying in patients with abnormal rwphageal motor function. For example, \vhr:n esophageal peristalsis is impaired, as in patients with scleroderma, barium is retained in the esophagus while the patient is recumbent, but empties rapidly into the stomach when the patient assumes the upright position (18). Some patients with rellux c>;ophagitis have abnormal esophageal peristalsis that ma\’ compromise esophageal emptying in the> recumbent position, and thereby prolong acid clearance (I 0). Elevation of the head of the b:ti oi;tc:n improves esophageal acid clearance in patients with reflux esophagitis (10,14.19). Thus. gra\,ity’ is important foi acid clearance in the patient l,vith int:tfecti\e esophageal emptying due to abnormal esophageal motor function. In contrast, when normal subjects assumed the sitting position. we found only a small improvement in esophageal acid clearanc:c:. A similar finding
612
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has been noted by Kjellen and Tibling (11).These investigators also reported that the 15” head-down position prolonged acid clearance as compared with the supine position. Studies monitoring spontaneous reflux episodes showed a slight tendency for the acid clearance time to decrease when normal subjects assumed the upright position (20).Thus, gravity would appear to influence esophageal acid clearance in the normal subject, but this influence is small. Perhaps the contribution of gravity to acid clearance in a healthy, upright subject is minimal because normal peristalsis alone efficiently strips fluid from the esophagus. Both esophageal peristalsis and the neutralization of acid by swallowed saliva are major determinants of esophageal acid clearance in normal subjects. Swallowing carries saliva into the esophagus and peristalsis empties intraesophageal fluid into the stomach. During conditions of minimal stimulation in awake subjects, a resting saliva flow of about 0.5 mlimin (3) is transported into the esophagus by spontaneous swallows that occur about once a minute (12,21,22). The neutralization of acid by saliva carried into the esophagus with each swallow accounts for the phenomenon of esophageal acid clearance by a series of step increases in pH. At night, gastroesophageal reflux and esophageal clearance of acid occur only during arousals from sleep (22). If esophageal acid clearance is not completed before a subject resumes sleep, acid clearance is delayed until another arousal from sleep, because during sleep peristalsis seldom occurs (22,23)and saliva flow virtually ceases (24).In theory, impaired salivation as well as abnormal peristalsis may prolong esophageal acid clearance.
References Helm JF, Riedel DR, Dodds WJ, Hogan WJ, Pate1 GW, Amdorfer RC. Determinants of esophageal acid clearance in normal subjects (abstr). Gastroenterology 1980;78:1181. Dodds WJ, Hogan WJ, Helm JF, Dent J. Pathogenesis of reflux esophagitis. Gastroenterology 1981;81:376-94. Helm JF, Dodds WJ, Hogan WJ, Soergel KH, Egide MS, Wood CM. Acid neutralizing capacity of human saliva. Gastroenterology 1982;83:69-74. Goldberg HI, Dodds WJ, Gee S, Montgomery C. Zboralske FF.
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Role of acid and pepsin in acute experimental esophagitis. Gastroenterology 1969;56:223-30. 5. Goldberg HI, Dodds WJ, Montgomery C, Baskin SA, Zboralske FF. Controlled production of acute esophagitis. Invest Radio1 1970;5:254-6. 6. Dodds WJ, Goldberg HI, Montgomery C, Ludemann WB. Zboralske FF. Sequential gross, microscopic, and roentgenographic features of acute feline esophagitis. Invest Radio1 1970;5:209-19. 7. Johnson LF, DeMeester TR, Haggitt RC. Esophageal epithelial response to gastroesophageal reflux. A quantitative study. Am J Dig Dis 1978;23:498-509. 8. Harmon JW, Johnson LF, Maydonovitch CL. Effects of acid and bile salts on the rabbit esophageal mucosa. Dig Dis Sci 1981;26:65-72. 9. Booth DJ, Kemmerer WT, Skinner DB. Acid clearing from the distal esophagus. Arch Surg 1968;96:731-4. 10. Stanciu C, Bennett JR. Oesophageal acid clearing: one factor in the production of reflux oesophagitis. Gut 1974;15:852-7. 11. Kjellen G, Tibling L. Influence of body position, dry and water swallows, smoking, and alcohol on esophageal acid clearing. Stand J Gastroenterol 1978;13:283-8. 12. Dodds WJ, Hogan WJ, Reid DP, Stewart ET, Arndorfer RC. A comparison between primary esophageal peristalsis following wet and dry swallows. J Appl Physiol 1973;35:851-7. KP, Dodds WJ, Hogan WJ, Barreras 13. Miller WN, Ganeshappa RF, Arndorfer RC. Effect of bethanechol on gastroesophageal reflux. Am J Dig Dis 1977:22:230-4. 14. Johnson LF, DeMeester TR. Evaluation of elevation of the head of the bed, bethanechol, and antacid foam tablets on gastroesophageal reflux. Dig Dis Sci 1981;26:673-80. 15. Humphries TJ, Caste11 DO. Effect of oral bethanechol on parameters of esophageal peristalsis. Dig Dis Sci 1981: 26:129-32. 16. Phaosawasdi K, Malmud LS, Tolin RD. Stelzer F. Applegate G, Fisher RS. Cholinergic effects on esophageal transit and clearance. Gastroenterology 1981;81:915-20. therapy and 17. Farrell RL, Roling GT, Caste11 DO. Cholinergic chronic heartburn. A controlled trial. Ann Intern Med 1974;80:573-6. disorders. In: Margulis A, 18. Dodds WJ. Hare11 GS. Motility Burhenne H, eds. Alimentary tract roentgenology. Vol 1, 2nd ed.. St. Louis: CV Mosby Co., 1973:461-83. on gastro-oesopha19. Stanciu C, Bennett JR. Effects of posture geal reflux. Digestion 1977;15:104-9. 20. DeMeester TK, Johnson LF, Joseph GJ. Toscano MS. Hall AW, Skinner DB. Patterns of gastroesophageal reflux in health and disease. Ann Surg 1976;184:459970. JB Jr, Moorrees CFA. The frequency of 21. Lear CSC, Flanagan deglutition in man. Arch Oral Biol 1965;10:83-100. RH. et al. Mechanism of gastro22. Dent J, Dodds WJ, Friedman esophageal reflux in recumbent asymptomatic subjects. J Clin Invest 1980;65:256-67. during sleep. 23. Lichter I, Muir RC. The pattern of swallowing Electroencephalogr Clin Neurophysiol 1975;38:427-32. LH, Pigman W, Hanahan L, Gilmore RW. Rate of 24. Schneyer flow of human parotid, sublingual, and submaxillary secretions during sleep. J Dent Res 1956;35:109-14.