Original Contribution
The role of perioperative chewing gum on gastric fluid volume and gastric pH: a meta-analysis☆ Jean-Pierre P. Ouanes DO (Assistant Professor)⁎,1,2 , Mark C. Bicket MD (Chief Resident)1,2 , Brandon Togioka MD (Assistant Professor)3 , Vicente Garcia Tomas MD (Assistant Professor)4 , Christopher L. Wu MD (Professor) 2 , Jamie D. Murphy MD (Assistant Professor)3 Department of Anesthesiology and Critical Care Medicine, Johns Hopkins University, Baltimore, MD Received 22 December 2013; revised 9 July 2014; accepted 10 July 2014
Keywords: Chewing gum; Gastrointestinal contents; Gastric pH; Gastric volume; Systematic review; Meta-analysis
Abstract Study objective: To determine if preoperative gum chewing affects gastric pH and gastric fluid volume. Design: Systematic review and meta-analysis. Methods: Data sources included Cochrane, PubMed, and EMBASE databases from inception to June 2012 and reference lists of known relevant articles without language restriction. Randomized controlled trials in which a treatment group that chewed gum was compared to a control group that fasted were included. Relevant data, including main outcomes of gastric fluid volume and gastric pH, were extracted. Results: Four studies involving 287 patients were included. The presence of chewing gum was associated with small but statically significant increases in gastric fluid volume (mean difference = 0.21 mL/kg; 95% confidence interval, 0.02-0.39; P = .03) but not in gastric pH (mean difference = 0.11 mL/ kg; 95% confidence interval, − 0.14 to 0.36; P = .38). Gastric fluid volume and gastric pH remained unchanged in subgroup analysis by either sugar or sugarless gum type. Conclusions: Chewing gum in the perioperative period causes small but statically significant increases in gastric fluid volume and no change in gastric pH. The increase in gastric fluid most likely is of no clinical significance in terms of aspiration risk for the patient. Elective surgery should not necessarily be canceled or delayed in healthy patients who accidentally chew gum preoperatively. © 2014 Elsevier Inc. All rights reserved.
☆ Funding: supported by the Department of Anesthesiology and Critical Care Medicine, The Johns Hopkins University. ⁎ Corresponding author. Department of Anesthesiology and Critical Care Medicine, Johns Hopkins University, Sheik Zayed Tower 8-120, 1800 Orleans St, Baltimore, MD. Tel.: +1 410 955 5608; fax: +1 443 287 4276. E-mail addresses: [email protected] (J.-P.P. Ouanes), [email protected] (M.C. Bicket), [email protected] (B. Togioka), [email protected] (V.G. Tomas), [email protected] (C.L. Wu), [email protected] (J.D. Murphy). 1 These authors contributed equally as first authors. 2 Role: This author helped design the study, conduct the study, analyze the data, and write the manuscript. 3 Role: This author helped conduct the study and write the manuscript. 4 Role: This author helped write the manuscript.
http://dx.doi.org/10.1016/j.jclinane.2014.07.005 0952-8180/© 2014 Elsevier Inc. All rights reserved.
2
1. Introduction Pulmonary aspiration of gastric contents is an uncommon event during the perioperative period with incidence, morbidity, and mortality estimates in the adult population of approximately 1 in 7000, 1 in 16,000, and 1 in 100,000, respectively [1]. Since Mendelson’s [2] original publication , several reports and studies of the incidence of aspiration have been published [3–7]. Raidoo et al. [8] reported that the volume of aspirate required to produce severe pneumonitis with significant mortality was 0.8 mL/kg and that aspirates up to 0.6 mL/kg did not cause severe pneumonitis in a primate model. These findings are in contrast to those of Roberts and Shirley [9] who quoted a risk reference range of greater than or equal to 25 mL (0.4 mL/kg) gastric volume and pH less than or equal to ≤2.5. The effect of chewing gum may influence gastric contents in a variety of ways, including increasing saliva and swallowing, increasing gastric secretions, and simultaneously increasing or causing no change to gastric emptying. Given that studies on gastric volume have found equivocal results [10–13], some anesthesiologists cancel or delay operative procedures if a patient has been chewing gum [14] because they believe that perioperative gum chewing increases saliva production, thereby increasing gastric volume and decreasing gastric pH. The concern is that gastric fluid volume would increase to a degree that would place the patient at risk for aspiration during induction of general anesthesia [10]. However, whether chewing gum ultimately increases the risk of aspiration has not yet been definitively addressed. The American Society of Anesthesiologists (ASA) has developed practice guidelines for preoperative fasting to reduce the risk of pulmonary aspiration in healthy patients undergoing elective procedures [15]. The most recent ASA guidelines fail to address directly the issue of preoperative gum chewing, potentially leading physicians to vary, delay, or cancel a case rather than proceeding when the patient has possibly violated the nil per os (NPO) directive by chewing gum. Guidelines on perioperative fasting for children and adults from the European Society of Anesthesiology include the recommendation that “patients should not have their operation cancelled or delayed just because they are chewing gum, sucking a boiled sweet or smoking immediately prior to induction of anesthesia” [16]. The objective of this systematic review and meta-analysis was to determine if preoperative gum chewing affects gastric pH and gastric fluid volume.
2. Methods 2.1. Data sources and searches This meta-analysis followed recent methodological guidelines Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) [17]. The aim of the study was to
J.-P.P. Ouanes et al. identify all relevant randomized controlled trials that evaluated the effect of chewing gum before anesthesia induction on gastric volume and pH by comparing patients who chewed gum preoperatively to fasted NPO controls. A search of the National Library of Medicine’s MEDLINE, Cochrane Collaboration’s CENTRAL, and the EMBASE databases was conducted on June 19, 2012, and updated on June 22, 2013. Search terms for each database were Chewing gum AND Gastrointestinal contents. We limited our searches to human subjects but did not limit them by publication date or language. Additional studies were identified through hand searches of reference lists. The search process was conducted iteratively until no duplicate citations were found in the reference lists of included articles (Fig. 1). Of the 24 potential eligible studies, 5 duplicate studies were excluded, leaving 19 studies for review, with another 15 excluded after abstract screening. This left 4 articles for review.
2.2. Study selection Inclusion and exclusion criteria were determined a priori. Included studies (1) were randomized controlled trials, (2) compared a treatment group in which patients were allowed to chew gum preoperatively to a control group that did not chew gum, and (3) included a documented assessment of gastric volume or gastric pH. Case reports, review articles, editorials, comments, and abstracts without sufficient detail for analysis were excluded. Using these criteria, all 4 studies for review were included in this analysis.
2.3. Data extraction Data extraction was completed independently by two authors (MB, JO). Disagreements were resolved by a third author (CW). Data were extracted by using a standard scoring sheet, which included the following variables: first author, publication year, type of study, study location, patient demographics, study size, exclusion criteria, gastric fluid volume, gastric fluid pH, and type of chewing gum. Data were extrapolated and adjusted from figures or tables as needed. We attempted to obtain unpublished data from the authors of one study [12] that did not present standard deviation measurements, but we were unsuccessful. Ratings of study methodology and risk of bias included both Cochrane quality ratings [18] and Jadad score [19]. Ratings were completed by 2 independent reviewers (MB and JO), with a third independent reviewer (CW) resolving disagreements.
2.4. Statistical analysis The primary outcome measures were gastric fluid volume and gastric fluid pH. Pooled effect estimates for primary measures were calculated as mean difference (MD) with 95% confidence intervals (CIs). Random effects models were used. Heterogeneity was assessed by calculation of I2, which assesses the variability among studies not attributable to chance alone.
Meta-analysis of perioperative gum chewing
Fig. 1
3
Flow diagram of study search results. RCT = randomized controlled trial.
Subgroup analysis by gum type was performed. I2 values greater than 50% were considered significant. Calculations were carried out with the Cochrane Collaboration’s RevMan version 5.0.25 (Cochrane Collaboration, Oxford, UK). Statistical significance for all tests was set at P ≤ .05.
3. Results In the systematic review, 4 articles provided data on 287 patients, with the gum-chewing and control groups including 174 and 113 patients, respectively [10–13]. Table 1 provides study characteristics, including details regarding assessment of gastric pH and/or volume. One treatment group chewing bicarbonate gum was excluded from analysis given expected changes in pH [13]. The gum-chewing group in each trial initiated chewing in the immediate preoperative period, chewed gum at intervals ranging from 20 to 240 minutes, and disposed of gum without swallowing it at 30 minutes to 0 minute before surgery. The NPO control groups in each study completed a typical preoperative fast, with 1 study fast lasting for more than 8 hours. Three studies examined adult populations [10,11,13], and 1 study examined children [12].
One study [10] was of low methodological quality by both Cochrane [18] and Jadad criteria [19] (Table 2). Schoenfelder et al [12] examined ASA I or II children 5 to 17 years of age after randomization to chewing sugar-free or sugared gum for 30 minutes or remaining NPO. Chewing either gum type resulted in significant increases in gastric volume and decreases in gastric pH compared to values of NPO controls. In another high-quality study, Soreide et al [11] examined ASA I or II female patients randomized to sugar-free gum, nicotine gum, or NPO. Because of the use of nicotine gum, patients were also analyzed by smoking status. Among nonsmokers, chewing sugar-free gum increased gastric volume but did not change gastric pH compared to that of NPO controls. No effect on gastric pH or volume was noted among smokers who chewed nicotine gum. Hamid et al [13] randomized ASA I or II patients to 1 of 4 groups, one of which chewed sugar-free gum. Neither gastric fluid volume nor gastric pH in the gum-chewing group was found to be statistically different from that of NPO groups. Dubin et al [10] studied ASA I or II patients randomized to sugarless gum or NPO. No difference was found between the 2 groups, and gastric fluid content and pH did not vary based on the length of time between cessation of gum chewing and surgery.
4
J.-P.P. Ouanes et al. Table 1
Characteristics of included studies
Author, year Study design Dubin et al, 1994 [10]
Patient population
Prospective 77 ASA I or RCT II standard fasting adults undergoing elective outpatient or same-day admission surgery Hamid et al, Prospective 150 ASA I 2012 [13] RCT or II standard fasting adults undergoing elective surgery Schoenfelder Prospective 46 ASA I or II standard et al, 2006 RCT fasting [12] children undergoing elective outpatient surgery
Results a
Comments
Treatment group
Control group
Group 2: 15 pts chewing sugarless gum up to 20 min preop Group 3: 46 pts chewing sugarless gum up to 0 min preop
Group 1: 16 standard Gastric fluid NPO pts volume (mL): no differences between groups Gastric pH: no differences between groups
Younger patients in group 1; no data for gastric pH in some group-3 pts (n = 11).
Group B: 30 pts chewing sugarless gum up to preop
Group E: 30 standard Gastric fluid NPO pts volume (mL): no differences between groups Gastric pH: no differences between groups
Pts instructed not to drink liquids for 3 h preop.
Group 1: 16 standard Gastric fluid NPO pts volume (mL, mL/kg): gum-chewing groups N NPO group; no difference among gum-chewing groups Gastric pH: gum-chewing groups N NPO group; no difference among proportion of patients with pH b 2.5 Soreide et al, Prospective 100 ASA I Group 3: 30 pts with no Group 1: 30 standard Gastric fluid volume 1995 [11] RCT or II (mL, mL/kg): NPO pts with no smoking history chewing standard nonsmokers: smoking history sugarless gum fasting Group 2: 21 standard gum chewing group (173 ± 76–min duration) female N NPO group; Group 4: 23 pts with smoking NPO pts with adults smokers: no difference smoking history history chewing nicotine undergoing gum (197 ± 87–min duration) between groups outpatient, Gastric pH: same-day no differences admission, between groups or alreadyhospitalized gynecologic surgery Group 2: 15 pts chewing sugarless gum preop (26 ± 6–min duration) Group 3: 15 pts chewing sugared gum preop (31 ± 6–min duration)
Mean weight of nonsmoking NPO group was greater than that of the gumchewing group.
preop = preoperatively; RCT = randomized clinical trial. a In all studies, gastric fluid volume was measured via aspiration of gastric sump, and pH was measured via pH monitor.
3.1. Gastric volume
3.2. Gastric pH
The meta-analysis of gastric volume found that chewing gum resulted in small but statistically significant increases in gastric fluid volume compared to strict NPO (MD = 0.21 mL/ kg; 95% CI, 0.02-0.39; I2 = 26%; P = .03) (Fig. 2).
Gum chewing resulted in no significant difference in gastric pH when compared to that in the NPO control group (MD = 0.11; 95% CI, − 0.14 to 0.36; I2 = 3%; P = .38) (Fig. 3). When the low-methodological-quality study [10]
Meta-analysis of perioperative gum chewing Table 2
5
Quality ratings of included studies
Study, year
Cochrane quality assessment
Dubin et al, 1994 [10] Hamid et al, 2012 [13] Schoenfelder et al, 2006 [12] Soreide et a, 1995 [11]
Random sequence generation
Allocation concealment
Blinding of participants and personnel
Blinding of outcome assessment
Incomplete outcome data
Selective reporting
? ? + +
? ? ? ?
+ + + +
– + + ?
? + + +
? + + +
Jadad Score
1 5 5 4
“+” = low risk; “?” = unclear risk; “–” = high risk.
10 13 12 11
Fig. 2 Forest plot of gastric volume comparing gum chewing to NPO control. SD = standard deviation. Diamond indicates the risk ratio, the overall summary estimate for the analysis (width of the diamond represents 95% CI). Boxes indicate the weight of individual studies in the pooled analysis. Whiskers indicate 95% CIs. Midline represents MD of 0. The entire diamond is to the right of midline, suggesting that the MD of gastric volume favors (ie, is less with) the NPO group (odds ratio = 0.21; 95% CI, 0.02-0.39; P = .03).
was excluded, the outcome did not change significantly (MD = 0.19; 95% CI, − 0.09 to 0.48; I2 = 0%; P = .18).
3.3. Subgroup analyses Subgroup analysis by type of chewing gum was performed on sugared gum [10,12] (30 patients chewed gum; 32 did not) and sugarless gum [10–13] (144 patients chewed gum; 113 did not). For gastric fluid volume, no differences were found for sugared gum (MD = 0.33 mL/kg; 95% CI, −0.04 to 0.70; I2 = 0%; P = .08) and sugarless gum (MD = 0.20 mL/kg; 95% CI, − 0.01 to 0.42; I2 = 24%; P = .06). For gastric pH, no significant difference for was found for sugared gum (MD = 0.05; 95% CI, −0.42 to 0.52; I2 = 58%; P = .83) or sugarless gum (MD = 0.12; 95% CI, − 0.15 to 0.40; I2 = 15%; P = .38).
For adult populations [10,11,13], chewing gum also resulted in small, statistically significant increases in gastric fluid volume compared to NPO (MD = 0.12; 95% CI, 0.01-0.23; I2 = 0%; P = .03), whereas no significant difference was noted in gastric pH (MD = −0.08; 95% CI, −0.42 to 0.26; I2 = 0%; P = .65).
4. Discussion In the first systematic review and meta-analysis on this topic, we found that chewing gum before induction of anesthesia resulted in small increases in gastric volume but no changes in gastric pH. For example, chewing gum would increase the gastric fluid volume in a 70-kg patient by approximately 9 to 15 mL. Though statistically significant, this increase in gastric fluid
10 13 12 11
Fig. 3 Forest plot of gastric pH comparing gum chewing to NPO control. Diamond indicates the risk ratio, the overall summary estimate for the analysis (width of the diamond represents 95% CI). Boxes indicate the weight of individual studies in the pooled analysis. Whiskers indicate 95% CIs. Midline represents MD of 0. The diamond touches midline, suggesting no difference between the two treatments (odds ratio = 0.11; 95% CI, −0.14 to 0.36; P = .38).
6 most likely is of no clinical significance in terms of risk for the patient. The exponential half time (t½) of gastric fluids is only 10 to 15 minutes [20,21], making changes due to intake or swallowing at the time of induction of anesthesia unlikely. The minimum gastric aspirate volume needed to cause complications from pulmonary aspiration remains to be defined. In an attempt to define “at-risk” criteria for pulmonary aspiration during a cesarean section, Roberts and Shirley [9] referred to unpublished data in the rhesus monkey which suggested that 0.4 mL/kg (approximately 28 mL in a 70-kg person) was the maximum acid aspirate that does not produce significant changes in the lung. Since then, numerous subsequent studies have considered a gastric volume of greater than 0.4 mL/kg to be a risk factor for aspiration in humans [12]. However, the relationship between gastric volume and aspiration volume remains to be defined. Thus, the clinical significance of the gastric volume increase found in this study likely poses no clinical significance. In those who chew gum, relatively small increases in gastric volume (Dubin et al = +5 mL, Soreide et al = +6 mL, Hamid et al = +10 mL) are unlikely to lead to a meaningful increase in risk of major morbidity from pulmonary aspiration. Such small increases in gastric fluid may theoretically be similar to that of patients taking medication with small sips of water in the preoperative period. Whether the presence of sugar alternatives in sugarless chewing gum or the act of mastication itself promotes intestinal motility and gastric empting is unknown [22]. Sham feeding (chewing food then expectorating it) produces electrical activity in the stomach, activates the cephalic phase of digestion, and leads to increased gastric motility [23,24]. Similar effects from chewing gum seem possible. Improved gastric throughput would theoretically decrease the risk of an aspiration event. Chewing gum increases the production of saliva, which has a pH range of 6.0 to 7.4 [25]. It seems likely that any increase in gastric volume should result in a higher pH if saliva contributes to the increased gastric volume. Although these results suggest that chewing gum in the perioperative period appears to be safe, the data originate from a small number of studies of healthy ASA I or II patients undergoing elective surgery. It may be difficult to draw conclusions regarding patients with greater disease burden, undergoing nonelective surgery, or who receive pharmacotherapy that delays gastric emptying. These results do not apply to patients who either forget to dispose of the gum prior to surgery or swallow the chewing gum itself. Caution regarding chewing gum becoming a “solid” food may continue to exist, as the gum may inadvertently find a path into the airway [21]. The variation in chewing times across the studies may be another limitation. Because chewing gum promotes salivary production at rates of up to 6.6 mL/min in the first minute and 1.5 mL/min within 15 minutes [26], differences in chewing gum times across the studies (20-240 minutes) may have a variable effect on total gastric volume at the time of anesthesia induction. In summary, chewing gum in the preoperative period results in a small but statistically significant increase in
J.-P.P. Ouanes et al. gastric fluid volume and no change in gastric pH. The increase in gastric fluid most likely is of no clinical significance in terms of aspiration risk for the patient. Elective surgery should not necessarily be canceled or delayed in healthy patients who accidently chew gum preoperatively.
Acknowledgements We would like to acknowledge Claire Levine, MS, ELS, for her help with the preparation of this manuscript.
References [1] Sakai T, Planinsic RM, Quinlan JJ, Handley LJ, Kim TY, Hilmi IA. The incidence and outcome of perioperative pulmonary aspiration in a university hospital: a 4-year retrospective analysis. Anesth Analg 2006;103:941-7. [2] Mendelson CL. The aspiration of stomach contents into the lungs during obstetric anesthesia. Am J Obstet Gynecol 1946;52:191-205. [3] Olsson GL, Hallen B, Hambraeus-Jonzon K. Aspiration during anaesthesia: a computer-aided study of 185,358 anaesthetics. Acta Anaesthesiol Scand 1986;30:84-92. [4] Warner MA, Warner ME, Weber JG. Clinical significance of pulmonary aspiration during the perioperative period. Anesthesiology 1993;78:56-62. [5] Mellin-Olsen J, Fasting S, Gisvold SE. Routine preoperative gastric emptying is seldom indicated. A study of 85,594 anaesthetics with special focus on aspiration pneumonia. Acta Anaesthesiol Scand 1996;40:1184-8. [6] Borland LM, Sereika SM, Woelfel SK, Saitz EW, Carrillo PA, Lupin JL, Motoyama EK. Pulmonary aspiration in pediatric patients during general anesthesia: incidence and outcome. J Clin Anesth 1998;10:95-102. [7] Warner MA, Warner ME, Warner DO, Warner LO, Warner EJ. Perioperative pulmonary aspiration in infants and children. Anesthesiology 1999;90:66-71. [8] Raidoo DM, Rocke DA, Brock-Utne JG, Marszalek A, Engelbrecht HE. Critical volume for pulmonary acid aspiration: reappraisal in a primate model. Br J Anaesth 1990;65:248-50. [9] Roberts RB, Shirley MA. Reducing the risk of acid aspiration during cesarean section. Anesth Analg 1974;53:859-68. [10] Dubin SA, Jense HG, McCranie JM, Zubar V. Sugarless gum chewing before surgery does not increase gastric fluid volume or acidity. Can J Anaesth 1994;41:603-6. [11] Soreide E, Holst-Larsen H, Veel T, Steen PA. The effects of chewing gum on gastric content prior to induction of general anesthesia. Anesth Analg 1995;80:985-9. [12] Schoenfelder RC, Ponnamma CM, Freyle D, Wang SM, Kain ZN. Residual gastric fluid volume and chewing gum before surgery. Anesth Analg 2006;102:415-7. [13] Hamid K, Masoud L, Reza FH, Mehran G, Karmella K. Comparison of different non-pharmacological preoperative preparations on gastric fluid volume and acidity: a randomized controlled trial. Anaesth Pain Intensive Care 2012;16:165-8. [14] Moore MW, Meyer RM. Elective ambulatory surgery: must the patient be NPO? JAMA 1990;263:733. [15] American Society of Anesthesiologists Committee. Practice guidelines for preoperative fasting and the use of pharmacologic agents to reduce the risk of pulmonary aspiration: application to healthy patients undergoing elective procedures: an updated report by the American Society of Anesthesiologists Committee on Standards and Practice Parameters. Anesthesiology 2011;114:495-511.
Meta-analysis of perioperative gum chewing [16] Smith I, Kranke P, Murat I, et al. Perioperative fasting in adults and children: guidelines from the European Society of Anaesthesiology. Eur J Anaesthesiol 2011;28:556-69. [17] Liberati A, Altman DG, Tetzlaff J, et al. The PRISMA statement for reporting systematic reviews and meta-analyses of studies that evaluate health care interventions: explanation and elaboration. Ann Intern Med 2009;151:W65-94. [18] Higgins JPT, Altman DG, Sterne JAC. Assessing risk of bias in included studies. In: Higgins JPT, Green S, editors. Cochrane handbook for systematic reviews of interventions. Copenhagen: Cochrane Collaboration; 2011. p. 1-53. [19] Jadad AR, Moore RA, Carroll D, et al. Assessing the quality of reports of randomized clinical trials: is blinding necessary? Control Clin Trials 1996;17:1-12. [20] Levy DM. Pre-operative fasting—60 years on from Mendelson. Contin Educ Anaesth Crit Care Pain 2006;6:215-8.
7 [21] Poulton TJ. Gum chewing during pre-anesthetic fasting. Paediatr Anaesth 2012;22:288-96. [22] Zaghiyan K, Felder S, Ovsepyan G, et al. A prospective randomized controlled trial of sugared chewing gum on gastrointestinal recovery after major colorectal surgery in patients managed with early enteral feeding. Dis Colon Rectum 2013;56:328-35. [23] Chang EB, Leung PS. Gastric physiology. In: Leung PS, editor. The gastrointestinal system. New York: Springer; 2014. p. 72. [24] Stern RM, Crawford HE, Stewart WR, Vasey MW, Koch KL. Sham feeding. Cephalic-vagal influences on gastric myoelectric activity. Dig Dis Sci 1989;34:521-7. [25] Guyton AC. Textbook of medical physiology. 8th ed. Philadelphia: W. B. Sanders Co.; 1991 709-25. [26] Dawes C, Macpherson LM. Effects of nine different chewing-gums and lozenges on salivary flow rate and pH. Caries Res 1992;26: 176-82.
The role of perioperative chewing gum on gastric fluid volume and gastric pH: a meta-analysis☆ Jean-Pierre P. Ouanes DO (Assistant Professor)⁎,1,2 , Mark C. Bicket MD (Chief Resident)1,2 , Brandon Togioka MD (Assistant Professor)3 , Vicente Garcia Tomas MD (Assistant Professor)4 , Christopher L. Wu MD (Professor) 2 , Jamie D. Murphy MD (Assistant Professor)3 Department of Anesthesiology and Critical Care Medicine, Johns Hopkins University, Baltimore, MD Received 22 December 2013; revised 9 July 2014; accepted 10 July 2014
Keywords: Chewing gum; Gastrointestinal contents; Gastric pH; Gastric volume; Systematic review; Meta-analysis
Abstract Study objective: To determine if preoperative gum chewing affects gastric pH and gastric fluid volume. Design: Systematic review and meta-analysis. Methods: Data sources included Cochrane, PubMed, and EMBASE databases from inception to June 2012 and reference lists of known relevant articles without language restriction. Randomized controlled trials in which a treatment group that chewed gum was compared to a control group that fasted were included. Relevant data, including main outcomes of gastric fluid volume and gastric pH, were extracted. Results: Four studies involving 287 patients were included. The presence of chewing gum was associated with small but statically significant increases in gastric fluid volume (mean difference = 0.21 mL/kg; 95% confidence interval, 0.02-0.39; P = .03) but not in gastric pH (mean difference = 0.11 mL/ kg; 95% confidence interval, − 0.14 to 0.36; P = .38). Gastric fluid volume and gastric pH remained unchanged in subgroup analysis by either sugar or sugarless gum type. Conclusions: Chewing gum in the perioperative period causes small but statically significant increases in gastric fluid volume and no change in gastric pH. The increase in gastric fluid most likely is of no clinical significance in terms of aspiration risk for the patient. Elective surgery should not necessarily be canceled or delayed in healthy patients who accidentally chew gum preoperatively. © 2014 Elsevier Inc. All rights reserved.
☆ Funding: supported by the Department of Anesthesiology and Critical Care Medicine, The Johns Hopkins University. ⁎ Corresponding author. Department of Anesthesiology and Critical Care Medicine, Johns Hopkins University, Sheik Zayed Tower 8-120, 1800 Orleans St, Baltimore, MD. Tel.: +1 410 955 5608; fax: +1 443 287 4276. E-mail addresses: [email protected] (J.-P.P. Ouanes), [email protected] (M.C. Bicket), [email protected] (B. Togioka), [email protected] (V.G. Tomas), [email protected] (C.L. Wu), [email protected] (J.D. Murphy). 1 These authors contributed equally as first authors. 2 Role: This author helped design the study, conduct the study, analyze the data, and write the manuscript. 3 Role: This author helped conduct the study and write the manuscript. 4 Role: This author helped write the manuscript.
http://dx.doi.org/10.1016/j.jclinane.2014.07.005 0952-8180/© 2014 Elsevier Inc. All rights reserved.
2
1. Introduction Pulmonary aspiration of gastric contents is an uncommon event during the perioperative period with incidence, morbidity, and mortality estimates in the adult population of approximately 1 in 7000, 1 in 16,000, and 1 in 100,000, respectively [1]. Since Mendelson’s [2] original publication , several reports and studies of the incidence of aspiration have been published [3–7]. Raidoo et al. [8] reported that the volume of aspirate required to produce severe pneumonitis with significant mortality was 0.8 mL/kg and that aspirates up to 0.6 mL/kg did not cause severe pneumonitis in a primate model. These findings are in contrast to those of Roberts and Shirley [9] who quoted a risk reference range of greater than or equal to 25 mL (0.4 mL/kg) gastric volume and pH less than or equal to ≤2.5. The effect of chewing gum may influence gastric contents in a variety of ways, including increasing saliva and swallowing, increasing gastric secretions, and simultaneously increasing or causing no change to gastric emptying. Given that studies on gastric volume have found equivocal results [10–13], some anesthesiologists cancel or delay operative procedures if a patient has been chewing gum [14] because they believe that perioperative gum chewing increases saliva production, thereby increasing gastric volume and decreasing gastric pH. The concern is that gastric fluid volume would increase to a degree that would place the patient at risk for aspiration during induction of general anesthesia [10]. However, whether chewing gum ultimately increases the risk of aspiration has not yet been definitively addressed. The American Society of Anesthesiologists (ASA) has developed practice guidelines for preoperative fasting to reduce the risk of pulmonary aspiration in healthy patients undergoing elective procedures [15]. The most recent ASA guidelines fail to address directly the issue of preoperative gum chewing, potentially leading physicians to vary, delay, or cancel a case rather than proceeding when the patient has possibly violated the nil per os (NPO) directive by chewing gum. Guidelines on perioperative fasting for children and adults from the European Society of Anesthesiology include the recommendation that “patients should not have their operation cancelled or delayed just because they are chewing gum, sucking a boiled sweet or smoking immediately prior to induction of anesthesia” [16]. The objective of this systematic review and meta-analysis was to determine if preoperative gum chewing affects gastric pH and gastric fluid volume.
2. Methods 2.1. Data sources and searches This meta-analysis followed recent methodological guidelines Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) [17]. The aim of the study was to
J.-P.P. Ouanes et al. identify all relevant randomized controlled trials that evaluated the effect of chewing gum before anesthesia induction on gastric volume and pH by comparing patients who chewed gum preoperatively to fasted NPO controls. A search of the National Library of Medicine’s MEDLINE, Cochrane Collaboration’s CENTRAL, and the EMBASE databases was conducted on June 19, 2012, and updated on June 22, 2013. Search terms for each database were Chewing gum AND Gastrointestinal contents. We limited our searches to human subjects but did not limit them by publication date or language. Additional studies were identified through hand searches of reference lists. The search process was conducted iteratively until no duplicate citations were found in the reference lists of included articles (Fig. 1). Of the 24 potential eligible studies, 5 duplicate studies were excluded, leaving 19 studies for review, with another 15 excluded after abstract screening. This left 4 articles for review.
2.2. Study selection Inclusion and exclusion criteria were determined a priori. Included studies (1) were randomized controlled trials, (2) compared a treatment group in which patients were allowed to chew gum preoperatively to a control group that did not chew gum, and (3) included a documented assessment of gastric volume or gastric pH. Case reports, review articles, editorials, comments, and abstracts without sufficient detail for analysis were excluded. Using these criteria, all 4 studies for review were included in this analysis.
2.3. Data extraction Data extraction was completed independently by two authors (MB, JO). Disagreements were resolved by a third author (CW). Data were extracted by using a standard scoring sheet, which included the following variables: first author, publication year, type of study, study location, patient demographics, study size, exclusion criteria, gastric fluid volume, gastric fluid pH, and type of chewing gum. Data were extrapolated and adjusted from figures or tables as needed. We attempted to obtain unpublished data from the authors of one study [12] that did not present standard deviation measurements, but we were unsuccessful. Ratings of study methodology and risk of bias included both Cochrane quality ratings [18] and Jadad score [19]. Ratings were completed by 2 independent reviewers (MB and JO), with a third independent reviewer (CW) resolving disagreements.
2.4. Statistical analysis The primary outcome measures were gastric fluid volume and gastric fluid pH. Pooled effect estimates for primary measures were calculated as mean difference (MD) with 95% confidence intervals (CIs). Random effects models were used. Heterogeneity was assessed by calculation of I2, which assesses the variability among studies not attributable to chance alone.
Meta-analysis of perioperative gum chewing
Fig. 1
3
Flow diagram of study search results. RCT = randomized controlled trial.
Subgroup analysis by gum type was performed. I2 values greater than 50% were considered significant. Calculations were carried out with the Cochrane Collaboration’s RevMan version 5.0.25 (Cochrane Collaboration, Oxford, UK). Statistical significance for all tests was set at P ≤ .05.
3. Results In the systematic review, 4 articles provided data on 287 patients, with the gum-chewing and control groups including 174 and 113 patients, respectively [10–13]. Table 1 provides study characteristics, including details regarding assessment of gastric pH and/or volume. One treatment group chewing bicarbonate gum was excluded from analysis given expected changes in pH [13]. The gum-chewing group in each trial initiated chewing in the immediate preoperative period, chewed gum at intervals ranging from 20 to 240 minutes, and disposed of gum without swallowing it at 30 minutes to 0 minute before surgery. The NPO control groups in each study completed a typical preoperative fast, with 1 study fast lasting for more than 8 hours. Three studies examined adult populations [10,11,13], and 1 study examined children [12].
One study [10] was of low methodological quality by both Cochrane [18] and Jadad criteria [19] (Table 2). Schoenfelder et al [12] examined ASA I or II children 5 to 17 years of age after randomization to chewing sugar-free or sugared gum for 30 minutes or remaining NPO. Chewing either gum type resulted in significant increases in gastric volume and decreases in gastric pH compared to values of NPO controls. In another high-quality study, Soreide et al [11] examined ASA I or II female patients randomized to sugar-free gum, nicotine gum, or NPO. Because of the use of nicotine gum, patients were also analyzed by smoking status. Among nonsmokers, chewing sugar-free gum increased gastric volume but did not change gastric pH compared to that of NPO controls. No effect on gastric pH or volume was noted among smokers who chewed nicotine gum. Hamid et al [13] randomized ASA I or II patients to 1 of 4 groups, one of which chewed sugar-free gum. Neither gastric fluid volume nor gastric pH in the gum-chewing group was found to be statistically different from that of NPO groups. Dubin et al [10] studied ASA I or II patients randomized to sugarless gum or NPO. No difference was found between the 2 groups, and gastric fluid content and pH did not vary based on the length of time between cessation of gum chewing and surgery.
4
J.-P.P. Ouanes et al. Table 1
Characteristics of included studies
Author, year Study design Dubin et al, 1994 [10]
Patient population
Prospective 77 ASA I or RCT II standard fasting adults undergoing elective outpatient or same-day admission surgery Hamid et al, Prospective 150 ASA I 2012 [13] RCT or II standard fasting adults undergoing elective surgery Schoenfelder Prospective 46 ASA I or II standard et al, 2006 RCT fasting [12] children undergoing elective outpatient surgery
Results a
Comments
Treatment group
Control group
Group 2: 15 pts chewing sugarless gum up to 20 min preop Group 3: 46 pts chewing sugarless gum up to 0 min preop
Group 1: 16 standard Gastric fluid NPO pts volume (mL): no differences between groups Gastric pH: no differences between groups
Younger patients in group 1; no data for gastric pH in some group-3 pts (n = 11).
Group B: 30 pts chewing sugarless gum up to preop
Group E: 30 standard Gastric fluid NPO pts volume (mL): no differences between groups Gastric pH: no differences between groups
Pts instructed not to drink liquids for 3 h preop.
Group 1: 16 standard Gastric fluid NPO pts volume (mL, mL/kg): gum-chewing groups N NPO group; no difference among gum-chewing groups Gastric pH: gum-chewing groups N NPO group; no difference among proportion of patients with pH b 2.5 Soreide et al, Prospective 100 ASA I Group 3: 30 pts with no Group 1: 30 standard Gastric fluid volume 1995 [11] RCT or II (mL, mL/kg): NPO pts with no smoking history chewing standard nonsmokers: smoking history sugarless gum fasting Group 2: 21 standard gum chewing group (173 ± 76–min duration) female N NPO group; Group 4: 23 pts with smoking NPO pts with adults smokers: no difference smoking history history chewing nicotine undergoing gum (197 ± 87–min duration) between groups outpatient, Gastric pH: same-day no differences admission, between groups or alreadyhospitalized gynecologic surgery Group 2: 15 pts chewing sugarless gum preop (26 ± 6–min duration) Group 3: 15 pts chewing sugared gum preop (31 ± 6–min duration)
Mean weight of nonsmoking NPO group was greater than that of the gumchewing group.
preop = preoperatively; RCT = randomized clinical trial. a In all studies, gastric fluid volume was measured via aspiration of gastric sump, and pH was measured via pH monitor.
3.1. Gastric volume
3.2. Gastric pH
The meta-analysis of gastric volume found that chewing gum resulted in small but statistically significant increases in gastric fluid volume compared to strict NPO (MD = 0.21 mL/ kg; 95% CI, 0.02-0.39; I2 = 26%; P = .03) (Fig. 2).
Gum chewing resulted in no significant difference in gastric pH when compared to that in the NPO control group (MD = 0.11; 95% CI, − 0.14 to 0.36; I2 = 3%; P = .38) (Fig. 3). When the low-methodological-quality study [10]
Meta-analysis of perioperative gum chewing Table 2
5
Quality ratings of included studies
Study, year
Cochrane quality assessment
Dubin et al, 1994 [10] Hamid et al, 2012 [13] Schoenfelder et al, 2006 [12] Soreide et a, 1995 [11]
Random sequence generation
Allocation concealment
Blinding of participants and personnel
Blinding of outcome assessment
Incomplete outcome data
Selective reporting
? ? + +
? ? ? ?
+ + + +
– + + ?
? + + +
? + + +
Jadad Score
1 5 5 4
“+” = low risk; “?” = unclear risk; “–” = high risk.
10 13 12 11
Fig. 2 Forest plot of gastric volume comparing gum chewing to NPO control. SD = standard deviation. Diamond indicates the risk ratio, the overall summary estimate for the analysis (width of the diamond represents 95% CI). Boxes indicate the weight of individual studies in the pooled analysis. Whiskers indicate 95% CIs. Midline represents MD of 0. The entire diamond is to the right of midline, suggesting that the MD of gastric volume favors (ie, is less with) the NPO group (odds ratio = 0.21; 95% CI, 0.02-0.39; P = .03).
was excluded, the outcome did not change significantly (MD = 0.19; 95% CI, − 0.09 to 0.48; I2 = 0%; P = .18).
3.3. Subgroup analyses Subgroup analysis by type of chewing gum was performed on sugared gum [10,12] (30 patients chewed gum; 32 did not) and sugarless gum [10–13] (144 patients chewed gum; 113 did not). For gastric fluid volume, no differences were found for sugared gum (MD = 0.33 mL/kg; 95% CI, −0.04 to 0.70; I2 = 0%; P = .08) and sugarless gum (MD = 0.20 mL/kg; 95% CI, − 0.01 to 0.42; I2 = 24%; P = .06). For gastric pH, no significant difference for was found for sugared gum (MD = 0.05; 95% CI, −0.42 to 0.52; I2 = 58%; P = .83) or sugarless gum (MD = 0.12; 95% CI, − 0.15 to 0.40; I2 = 15%; P = .38).
For adult populations [10,11,13], chewing gum also resulted in small, statistically significant increases in gastric fluid volume compared to NPO (MD = 0.12; 95% CI, 0.01-0.23; I2 = 0%; P = .03), whereas no significant difference was noted in gastric pH (MD = −0.08; 95% CI, −0.42 to 0.26; I2 = 0%; P = .65).
4. Discussion In the first systematic review and meta-analysis on this topic, we found that chewing gum before induction of anesthesia resulted in small increases in gastric volume but no changes in gastric pH. For example, chewing gum would increase the gastric fluid volume in a 70-kg patient by approximately 9 to 15 mL. Though statistically significant, this increase in gastric fluid
10 13 12 11
Fig. 3 Forest plot of gastric pH comparing gum chewing to NPO control. Diamond indicates the risk ratio, the overall summary estimate for the analysis (width of the diamond represents 95% CI). Boxes indicate the weight of individual studies in the pooled analysis. Whiskers indicate 95% CIs. Midline represents MD of 0. The diamond touches midline, suggesting no difference between the two treatments (odds ratio = 0.11; 95% CI, −0.14 to 0.36; P = .38).
6 most likely is of no clinical significance in terms of risk for the patient. The exponential half time (t½) of gastric fluids is only 10 to 15 minutes [20,21], making changes due to intake or swallowing at the time of induction of anesthesia unlikely. The minimum gastric aspirate volume needed to cause complications from pulmonary aspiration remains to be defined. In an attempt to define “at-risk” criteria for pulmonary aspiration during a cesarean section, Roberts and Shirley [9] referred to unpublished data in the rhesus monkey which suggested that 0.4 mL/kg (approximately 28 mL in a 70-kg person) was the maximum acid aspirate that does not produce significant changes in the lung. Since then, numerous subsequent studies have considered a gastric volume of greater than 0.4 mL/kg to be a risk factor for aspiration in humans [12]. However, the relationship between gastric volume and aspiration volume remains to be defined. Thus, the clinical significance of the gastric volume increase found in this study likely poses no clinical significance. In those who chew gum, relatively small increases in gastric volume (Dubin et al = +5 mL, Soreide et al = +6 mL, Hamid et al = +10 mL) are unlikely to lead to a meaningful increase in risk of major morbidity from pulmonary aspiration. Such small increases in gastric fluid may theoretically be similar to that of patients taking medication with small sips of water in the preoperative period. Whether the presence of sugar alternatives in sugarless chewing gum or the act of mastication itself promotes intestinal motility and gastric empting is unknown [22]. Sham feeding (chewing food then expectorating it) produces electrical activity in the stomach, activates the cephalic phase of digestion, and leads to increased gastric motility [23,24]. Similar effects from chewing gum seem possible. Improved gastric throughput would theoretically decrease the risk of an aspiration event. Chewing gum increases the production of saliva, which has a pH range of 6.0 to 7.4 [25]. It seems likely that any increase in gastric volume should result in a higher pH if saliva contributes to the increased gastric volume. Although these results suggest that chewing gum in the perioperative period appears to be safe, the data originate from a small number of studies of healthy ASA I or II patients undergoing elective surgery. It may be difficult to draw conclusions regarding patients with greater disease burden, undergoing nonelective surgery, or who receive pharmacotherapy that delays gastric emptying. These results do not apply to patients who either forget to dispose of the gum prior to surgery or swallow the chewing gum itself. Caution regarding chewing gum becoming a “solid” food may continue to exist, as the gum may inadvertently find a path into the airway [21]. The variation in chewing times across the studies may be another limitation. Because chewing gum promotes salivary production at rates of up to 6.6 mL/min in the first minute and 1.5 mL/min within 15 minutes [26], differences in chewing gum times across the studies (20-240 minutes) may have a variable effect on total gastric volume at the time of anesthesia induction. In summary, chewing gum in the preoperative period results in a small but statistically significant increase in
J.-P.P. Ouanes et al. gastric fluid volume and no change in gastric pH. The increase in gastric fluid most likely is of no clinical significance in terms of aspiration risk for the patient. Elective surgery should not necessarily be canceled or delayed in healthy patients who accidently chew gum preoperatively.
Acknowledgements We would like to acknowledge Claire Levine, MS, ELS, for her help with the preparation of this manuscript.
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