Gastric ultrasound and preoperative clear liquid fasting
Libby Elliott MD, Suryakumar Narayanasamy MD, Myron Yaster MD, and Lynne G. Maxwell MD
Preoperative fasting guidelines for carbohydrate containing clear liquids has been a frequent issue discussed and debated in the PAAD and amongst our readers. It was a highlight of SPA’s 36th Annual Meeting held in New Orleans in October 2022 (The Case for Change in Clear Fluid Fasting Policy - Panel Discussion: Mark Thomas, BSc MBBChir FRCA; Alexander Schmidt, MD; Sulpicio G. Soriano, III, MD, FAAP) and has also been the subject of discussion in recent updates of our major Societies’ Practice Guidelines.1, 2 The new ASA guidelines1 still suggest a 2 hour fast for clear liquids and differs from the new European Society of Anesthesiology and Intensive Care guidelines which have reduced clear liquid fasting to one hour.2 “What to do, what to do”? In today’s PAAD, Sarhan et al.3, used ultrasound to assess gastric volume prospectively in a randomized controlled trial to study and provide evidence in this debate. The article is primarily reviewed by Dr. Libby Elliott of Nemours Children’s Health who has taken the lead in reviewing articles on this subject for the PAAD. She asked Dr Suryakumar Narayanasam of Cincinnati Children's Hospital to assist. Myron Yaster MD
Original article
Sarhan, K. , Hasaneen, H. , Hasanin, A. , Mohammed, H. , Saleh, R. & Kamel, A. (2023). Ultrasound Assessment of Gastric Fluid Volume in Children Scheduled for Elective Surgery After Clear Fluid Fasting for 1 Versus 2 Hours: A Randomized Controlled Trial. Anesthesia & Analgesia, 136 (4), 711-718. PMID: 35881513
“What to do, what to do”? “A one hour clear liquid preoperative fast or two”? “Are we putting ourselves at professional risk if we adopt the European Guidelines (1 hour) instead of the ASA’s (2 hour””?1, 2 We are all scrambling to piece together meaningful data regarding the safe time for children to stop drinking clear, carbiohydrate containing liquids before anesthesia. In the April 2023 issue of Anesthesia and Analgesia, Sarhan et al.3 prospectively evaluated by ultrasound the differences in gastric fluid volumes in children aged 3-10 who had fasted either 1 or 2 hours for clear fluids before elective surgery. Their study is JUST WHAT WE HAVE BEEN CALLING FOR! More RCTs! Let’s be scientific about this! Or can we?
The study was a well designed, randomized, prospective, controlled trial comparing 116 healthy children who received 3ml/kg apple juice one hour prior to ultrasound evaluation of gastric fluid volume to 111 healthy children who received the same volume 2 hours prior to ultrasound evaluation. All children had fasted at least 6 hours for solid foods in preparation for surgical procedure. Neither group reported thirst, thus disproving the idea that children fasted for more than an hour arrive to the preop area in distress because of their thirst. Thus, 2 hours is perfectly fine! And lo and behold, the children who had drunk only one hour prior to evaluation had higher gastric volumes than the children who had fasted 2 hours (0.61 mL/kg vs 0.32 mL/kg). Game over. One hour is just not as safe.
Wait! Not so fast!! (pun intended)
In this study of 227 patients, while demonstrating that gastric volumes (primary outcome) are higher in patients fasted one hour compared to those fasted for two hours, neither group had any patients who aspirated. And aspiration is the safety question we are really interested in, not the volume of gastric contents. Indeed, gastric volume is simply a surrogate marker for the risk of aspiration. “Surrogate endpoints are frequently used in place of clinical events because the variable measured is believed to be correlated to the clinical outcome of interest and because of its perceived utility in detecting treatment differences. Surrogates may be easier to assess but unless proven to predict the outcome of interest, they are poor substitutes for pivotal trials”.5
For example, “In a hypothetical clinical trial designed to determine which of several different anesthetics had the best outcome for children with upper respiratory infection, hemoglobin-oxygen saturation < 94% would be considered a surrogate measure of adverse respiratory outcome compared to pneumonia requiring hospitalization, brain injury or death. However, the relationship of hemoglobin-oxygen saturation < 94% to death, morbidity, quality of life or cost is in fact very tenuous. Therefore, it is always better to select an outcome directly related to something that matters to the patient or the health care system (cost)”.4
Thus, what we really need to know is the actual risk of aspiration following a one or two hour fast, not the surrogate marker, gastric volume. Further, gastric volume may not even matter. We do know that gastric pH and the presence of particulate matter is important if a patient aspirates, but volume? Maybe, but probably not. Thus, what we really want to know is the number of patients who aspirated which was zero in both groups in this study and that is not surprising. The number needed to treat (NNT) to find out if the one hour fasting increases the risk of aspiration is very high due to the extremely low incidence of aspiration events and designing a RCT looking at aspiration as an outcome is logistically impossible (think of >100000 patients).5
A couple of comments on the methodology: 1. The authors have used either Aorta or IVC as the landmark to locate the gastric antrum. Since the IVC is to the right and anatomically close to the pyloric sphincter than to the antrum, using IVC as a landmark can underestimate the circumference (i.e: gastric fluid volume) and could lower the grading (likely in both groups since it is RCT) . 2. In this study, based on qualitative grading 66 patients (56.9%) in 1 hr fasting group are at high-risk category, however, quantitative assessment showed only 2 patients in 1 hr group > 1.25 ml/kg and none had >1.5 ml/kg. It is preferable to use quantitative measurements in pediatric patients over qualitative grading if your ultrasound has the capability to do it.
So what is the volume of clear fluid that is OK for a patient to have in their stomach and does it matter? We know that 0 cannot be right, because in patients fasted >8 hours many will have some gastric volume and they do not aspirate. How about 0.8 ml/kg? 1.25 ml/kg? 1.5ml/kg (which was not seen in any patients in this study)?
Other than Myron, Is anyone in this forum old enough to remember the Roberts and Shirley study from 1974?6 How about this: 25ml of acid in the stomach is too much? For those who don’t know, the number 25 ml of gastric fluid was derived from the volume of acid (0.4 ml/kg extrapolated to the average adult human) that Roberts and Shirley directly infused into the right mainstem bronchus of rhesus macaques. You read that right… These monkeys did not have 0.4 ml/kg of acid in their stomachs, but rather suffered severe aspiration injury due to intentionally squirting gastric juice with a pH of 1.62 into their anesthetized lungs. The conservative approach calling for 0.8 ml/kg limit (about 50 ml for average human adult) is derived from a classic study by Raidoo et al.7 where they injected the gastric aspirate fluid with HCl added to adjust the pH to 1.0 into the anesthetized intubated animal trachea. Further, we would contend that it is the ACID and not the volume that is the risk factor for injury.
With ultrasound, we now know that there are a significant proportion of normal healthy fasted individuals have residual gastric content more than 0.8 ml/kg (including this study) and close to 1.5 ml/kg. Do we need to have a separate volume cut-off based on how long the patient was fasting – with prolonged fasting a larger volume of low pH stomach juice might result that might do more damage to the lungs and liberal drinkers may have higher pH due to dilution from the drink: anyone interested doing a study on this? Please note that about 2% of patients with more than 2 hrs of fasting might still have >1.5 ml/kg of residual gastric volume. Regardless of the volume cutoff, it is not feasible to do an ultrasound exam of all the patients before surgery to find out who has more than 0.8 ml/kg (or >1.5 ml/kg) of fluid in the stomach. With Sarhan et al’s study, if you’re in the conservative camp of 0.8 ml/kg, 2 hrs seems to satisfy the threshold. For the liberals who are tolerant up to 1.5 ml/kg, you’ll find comfort that none of the 1 hr patients exceeded this threshold. Until we have a consensus and clear evidence on what is the cut off volume, the debate will continue.
Given the rarity of aspiration and the practical and logistical impossibility of achieving adequate numbers of patients to do an RCT for the primary outcome of aspiration, let’s ask another question. How many serious aspiration events have you seen with clear liquids? (In all my (LE) conversations with anesthesia providers at two hospitals the answer is zero.) How about solids? How long did the patient fast prior to the case? Was it an emergency? What induction technique and airway device were used? Could it have been improved?
As the safety officer for my prior department for a number of years (LE), I reviewed a very small handful of serious aspiration events. All involved solid particulate matter. Most of the patients fasted well over 8 hours. Not ONE single serious aspiration event occurred with clear fluids. Tens of thousands of patients over four years. Not one. With my limited experience, I agree on this one (SN).
So while we commend Sarhan et al.3, the use of a surrogate marker, gastric volume rather than the endpoint of aspiration is unconvincing to us. What are you doing in your practice? Finally, we shouldn’t have to guess. With the use of electronic medical records, we should be able to get a numerator and denominator to study this. Let us know your thoughts and Myron will post in the Friday Reader Response.
PS: I (SN) would like to thank Dr. Anahi Perlas, MD, who has extensively studied and published on point of care gastric ultrasound for her expertise and insights in reviewing this article.
References
1. Joshi GP, Abdelmalak BB, Weigel WA, et al. 2023 American Society of Anesthesiologists Practice Guidelines for Preoperative Fasting: Carbohydrate-containing Clear Liquids with or without Protein, Chewing Gum, and Pediatric Fasting Duration-A Modular Update of the 2017 American Society of Anesthesiologists Practice Guidelines for Preoperative Fasting. Anesthesiology. Feb 1 2023;138(2):132-151. doi:10.1097/aln.0000000000004381
2. Frykholm P, Disma N, Andersson H, et al. Pre-operative fasting in children: A guideline from the European Society of Anaesthesiology and Intensive Care. European journal of anaesthesiology. Jan 1 2022;39(1):4-25. doi:10.1097/eja.0000000000001599
3. Sarhan KA, Hasaneen H, Hasanin A, Mohammed H, Saleh R, Kamel A. Ultrasound Assessment of Gastric Fluid Volume in Children Scheduled for Elective Surgery After Clear Fluid Fasting for 1 Versus 2 Hours: A Randomized Controlled Trial. Anesthesia and analgesia. Apr 1 2023;136(4):711-718. doi:10.1213/ane.0000000000006157
4. Yaster M, Galinkin J, Schreiner M. An Introduction to the Ethical Design, Conduct, and Analysis of Pediatric Clinical Trials. In: Andropolous D, Gregory G, eds. Gregory's Pediatric Anesthesia. 6th ed. Wiley-Blackwell; 2020:56-74:chap 4.
5. Schmitz A, Schmidt AR. Can we use ultrasound examination of gastric content as a diagnostic test in clinical anaesthesia? Pediatric Anesthesia. 2019;29(2):112-113. doi:https://doi.org/10.1111/pan.13555
6. Roberts RB, Shirley MA. Reducing the risk of acid aspiration during cesarean section. Anesthesia and analgesia. Nov-Dec 1974;53(6):859-68. doi:10.1213/00000539-197453060-00010
7. Raidoo DM, Rocke DA, Brock-Utne JG, Marszalek A, Engelbrecht HE. Critical volume for pulmonary acid aspiration: reappraisal in a primate model. British journal of anaesthesia. Aug 1990;65(2):248-50. doi:10.1093/bja/65.2.248