Flow rate, syringe size and architecture are critical to start-up performance of syringe pumps

BACKGROUND AND OBJECTIVE: Significant start-up delays are inherent to syringe infusion pumps, particularly at low infusion rates, as routinely used in children's anaesthesia and intensive care. Such delays are mainly the result of engagement of gears in the mechanical drive or compliance of the syringe assembly. The purpose of the present study was to determine the effect of flow rate, syringe size and syringe architecture on fluid delivery during infusion start-up. METHODS: Elapsed time from infusion start to achievement of steady-state flow was gravimetrically determined for various infusion rates (0.1, 0.5, 1 mL h-1), different syringe sizes (10-, 20-, 30-, 50-mL) and syringes of two different brands (BD and Codan). Four measurements for each condition were performed with two identical Alaris Asena GH syringe infusion pumps (total of eight experiments). Statistical analysis was done by two-way ANOVA with Bonferroni's post-test; P < 0.05 was considered significant. RESULTS: Start-up time was from 3.6 +/- 0.9 min (BD 10-mL syringe, 1.0 mL h-1) to 74.5 +/- 26.6 min (BD 50-mL syringe, 0.1 mL h-1). Overall, the start-up time markedly increased with lower flow rate (0.1 mL h-1 vs. 1 mL h-1; P < 0.0001), larger syringe size (50 mL vs. 10 mL; P < 0.01), and the BD brand in comparison with the Codan syringes (P < 0.01). CONCLUSIONS: Highest possible flow rate, smaller sized syringes and syringe plungers with reduced compressibility should be preferred in order to avoid significant start-up delays in fluid delivery.     

Brand and size matter when choosing a syringe to relieve pressure in a tracheal tube cuff

We studied the use of an inline syringe as a pressure relief valve for tracheal tube cuffs during exposure to nitrous oxide to see if the technique works. Bench testing was done to determine the stick and slip characteristics of syringes of different brands and sizes. Cuffs were inflated with 20 mL of air, producing a cuff pressure of 100-120 mm Hg. Then the plunger of the syringe was allowed to passively rebound to a steady pressure at which the plunger stopped ("stick pressure"). After several minutes, pressure in the syringe was forcibly increased with a second syringe until the plunger started moving again ("slip pressure"). Stick pressure varied from 18 to 82 mm Hg depending on the brand and size of syringe used. Slip pressures exceeded stick pressures by 20-120 mm Hg. Cuff pressure increased in a linear fashion during nitrous oxide exposure, and no syringe demonstrated automatic pressure reduction. We conclude that a syringe attached to the pilot balloon connector can be used to control tracheal tube cuff pressure during nitrous oxide anesthesia. However, not all syringes are suitable for this purpose: large syringes are better than small syringes, and the Terumo brand is more suitable than BD or Monoject. The system does not work automatically, and intermittent compression of the syringe plunger to overcome static friction is required to avoid overdistension.     

Syringe size and flow rate affect drug delivery from syringe pumps

PURPOSE: To evaluate the effect of syringe size and infusion rate on drug delivery after vertical displacement of syringe pumps. METHODS: Four syringes (10 ml, 20 ml, 30 ml, 50 ml) were studied at three infusion speeds (2 ml x hr(-1), 1 ml x hr(-1) and 0.5 ml x hr(-1)). Fluid delivery was measured gravimetrically using an electronic balance. Aspiration volume (amount of fluid retracted into the syringe-infusion line assembly) and zero-drug delivery time (time between the lowering of the syringe pump and reattainment of the initial weight) were determined after lowering the syringe pump 130 cm or 50 cm. Then, infusion bolus was measured after elevating the pump to its initial position. Syringe compliance was calculated from the occlusion release bolus at an occlusion pressure of 300 mmHg. RESULTS: Zero-drug delivery time, aspiration volume and infusion bolus increased with syringe size (P < 0.0001). At 0.5 ml x hr(-1), a zero-drug delivery time of 29.7 +/- 0.6 min was recorded for the 50 ml syringe and 5.5 +/- 0.15 min for the 10 ml syringe. As infusion rates increased, zero-drug delivery times decreased following a 1/x ratio. Zero-drug delivery time highly correlated with calculated syringe compliance (Pearson's r = 0.97, P < 0.001), which was closely related to syringe plunger area (R2 = 0.93, P < 0.001). CONCLUSIONS: Vertical displacement of syringe pumps causes flow irregularities which correlate directly with syringe size and indirectly with infusion rate. Extreme vertical maneuvering of syringe pumps should be avoided. Infusion rates > or = 1 ml x hr(-1) and low-compliant, small syringes are recommended when highly concentrated drugs are administered.     

Technical communication: design and in vitro testing of a pressure-sensing syringe for endotracheal tube cuffs

Endotracheal intubation is a frequently performed procedure in the prehospital setting, intensive care unit, and for patients undergoing surgery. The endotracheal tube cuff must be inflated to a pressure that prevents air leaks without compromising tracheal mucosal blood flow. For simultaneous endotracheal tube cuff inflation and measurement, we designed and tested a novel pressure-sensing syringe in vitro. The prototype was developed using a standard 10-mL polycarbonate syringe body that houses a plunger and a silicone rubber bellows, the pressure-sensing element. Bellow feasibility was determined and modeled using finite element analysis. Repeatability testing at each pressure measurement for each bellows (pressure versus deflection) was within an average standard deviation of 0.3 cm to 1.61 cm (1%-5% error). Using an aneroid manometer for comparison, there was excellent linear correlation with a Spearman rank of 0.99 (P < 0.001), up to 30 cm H(2)O.     

Do antisiphon valves reduce flow irregularities during vertical displacement of infusion pump systems?

Vertical displacement of syringe pumps may cause irregular drug delivery due to hydrostatic pressure changes in the infusion line. The extent of flow fluctuations depends on the internal compliance of infusion lines, syringes and syringe pumps. We evaluated whether pressure regulation by antisiphon valves (ASV) reduces the flow variation during vertical displacement of 50 ml standard syringes and infusion pumps. An infusion assembly comprising a standard 50 ml BD Plastipak or a 50 ml Fresenius syringe, with syringe pump and 2 m low compliance infusion line was used for in vitro measurement of fluid delivery. The assembly was tested without ASV, and with two ASVs of differing operating pressure (ASV:75: valve opening pressure = 75 mmHg; ASV:155, 155 mmHg). Retrograde aspiration volume, zero-drug delivery time and valve opening bolus were determined after lowering the syringe pump by 50 cm. After elevating the syringe pump to its initial position the ensuing infusion bolus was recorded. Without an antisiphon valve the observed zero-drug delivery times after lowering the syringe pump were (mean +/- SD) 2.4 +/- 0.2 min using the BD Plastipak syringe and 4.09 +/- 0.55 min using the Fresenius syringe. Introduction of the antisiphon valve prolonged the zero drug delivery time 58% (ASV:75) and 88% (ASV:155) in the BD Plastipak syringe assembly and 43% (ASV:75) and 81% (ASV:155) in the Fresenius syringe assembly (P < 0.001). Antisiphon valves worsen flow irregularities caused by vertical displacement of syringe pumps and when used with delivery of concentrated inotropic drugs at low infusion rates, they may aggravate haemodynamic consequences of inconstant drug delivery.     

Compressed air injection technique to standardize block injection pressures

PURPOSE: Presently, no standardized technique exists to monitor injection pressures during peripheral nerve blocks. Our objective was to determine if a compressed air injection technique, using an in vitro model based on Boyle's law and typical regional anesthesia equipment, could consistently maintain injection pressures below a 1293 mmHg level associated with clinically significant nerve injury. METHODS: Injection pressures for 20 and 30 mL syringes with various needle sizes (18G, 20G, 21G, 22G, and 24G) were measured in a closed system. A set volume of air was aspirated into a saline-filled syringe and then compressed and maintained at various percentages while pressure was measured. The needle was inserted into the injection port of a pressure sensor, which had attached extension tubing with an injection plug clamped "off". Using linear regression with all data points, the pressure value and 99% confidence interval (CI) at 50% air compression was estimated. RESULTS: The linearity of Boyle's law was demonstrated with a high correlation, r = 0.99, and a slope of 0.984 (99% CI: 0.967-1.001). The net pressure generated at 50% compression was estimated as 744.8 mmHg, with the 99% CI between 729.6 and 760.0 mmHg. The various syringe/needle combinations had similar results. CONCLUSION: By creating and maintaining syringe air compression at 50% or less, injection pressures will be substantially below the 1293 mmHg threshold considered to be an associated risk factor for clinically significant nerve injury. This technique may allow simple, real-time and objective monitoring during local anesthetic injections while inherently reducing injection speed.     

The 50-millilitre syringe as an inexpensive training aid in the application of cricoid pressure

A recent study in our department demonstrated that depressing the plunger of a 50-mL syringe was reliably and linearly related to the force applied between 20 N and 50 N. Using a 50-mL syringe we constructed a simple device to help train anaesthetic assistants to apply cricoid pressure correctly. We then tested anaesthetists, operating department practitioners (non-physicians) and nurses in our hospital to see if they could correctly apply forces of 20 and 40 N. All subjects were then trained using this apparatus and once confident were retested immediately afterwards, and again 1 week and 1 month later. The results show a wide variation in the force applied with only 30% of subjects applying appropriate force at 20 N, and 40% at 40 N. Training leads to a significant improvement in performance (P < 0.005 at 20 N and P < 0.001 at 40 N) which is maintained for 1 week for both 20 N (P < 0.05) and 40 N (P < 0.05) but not for 1 month. Therefore training should be practised on a weekly basis. This is an inexpensive and simple device that we believe to be useful in helping anaesthetic assistants to apply effective cricoid pressure.     

The Panomat® P-10 micro-volumetric infusion pump is suitable for continuous drug administration at minimal flow rates

PURPOSE: To evaluate the performance of the Panomat P-10 micro-volumetric infusion pump for its use in drug administration at minimal flow rates (microL x hr(-1); e.g., intrathecal application). METHODS: Fluid delivery at steady state conditions, and after vertical displacement of the syringe pump by -50 cm was determined gravimetrically. The Panomat P-10 infusion pump was evaluated at 4, 10, 20, 50 and 100 microL x hr(-1), and compared to a conventional syringe pump assembly at 100, 200, 500 and 1000 microL x hr(-1). Measurements were repeated twice with two different devices of each syringe pump system, and with two syringes. Data are reported as mean +/- SD. RESULTS: Steady state fluid delivery of the Panomat P-10 infusion pump revealed less than 5% deviation to set flow rate at 10, 20, 50 and 100 microL x hr(-1), and 12% deviation at 4 microL x hr(-1). Mean zero-drug delivery time (ZDDT) after lowering the pump by 50 cm at 4 microL x hr(-1) flow rate was 38.4 +/- 7.3 min. At 100 microL x hr(-1) and with original infusion line ZDDT was almost 20 times shorter when compared to the conventional syringe pump assembly (1.5 +/- 0.5 min vs 28.5 +/- 5.0 min). CONCLUSION: The tested Panomat P-10 micro-volumetric pump shows an acceptable flow accuracy as well as a low susceptibility to vertical displacement, and is therefore suitable for continuous drug administration at minimal flow rates. The technology used in this pump carries potential implications for a new generation of syringe pumps.     

Störung der kontinuierlichen,pumpengesteuerten Applikation kardiovaskulär wirksamer Pharmaka durch den hydrostatischen Druck

Administration of highly concentrated, highly potent, and therefore highly dangerous drugs with syringe pumps is common in modern anaesthesia as well as in intensive care and emergency medicine. Because of their exact flow rates down to <1 ml/h, these pumps are predestined for delivery of drugs with short half-lives, such as catecholamines and vasodilators. But intravenous application of drugs with syringe pumps is not without problems. While it is well known that syringes not fixed correctly into the pump can empty themselves by the influence of gravity, it seems not to be known that hydrostatic pressure can influence the flow rate of a correctly connected system even during continuous infusion. In this situation a change of height of the syringe pump in relation to the patient's position can have tremedous effects on hemodynamics due to unintended acceleration or deceleration of the flow rate. This case report demonstrates that the elevation of a connected epinephrine pump while moving a cardiac surgery patient after ACB operation from the operation table into his bed led to critical increases of heart rate, blood pressure and left atrial pressure. In order to quantify the problem we repeated the situation experimentally. It could be demonstrated that the elevation of the syringe pump by 80-100 cm delivers an additional bolus of 4–5 drops as the central venous catheter outlet. Lowering the pump consecutively leads to the opposite effect. In the case reported, the accidentally administered bolus of epinephrine was 12–15 μg (we use a concentration of 60 μg/ml epinephrine for continuous infusion with syringe pumps). From this accidental observation the following conclusion can be drawn: The change of height, in relation to the patient's position, of a running syringe pump during continuous infusion of highly concentrated cardiovascular drugs may cause considerable, even life-threatening hemodynamic disorders. Even in a closed infusion system (syringe-extension-central venous catheter), hydrostatic pressure influences infusion rate. Elevation of the pump leads to unintended bolus administration, and lowering of the pump is followed by an interruption of the infusion. In the knowledge of this phenomenon, unexpected hemodynamic reactions during transport of critically ill patients cannot always be interpreted as a result of inadequate anesthesia or volume load, but may be a consequence of incorrect handling of the syringe pumps as described in this report.     

Intravenous infusions in hyperbaric chambers: effect of compression on syringe function

Haemodynamic instability is a recognised phenomenon in critically ill patients undergoing hyperbaric therapy. Instability may result from the effects of ambient pressure on the cardiovascular system, devices involved in infusion of drugs and fluids, or a combination of the two. The effect of hyperbaric pressure on air-containing spaces in syringes has not been previously measured. We connected 60-ml syringes (Terumo Corporation, Tokyo, Japan) containing coloured water to low volume extensions via three-way taps. We examined the effect of pressurisation to 2.4 and 2.8 atmospheres absolute (ATA) on the syringes by measuring the displacement of the coloured water in the low volume extension set. There was compression of air spaces within the syringe causing retrograde flow of fluid within the low volume extension set. The mean (95% CI) change in volume was 154 (141-168) microl at 2.4 ATA, and 197 (183-212) microl at 2.8 ATA (both p < 0.0001). We conclude that hyperbaric exposure may cause clinically significant changes in syringe function at infusion rates < 100 ml. h(-1).     

Syringe pumps take too long to give occlusion alarm

BACKGROUND: The aim of this study was to assess the time needed to trigger an occlusion alarm, and the influence of the type of infusion pump, type and size of the syringe, and the set infusion rate. METHODS: Forty syringe pumps (20 JMS SP-100 and 20 JMS SP-500 machines) were tested using two types (JMS and Hayat) and two sizes (20 and 50 ml) of syringes at four infusion rates (0.5, 1, 2 and 5 ml x h(-1)). Syringes filled with saline were occluded with a stopcock, and times to activation of the occlusion alarm were recorded. Statistical analysis was performed with Wilcoxon and Mann-Whitney U-tests, P < 0.05 was considered significant. RESULTS: The mean time to alarm activation was longer with lower infusion rates and larger syringes (P < 0.05). We recorded delays up to 117.3 +/- 9.4 min with 50 ml syringe at 0.5 ml x h(-1) and 15.0 +/- 7.1 min at 5 ml x h(-1). Syringe type had no effect on time to alarm activation (P > 0.05). The alarm on the JMS SP-500 pump was activated faster than the JMS SP-100 pump (P < 0.05). CONCLUSION: Our results showed that activation of occlusion alarms on both pumps takes a considerable time, and that the mean time to alarm activation was longer with low infusion rates and larger syringes. To reduce occlusion alarm delays smaller sized syringes with low compliance should be used and staff be alerted when using low flow rates with highly concentrated potent drugs.     

Suction Blister Induction Time: 15 minutes or 150 minutes?

BACKGROUND: The literature on suction blister is somewhat confusing regarding suction blister induction time (SBIT). To date, various factors influencing the SBIT have not been discussed comprehensively. OBJECTIVE: To determine the effect of diameter of suction syringes/cups on SBIT and review the factors influencing SBIT. METHODS: Five young patients (four females, one male; age range 16-27 years, average age 23 years) were selected for the study. Five suction syringes of 2, 5, 10, 20, and 50 ml capacity (diameter 1, 1. 4, 1.7, 2.2, and 3.2 cm, respectively) were applied on the lateral aspect of the thigh and observed for production of unilocular blister at 15 minute intervals. RESULTS: The average SBIT was 63, 72, 98, 123, and 208 minutes in 2, 5, 10, 20, and 50 ml syringes, respectively. CONCLUSIONS: The SBIT is directly proportional to the diameter of the suction syringe. A literature review suggests that the site of the suction blister, age of the subject, amount of vacuum created, temperature, intradermal injection of saline, corticosteroid-induced atrophy, and pretreatment of the site with PUVA are the important factors influencing the SBIT. Use of 10 or 20 ml syringes as suction cups and -300 mmHg pressure are appropriate choices for suction blister induction and produce blisters in about 1-2 hours.     

The effects of syringe plunger design on drug delivery during vertical displacement of syringe pumps

Fluid delivery from four types of commercially available 50-ml syringes was measured using an electronic balance at an infusion rate of 1 ml.h(-1). Retrograde aspiration volume and zero-drug delivery time were recorded after lowering the syringe pump by 50 cm. Syringe compliance was calculated from the volume of bolus released after occlusion at 100 mmHg. Zero-drug delivery times differed significantly between syringes, ranging from [mean (SD)] 3.26 (0.40) min to 6.38 (0.56) min (F = 55.5, d.f. = 3/20, p < 0.0001). Syringe compliance correlated well with aspiration volume (Pearson r(2) = 0.92, p < 0.001) and zero-drug delivery time (r(2) = 0.90, p < 0.001). Syringe design affected the internal syringe compliance. All syringes were associated with potentially relevant zero-drug delivery times after moderate vertical displacement. To minimise this risk, vertical displacement of syringe pumps delivering highly vasoactive drugs should be avoided.     

Active warming of saline or blood is ineffective when standard infusion tubing is used: an experimental study

PURPOSE: To determine the effect of infusion rate, tubing length and fluid composition on the temperature of the infusate reaching the distal end of an infusion tubing with and without active fluid warming. METHODS: Warmed normal saline (W-NS) and packed red blood cells (W-PRBC), were infused with a fluid warmer through a modified infusion set. The fluids were delivered at eight infusion rates from 50 to 999 mL x hr(-1). The infusate temperature was monitored at 20 cm intervals on the iv tubing. The same temperature monitoring protocol was applied to PRBC without warmer (NoW-PRBC). RESULTS: In W-NS and W-PRBC groups, there was a decrease in the infusate temperature, at each flow rate, from the drip chamber to the distal end of tubing ( P <0.001). In NoW-PRBC group, there was a rapid increase in the infusate temperature from the bag to the drip chamber ( P <0.001). Thereafter, there was no change in temperature, except at the 999 mL x hr(-1) infusion rate, where a slight increase in the infusate temperature throughout the tubing was shown. In W-NS and W-PRBC groups increasing the flow rate produced a significant increase in the infusate temperature, at each measurement point ( P <0.001). In the NoW-PRBC group, increasing the flow rate did not alter the infusate temperature. The fluid composition did not influence the infusate temperature. CONCLUSION: There is an important heat exchange within the tubing, which is aggravated at low flow rates. At infusion rates appropriate for pediatric anesthesia the clinical and economic value of fluid warming without the use of heated extension tubing is questionable.     

Alteration in flow delivery with antisyphon devices

The aim of this study was to determine whether infusion sets containing antisyphon devices increased the time to initial flow from syringe drivers. The antisyphon devices assessed were those manufactured by B Braun, Wescott and Vygon. Each device was placed between a 50-ml syringe and a spiral extension set and primed with saline. A fourth syringe and spiral extension set acted as a control. The infusion sets were placed in four identical syringe drivers and started simultaneously. The time from pressing the start button until the initial flow for each infusion set (start-up time) was recorded. The test was conducted 15 times each at 2 ml.h-1, 10 ml. h-1 and 50 ml.h-1. At 2 ml.h-1 the start-up time was significantly longer with all the antisyphon sets compared with the control (p < 0. 0001). At higher infusion rates the differences between the antisyphon sets and the control were less pronounced. Clinicians who use syringe driver infusions should be aware of this delay between the activation of the infusion pump and the onset of flow and take steps to prevent it.     

Auswirkungen von Luftbläschen in der Infusionsspritze auf die Funktion von Spritzenpumpen

Application of highly concentrated short-acting vasoactive drugs in the critically ill patient requires precisely working syringe pump systems for continuous intravenous drug delivery. We performed a bench study to investigate the consequences of small amounts of air entrapped within a 50-ml infusion syringe. In particular we studied the effect of entrapped air on drug delivery after moderate vertical displacement of the pump by 50 cm (e.g. in preparation for transport) and the effect on the time required to trigger the pressure alarm after occlusion of the infusion line. At a flow rate of 1 ml/h, lowering the syringe pump prolonged the zero-drug delivery time from (mean +/- SD) 4.1 +/- 0.8 min (without air) to 6.2 +/- 0.9 (with 1 ml air) and to 13.1 +/- 0.9 min (with 2 ml of air, p < 0.001 for all comparisons). Entrapping of 2 ml of air within the syringe resulted in a 2.6-fold prolongation of the occlusion alarm time after accidental occlusion of the infusion line and a 3-fold increase of the resulting infusion bolus after occlusion. Enclosed air within infusion syringes considerably affects the syringe compliance. It increases the susceptibility of constant drug delivery to vertical displacement of syringe pumps and impairs the occlusion alarm function. Therefore, any air in syringe of infusion pump systems should be carefully removed. To avoid infusion boluses of short-acting vasoactive drugs after accidental occlusions, the occluded infusion line should be released to ambient pressure first.     

A simple device for monitoring neuromuscular blockade in children

A simple instrument for measuring adductor pollicis tension in children has been devised. The thumb sensor utilizes a thin conical rubber seal obtained from a 5, 10 or 20 ml Becton Dickinson syringe. A syringe barrel of similar size, fixed at right angles to the sensor, forms a handle for the fist. The sensor is linked to a pressure transducer with non-compliant tubing. Pressure output of the saline-filled system was found to be linearly related to the force exerted on each size of the sensor. In 20 children, aged two to eight years, the device, fitted in turn with the 5, 10 and 20 ml syringe seal sensors, gave train-of-four (TOF) ratios which showed close correlation (coefficient r = 0.96, 0.93, 0.95) and agreement (regression slope b = 0.97, 0.95, 0.98) with those of a force transducer. Thus, inspection of the displayed TOF pressure pulses gives a rapid and reliable estimate of neuromuscular blockade.     

Perfusion des médicaments vasoactifs à très faible débit : influence des modalités du relais et du volume des seringues (étude expérimentale)

ObjectiveThe flow rate of intravenous administration of vasoactive agents should be steady in order to prevent changes in hemodynamics. In the newborns, because the flow rate is often lower than 1 mL/h, it is difficult to switch of the syringe. The aim of our study was to compare the variations of concentration of active substance delivered after a manual or an automatic switch off the syringe with three different volumes (10 mL, 20 mL and 50 mL).Materials and methodsGlucose solution (10 g/L) was used to simulate the administered substance. Saline was administered in “Y” simultaneously with the glucose solution through a catheter. The infused substance was collected at the tip of the catheter. The glucose concentration was measured at 15min-interval for 3 hours.ResultsThe manual switch of the syringe was associated with a significant alteration of the flow rate, lasting more than 15 min. In contrast, the automatic switch of syringes was associated with no change of the flow rate, especially with small-volume syringes (10 mL).ConclusionIn newborns, in order to prevent the change in flow rate of drugs after a switch of syringes, our results suggest the use of syringe-pump with built-in automatic switch and small-volume syringes.     

Do laryngeal mask airway devices attenuate liquid flow between the esophagus and pharynx? A randomized, controlled cadaver study

In this randomized, controlled cadaver study, we tested the hypothesis that the standard laryngeal mask airway (LMA) and flexible laryngeal mask airway (FLMA) attenuate liquid flow between the esophagus and pharynx. Fifty fresh cadavers were studied in four LMA groups. Ten female cadavers had a size 4 LMA and 10 had a size 4 FLMA; 10 male cadavers had a size 5 LMA and 10 had a size 5 FLMA; 5 male and 5 female cadavers functioned as controls. The chest was opened, and the infusion set of a pressure-controlled, continuous flow pump was inserted into the esophagus and ligated into place. Esophageal pressure was increased in 2-cm H2O increments. Regurgitation pressure was the esophageal pressure at which fluid was first seen with a fiberoptic scope in the hypopharynx (control group) and above the cuff or within the bowl (LMA groups). This was performed in the LMA groups at 0-40 mL cuff volume in 10-mL increments. Mean (95% confidence interval) regurgitation pressure for the control group was 7 (6-8) cm H2O and for the LMA groups combined was 19 (17-20) cm H2O at 0 mL cuff volume, 47 (41-52) cm H2O at 10 mL, 51 (44-55) cm H2O at 20 mL, 52 (45-56) cm H2O at 30 mL, and 52 (45-55) cm H2O at 40 mL. The increase in regurgitation pressure with increasing cuff volume from 0 to 10 mL was statistically significant (P < 0.0001). Regurgitation pressure was higher for the LMA groups at all cuff volumes compared with the control group (P < 0.0001). There were no differences in regurgitation pressure among the LMA groups. We conclude that the correctly placed LMA and FLMA attenuate liquid flow between the esophagus and pharynx. Implications: We have shown, in cadavers, that the correctly placed standard and flexible laryngeal mask airways attenuate liquid flow between the pharynx and esophagus.     

The accuracy and variability of bolus injections with different sized syringes

The accuracy of bolus injections using different sized syringes was studied. A 1 ml bolus was delivered from a 1 ml, 5 ml, 10 ml, and a 20 ml syringe (n = 205). The 1 ml syringe was the most accurate (P less than 0.001) and the bolus delivered was the least variable (P less than 0.001). Variability increased as syringe size increased. A 3 ml bolus was delivered from a 3 ml, 5 ml, 10 ml, and a 20 ml syringe (n = 164). The 5 ml syringe was the most accurate (P less than 0.05) and the 3 ml and 5 ml syringes delivered a bolus with the least variability (P less than 0.001). A 5 ml bolus was delivered from a 5 ml, 10 ml, and a 20 ml syringe (n = 123); in this case there was no significant difference in the accuracy or variability of bolus among the three syringes. We conclude that for accuracy of small volume boluses (less than 5 ml), small-sized syringes should be used.