Influence of infusion pumps on the pharmacologic response to nitroprusside

OBJECTIVE: To compare the relationship between variability in nitroprusside delivery from five infusion pumps and the resulting variability in mean arterial pressure (MAP). DESIGN: Randomized, crossover study design. SETTING: A pediatric ICU in a university hospital. PATIENTS: Informed parental consent was obtained for six patients who were hemodynamically stable and receiving a continuous nitroprusside infusion for a clinical application. Subjects ranged in age from 11 months to 9 yr. INTERVENTIONS: All of the subjects were administered nitroprusside using selected infusion pumps, which included Abbott (Micro), 3M/AVI (210), IMED (965), IVAC (565), and Kendall McGaw (MicroRate). MEASUREMENTS AND MAIN RESULTS: After an initial equilibration interval for each device, MAP was measured and recorded at 10-sec intervals for greater than or equal to 90-min intervals using a computerized data collection technique. Variation in nitroprusside administration (flow continuity) for each infusion pump was determined in vitro using a computerized gravimetric technique. Variation in both MAP and flow continuity was mathematically expressed as the coefficient of variance (CV) of the measured values for each of the respective infusion pumps. For the Abbott, IMED, 3M/AVI, IVAC, and Kendall McGaw, infusion pumps, mean +/- SD continuity CV values were 85 +/- 31%, 39 +/- 26%, 19 +/- 8%, 17 +/- 3%, and 12 +/- 3%, respectively, and MAP CV values were 18 +/- 21%, 15 +/- 11%, 8 +/- 2%, and 16 +/- 10%, respectively. CONCLUSIONS: An apparent direct relationship between MAP variability and flow continuity was observed. We speculate that variation in effect of potent short-acting drugs may, in part, be due to infusion pump operation.     

A comparison of infusion devices at 1 ml/hr

Delivery of medications by some infusion devices is irregular. This study investigated instantaneous flow in several infusion devices set at a rate of 1 ml/hr. The following devices were tested: Infusion Pumps: IMED 956A, IVAC 570, IVION "Kids Pump." Syringe Pumps: Medfusion, Baxter, Baxter OR. Tests were performed using a Bio-Tek Infusion Device Analyzer (Model IDA-1). Instantaneous flow rate was defined as Q1/T1 where: Q=sample volume and T=time required to deliver sample volume. The infusion devices were received directly from their respective manufacturers and had not seen clinical service before testing. The units were fully charged and were tested while on AC power. The tests were conducted by the authors, using standard infusion sets and commercially prepared 5% dextrose and 0.45% sodium chloride solution. Each pump was tested for several hours and multiple trials were performed on each pump. The infusion pumps, IMED, IVION, and IVAC all demonstrated deviations from the desired flow rate. The IVAC pump had a greater fluctuation in flow from the set value of 1 ml/hr (p less than 0.02). Variances from mean +/- standard error for each device are shown in parenthesis. IMED 965A (0.005+/-0.014), IVION Kid's Pump (0.002+/-0.009), IVAC 570 (0.001+/-0.006). The Baxter syringe pump (0.002+/-0.009) also had a wide variance in flow. The Baxter OR (0.001+/-0.005) and the Medfusion (0.001+/-0.008) syringe pumps maintained the most consistent flow and showed less variance than the other devices tested.(ABSTRACT TRUNCATED AT 250 WORDS)     

Delivery of tobramycin by three infusion systems

The delivery of tobramycin was studied at the flow rates of 2, 4 and 6 ml/h after injection into the various sites of IVAC, IMED and Auto Syringe infusion systems. The actual times were markedly longer than the predicted times of delivery, especially for all sites of IVAC system and Buretrol site of IMED system. About 20% of tobramycin was never delivered by the Buretrol sites of IVAC and IMED during a 12-hour period at the flow rate of 2 ml/h. If tobramycin delivery is to be assured within 30 min after the start of infusion, at 2-6 ml/h, proximal y-site of IMED or Auto Syringe systems must be utilized. These data suggest the need for considering the method of tobramycin infusion when monitoring and interpreting its serum concentrations.     

Infusion pump performance with vertical displacement: effect of syringe pump and assembly type

OBJECTIVE: To evaluate the effect of different infusion pump models on continuity of drug delivery during vertical displacement of syringe pumps. DESIGN: Zero-drug delivery time (ZDDT), retrograde aspiration volume, and infusion bolus were recorded using the same syringe in three different models of syringe pump after lowering and elevating the pump. Compliance of each infusion assembly was measured using the occlusion release technique at 38 mmHg. RESULTS: Lowering the pump by 50 cm at an infusion rate of 1 ml/h resulted in ZDDT values ranging from 2.78 +/- 0.29 to 5.99 +/- 1.09 min. Elevating the syringe pump to its original position caused infusion boluses between 44.1 +/- 3.2 and 77.1 +/- 5.1 microl. The results demonstrated that there are large differences between syringe pump models (F = 66.8, df = 2/33, p < 0.0001) and between pumps of the same model (F = 21.3, df = 1/34, p < 0.0001). A similar pattern was found in retrograde aspiration volume and infusion bolus. CONCLUSION: All tested pumps led to clinically relevant flow irregularities during vertical displacement of the syringe pump. Thus, vertical displacement of any syringe pump connected to an infusion line delivering highly potent drugs at low infusion rates should be avoided. The variability across syringe pumps indicates that syringe pump design remains an area of potential further improvement for reducing the risk of adverse patient events.     

Central beta adrenoceptor mediation of the antinatriuretic response to behavioral stress in conscious dogs

This study examined the renal excretory response to behavioral stress (aversive conditioning) in conscious dogs during infusion of beta adrenoceptor antagonists that either readily cross, or that do not readily cross the blood-brain barrier. With saline infusion alone (n = 7), behavioral stress decreased sodium excretion (-47 +/- 7% from 345 microEq/min) and urine flow rate (-48 +/- 5% from 2.1 ml/min). In the same dogs, infusion of propranolol, which readily crosses the blood-brain barrier, abolished the sodium excretion and urine flow rate responses to behavioral stress (-2 +/- 6% from 527 microEq/min and -5 +/- 3% from 3.0 ml/min, respectively). In a different group of dogs (n = 5), infusion of timolol or oxprenolol, antagonists that cross the blood-brain barrier less readily than propranolol, did not abolish the excretory response to behavioral stress. Behavioral stress decreased sodium excretion and urine flow rate during timolol infusion (-38 +/- 6% from 452 microEq/min and -36 +/- 6% from 2.4 ml/min, respectively) and oxprenolol infusion (-25 +/- 9% from 328 microEq/min and -19 +/- 8% from 1.8 ml/min, respectively). With saline infusion alone in these same dogs, behavioral stress decreased sodium excretion (-48 +/- 7% from 316 microEq/min) and urine flow rate (-40 +/- 7% from 1.7 ml/min). Mean arterial pressure increased similarly among all conditions in this study. Since the three antagonists accumulate to a high degree in the kidney, but only propranolol abolished the excretory response to behavioral stress, renal beta adrenoceptors did not appear to mediate the excretory response.(ABSTRACT TRUNCATED AT 250 WORDS)     

Patient and nurse evaluation of paient-controlled analgesia delivery systems for postoperative pian management

Five different patient-controlled analgesia (PCA) delivery systems were evaluated for the treatment of acute postoperative pain in 423 patients undergoing elective operations at a large tertiary care hospital The PCA trial was conducted on four different postsurgical wards over a 5-mo period. All five devices were utilized on each ward for a 1-mo period. According to the nurses, the mean time (± SD) required to become comfortable using the Pharmacia Deltec CADD-PCA was significantly longer (50 ± 37 min) than that using the Abbott Lifecare Plus (19 ± 17 min), Bard PCA I (17 ± 14 min), IVAC PCA (17 ± 14 min), or Baxter PCA Infusor (7 ± 8 min). With respect to ease of documentation by the nursing staff, the Baxter device was superior to the Pharmacia device. Similarly, mechanical problems were less frequent with the Baxter (6%) compared with the Pharmacia device (71%). The patients felt that the nurses were more comfortable using the Baxter device than the Pharmacia device. The patients also found the Baxter device easier to use, especially at night, and the least likely to interfere with ambulation. In conclusion, 80% of the nurses at this teaching center preferred the Baxter PCA Infusor over four widely used electronic PCA devices for the management of acute postoperative pain. The Pharmacia device was felt by the nurses to be less user friendly than She other programmable PCA devices used in this trial. Of the electronic devices we studied, the Bard and IVAC devices were the most cost-effective.     

Red cell damage after pumping by two infusion control devices (Arcomed VP 7000 and IVAC 572)

The aim of this study was to assess the performance in terms of red cell damage of two peristaltic volumetric infusion pumps - the Alaris IVAC 572 (San Diego, CA, USA) and Arcomed Volumed VP7000 (Regensdorf, Switzerland). Various infusion pumps are available to transfuse blood at a predetermined rate. It is recommended that each machine should be individually assessed. This experiment used six units of single-donor-transfusable packed red cells and ran each unit through both pumps. This was carried out at 9, 28 and 35 days post-donation at rates from 2 to 150 mL h(-1). Post-pumping samples from these experiments, and a pre-pumping sample in each case, were analysed for levels of potassium and free haemoglobin (Hb). They were also examined microscopically for evidence of cell damage. Potassium levels showed no significant change with pumping on any occasion, but rose significantly as the samples aged. Free Hb showed some variation, but the only consistent finding was a similar rise in value with increasing pack age. Microscopic examination revealed no cell damage under any condition. Both pumps performed to an acceptable level and appear safe to be used for red cell transfusion.     

Detection of intravenous fluid extravasation using resistance measurements

Resistance to fluid infusion can be derived from measurements of pressure at two or more flow rates. We measured resistance in 31 patients using a pressure-monitoring infusion pump (Model 560, IVAC) by recording pressure at five flow rates (0, 50,100, 200, and 300 mL/hr), and computing resistance as the slope of the pressure versus flow curve. Resistance was measured subcutaneously (R_tissue) and intravenously (R_vein) immediately after unsuccessful or successful IV catheter placement. In all patients, R_tissue was always greater than R_vein. The difference ranged from 23 resistance units (RU) to 4166 RU, with a mean difference of 1147 RU (p < 0.0001, Student'st-test). Unpaired analysis of the data was performed to assess the ability of resistance to indicate extravasation in the absence of prior R_vein measurement. The median value for R_vein was 62 RU (range ?13.6 to 420 RU), and for R_tissue, 544 RU (range 65.7 to 4170 RU). Receiver operating characteristic (ROC) analysis revealed that a 200-RU threshold detected infiltration with 0.90 sensitivity and 0.91 specificity. We conclude that elevated resistance during fluid infusion is an important early and easily measurable finding in fluid extravasation.     

Evaluation of a spring-loaded syringe driver for the subcutaneous administration of narcotics

A simple spring-loaded syringe driver was tested for the subcutaneous administration of narcotic analgesics and antiemetics. With concentrations of 2 to 10 mg/mL of hydromorphone and 10 to 50 mg/mL of morphine, the infusion rate during preclinical testing was 1.01 +/- 0.1 mL/hr (range 0.70-1.2 mL/hr). The rate of infusion was not modified by the concentration of narcotic in solution. Clinical trials were performed with morphine in 17 patients, and with hydromorphone in 11 patients. The duration of the infusion was 21 +/- 11 days. The most frequent reason for discontinuation was death (22 cases). The average duration of the site of infusion was 6.3 +/- 4 days. When used subcutaneously, the rate of infusion of the device was 1 +/- 0.15 mL/hr (range 0.70-1.30 mL/hr). Patients and nurses were satisfied with the simplicity and safety of the device. Cost analysis shows that this device is significantly less expensive than currently available portable infusion devices. We conclude that the Medifuse Pump is an inexpensive, safe and effective device for the subcutaneous infusion of narcotics and antiemetics.     

Rarity of a marked "dawn phenomenon" in diabetic subjects treated by continuous subcutaneous insulin infusion

We assessed the quality of overnight glycemic control and the frequency of the "dawn phenomenon" (nadir-0800 h glycemic increase) in 41 insulin-dependent diabetic patients treated by continuous subcutaneous insulin infusion (CSII). Mean plasma glucose levels were near-normal during the 24 h and, in particular, constant throughout the night. In a subset of six patients overnight plasma free insulin concentrations were also constant during CSII. The majority of profiles (88%) showed a glucose nadir from 2.0 to 5.9 mmol/L (most frequently at 0600 h) and had an 0800 h value from 2.0 to 6.9 mmol/L (92%). A large proportion (46%) of profiles showed a zero or negative nadir-0800 h glycemic increase. In 22 patients with three or more profiles recorded at the same basal insulin infusion rate, only one of 103 profiles had a fasting glycemic increase greater than an arbitrary value of 5.0 mmol/L (5.3), although many patients exhibited small dawn glycemic increases (e.g., 14 of 22 had a mean increase of from 0 to 2 mmol/L). In 12 subjects a 15% reduction in basal insulin infusion rate increased the mean +/- SEM dawn glycemic increase from 0.58 +/- 0.25 mmol/L to 2.7 +/- 0.76 mmol/L (P less than 0.025) as well as significantly increasing the nocturnal nadir and 0800 h plasma glucose concentrations. Thus, a marked dawn phenomenon is rare when a single but adequate basal infusion rate is used for CSII, and this questions the need in the majority of patients for preprogrammable pumps with nocturnal infusion rate changes.     

In vitro function of granulocyte concentrates following passage through an electromechanical infusion pump

Six units of granulocytes stored for 18 hours at 20 to 24 degrees C without agitation were passed through an electromechanical infusion device pump system (test) (Abbott) and through a 170-micron filter (control). To determine the effect of needle gauge, a 19 or 23 gauge needle was attached to the end of the pump administration set. After passage through the pump, granulocytes were evaluated for signs of cytolysis and functional loss, red cells for hemolysis, platelets for release of beta-thromboglobulin, and plasma for C3a and C5a complement activation. There was no evidence (mean +/- 1 SD) of hemolysis (less than 1.0 vs. less than 1.0 mg/dl), loss of granulocytes (123 +/- 38 X 10(3)/microliters vs. 118 +/- 29 X 10(3)/microliters), changes in any of several tests of neutrophil function, increased release of beta-thromboglobulin (15.8 +/- 5.8 vs. 17.0 +/- 6.6%), or C3a complement activation (483 +/- 221 vs. 500 +/- 200 ng/ml) after passage through the pump system (p greater than 0.05). No significant differences were seen between the use of a 19 or 23 gauge needle. Based on these in vitro data, we conclude that this pump system is acceptable for use in clinical practice when control over rate and volume of granulocyte administration is important.     

Long-term ambulatory peritoneal insulin infusion of brittle diabetes with portable pumps: comparison with intravenous and subcutaneous routes

This work compares different routes of insulin infusion via portable pumps with chronically implanted catheters and evaluates the long-term feasibility of the technique. Six severely unstable (i.e., uncontrolled by optimized intensive insulin therapy) diabetic individuals (age range: 35 +/- 4 yr; duration: 11 +/- 2 yr) were selected. Promedos pumps (Siemens A. G., Erlangen, West Germany) were exclusively used because of their portability and long-life insulin reservoir (1-mo duration with U40 acidic Hoechst insulin). Each patient underwent three randomized 1-mo periods of insulin infusion: subcutaneous (s.c.), intravenous (i.v.), and intraperitoneal (i.p.) before the catheter was left indefinitely in one of these sites. Diabetic control was improved and insulin doses reduced whatever the route of infusion, although the s.c. route gave slightly higher values. These results did not deteriorate with time: mean blood glucose was 126 +/- 3 mg/dl and HbA1 was 8.3 +/- 0.6% after 10-18 mo of constant infusion versus 237 +/- 35 mg/dl and 10.0 +/- 0.8%, respectively, under conventional therapy. From a practical point of view, the i.p. route seems preferable since all s.c. catheters provoked local reactions after less than 1 mo and the two chronic i.v. catheters obstructed after 8 and 9 mo. All other incidents were minor and curable without removal of the catheters. All patients argued improvement of both diabetes and quality of life and no one has resigned so far. Thus, the i.p. infusion technique seems beneficial to unstable diabetic individuals and adaptable to long-term therapy, although intensive education and careful follow-up are necessary.     

A regulatory role for large vessels in organ circulation. Endothelial cells of the main renal artery modulate intrarenal hemodynamics in the rat.

After arterial denudation by external rubbing of the left main renal artery, we assessed renal plasma flow rate (RPF) and glomerular filtration rate (GFR) in left and right kidneys of Munich-Wistar rats before and after intravenous infusion of acetylcholine (ACH), atrial natriuretic peptide (ANP), or nitroprusside (NP). In the right kidney RPF and/or GFR increased in response to both endothelium-derived relaxing factor (EDRF)-dependent (i.e., ACH) and -independent vasodilators (i.e., ANP and NP); on average, RPF rose by 22 +/- 4% (P less than 0.005), 19 +/- 10% (P less than 0.005), and 37 +/- 12% (P greater than 0.05), respectively. By contrast, in the left kidney RPF failed to increase after ACH (falling by 23 +/- 10%, P less than 0.001) and rose only in response to ANP and NP. To further evaluate the main renal artery's contribution to renal vasodilation, ACH and another EDRF-dependent agent, histamine, were infused through a micropipette into either the proximal or distal portions of the endothelium-intact renal artery. Proximal infusion of ACH led to increases in RPF and GFR, on average by 8 +/- 2% (P less than 0.025) and 10 +/- 3% (P less than 0.01), while bypassing the arterial endothelium by distal infusion failed to increase RPF and GFR, which fell by 24 +/- 6% (P less than 0.025) and 22 +/- 6% (P less than 0.005), respectively. Similarly, proximal infusion of histamine increased RPF by 12 +/- 3% (P less than 0.05), while distal infusion was virtually without effects on plasma flow. Micropuncture study during intravenous ACH infusion revealed significantly higher afferent and efferent arteriolar resistances and lower ultrafiltration coefficients in denuded versus nondenuded kidneys. These data indicate that the main renal artery is a major regulator of renal blood flow and vascular resistances. Similar to other endothelium-derived substances, EDRF may be elaborated mainly by large vessels and may act on the downstream microcirculatory systems, which determine organ blood flow and transcapillary fluid transfer.     

Comparison of insulin aspart with buffered regular insulin and insulin lispro in continuous subcutaneous insulin infusion: a randomized study in type 1 diabetes

OBJECTIVE: To compare the safety and efficacy of insulin aspart (IAsp), buffered regular insulin (BR), and insulin lispro administered by continuous subcutaneous insulin infusion (CSII) in patients with type 1 diabetes. RESEARCH DESIGN AND METHODS: After completing a 4-week run-in period with BR, 146 adult patients with type 1 diabetes (with pretrial CSII experience) were randomly assigned (2:2:1) to CSII treatment with IAsp, BR, or lispro for 16 weeks in a multicenter, open-label, randomized, parallel-group study. Bolus insulin doses were administered 30 min before meals (BR) or immediately before meals (IAsp or lispro). RESULTS: Treatment groups had similar baseline HbA(1c) (7.3% +/- 0.7 for IAsp, 7.5% +/- 0.8 for BR, and 7.3% +/- 0.7 for lispro). After 16 weeks of treatment, HbA1c values were relatively unchanged from baseline, and the mean changes in baseline HbA1c values were not significantly different between the three groups (0.00 +/- 0.51, 0.15 +/- 0.63, and 0.18 +/- 0.84 for the IAsp, BR, and lispro groups, respectively). The rates of hypoglycemic episodes (blood glucose <50 mg/dl) per patient per month were similar (3.7, 4.8, and 4.4 for the IAsp, BR, and lispro groups, respectively). Clogs/blockages in pumps or infusion sets were infrequent; most subjects (76, 83, and 75% in the IAsp, BR, and lispro groups, respectively) had < or = 1 clog or blockage per 4 weeks during the trial. CONCLUSIONS: Insulin aspart in CSII was as efficacious and well tolerated as BR and lispro and is a suitable insulin for continuous subcutaneous insulin infusion using external pumps.     

Simulation study on flow rate accuracy of infusion pumps in vibration conditions during emergency patient transport

Infusion pumps are frequently used when transferring critically ill patients via patient transport cart, ambulance, or helicopter. However, the performance of various infusion pumps under these circumstances has not been explored. The aim of this study was to evaluate the flow rate accuracy of infusion pumps under various clinical vibration conditions. Experiments were conducted with four different types of pumps, including two conventional syringe pumps (Injectomat MC Agilia, Fresenius Kabi and TE-331, Terumo), one conventional peristaltic pump (Volumed μVP7000; Arcomed), and one new cylinder pump (H-100, Meinntech). The flow rate was measured using an infusion pump analyzer on a stable table (0 m/s²) for 1 h with 1 ml/h and 5 ml/h. Experiments were repeated in mild vibration (2 m/s²) (representing vibration of patients in a moving stretcher or ambulance), and in moderate vibration (6 m/s²) (representing vibration in helicopter transport). Any accidental bolus occurrence in extreme vibration situations (20 m/s²) was also analyzed. Simulated vibrations were reproduced by a custom-made vibration table. In the resting state without vibration and in mild vibration conditions, all pumps maintained good performance. However, in moderate vibration, flow rates in syringe pumps increased beyond their known error ranges, while flow rates in peristaltic pumps remained stable. In extreme vibration, accidental fluid bolus occurred in syringe pumps but not in peristaltic pumps. The newly developed cylinder pump maintained stable performance and was unaffected by external vibration environments.

     

Caffeine and theophylline attenuate adenosine-induced vasodilation in humans

In this study the local vasoactive effects of adenosine were explored in the human forearm. Adenosine (15 micrograms/100 ml forearm/min) infused into the brachial artery (n = 6) increased forearm blood flow by 572% +/- 140%, versus - 0.5% +/- 5.8% during placebo infusion (p less than 0.01). Lower adenosine infusion rates (5 micrograms/100 ml forearm/min, three times) induced forearm blood flow increments to 330% +/- 94%, 339% +/- 67% and 330% +/- 79%, respectively (n = 8). These forearm blood flow responses were reduced (p = 0.02) during concomitant intra-arterial infusion of two doses of caffeine (30 and 90 micrograms/100 ml forearm/min) to 150% +/- 45% and 98% +/- 28%, respectively. Theophylline (30 micrograms/100 ml forearm/min; n = 6) also significantly attenuated the adenosine-induced increase in forearm blood flow. Enprofylline (30 micrograms/100 ml forearm/min), a related xanthine with a low affinity to adenosine receptors in vitro, did not change the response to adenosine. Nonspecific vasodilation by sodium nitroprusside infusion (50 ng/100 ml forearm/min) was not inhibited by caffeine compared with placebo (forearm blood flow responses were 202% +/- 21% versus 216% +/- 40%; n = 6). This study demonstrated that caffeine and theophylline specifically reduce adenosine-induced vasodilation in humans, supporting the existence of functional human vascular adenosine receptors.     

Comparative evaluation of the Abbott TDX, the Abbott ABA200, and the Syva LAB5000 for assay of serum gentamicin

Three automated chemical assays for serum gentamicin were compared for accuracy, reproducibility and cost. One method utilized fluorescence polarization (Abbott TDX), and the other two enzyme-multiplied immunoassay (Abbott ABA200, and the Syva Lab5000). All three systems produced a high degree of accuracy and reproducibility with spiked samples when the concentrations of gentamicin were within the range of 3.0-8.0 mg/l. However, with concentrations below 2.0 mg/l or above 8.0 mg/l, only the TDX system gave acceptable coefficients of variation and accurate recoveries. Similarly, excellent correlations were obtained between all three systems for assays of clinical specimens containing 2.0-8.0 mg/l gentamicin, but above and below this range, the correlations were poor except between TDX and Lab5000 within the range of 0.0-2.0 mg/l. The Abbott TDX was thus the most accurate and reproducible of the three systems for the assay of serum gentamicin in the critical concentrations below 2.0 mg/l and above 8.0 mg/l. The cost per assay by the Abbott TDX was US$4.57 compared with US$5.40 for the Abbott ABA200, and US$3.20 for the Syva Lab5000.     

An evaluation of water-vapor output from four brands of unheated, prefilled bubble humidifiers.

We evaluated the water-vapor output from four brands of unheated, prefilled bubble humidifiers-the Aerwey 300, the Aquapak 301, the McGaw 250, and the Travenol 500-at oxygen flowrates of 2, 4, 6, and 81/min. We recorded relative humidity (RH) and temperature in a test chamber at intervals over a minimum period of 24 h, and we calculated water-vapor content (WVC) and corresponding RH at 37 degrees C. Ambient RH was 53.4% +/- 5.25% (mean +/- 1 SD) at a test-chamber temperature of 26.2 degrees C +/- 0.79 degrees C. Mean 24-h WVC at 2 1/min (n = 20) ranged from 20.4 +/- 0.63 mg/l (McGaw) to 17.2 +/- 0.76 mg/l (Aerwey). This corresponded to a range of RH at 37 degrees C of 46.6% +/- 1.43% (McGaw) to 39.1 +/- 1.72% (Aerwey). A statically significant difference in mean WVC existed between brands of humidifiers (P less than 0.001). The McGaw and Travenol humidifiers consistently delivered hydrated gas with a greater WVC than did either the Aerwey or Aquapak humidifiers (P less than 0.001). A statistically significant decrease in mean WVC and RH at 37 degrees C occurred as the oxygen flowrate increased (P less than 0.001). None of the humidifiers was able to hydrate the source gas to a mean equivalent of 50% RH at 37 degrees C, at any flowrate. The design of the humidifier bubble diffuser and the source-gas flowrate appear to be the prime determinants of the humidification efficiency of unheated, prefilled bubble humidifiers.