The nasal response to exercise and exercise induced bronchoconstriction in normal and asthmatic subjects.

Two studies were carried out to test the hypothesis that the fall and recovery of nasal resistance after exercise in asthmatic and non-asthmatic subjects are related to the development of bronchoconstriction after exercise. In study 1 nasal resistance (posterior rhinomanometry) and specific airway resistance (sRaw) were measured before challenge and one, five, 10 and 30 minutes after four minutes of exhausting legwork exercise in nine asthmatic subjects and nine age matched healthy subjects. One minute after exercise there was a reduction in nasal resistance of 49% (SD 15%) from baseline in the healthy subjects and of 66% (17%) in the asthmatic subjects. This response and the subsequent return of nasal resistance to baseline values did not differ significantly between the two groups despite a substantial difference in the change in sRaw, an increase of 74% (45%) in the asthmatic subjects 10 minutes after exercise, and no change in the non-asthmatic subjects. In study 2, nasal and specific airway resistances were monitored according to the same measurement protocol in six subjects with increased airway reactivity. Subjects exercised on two occasions, wearing a noseclip, once while breathing cold, dry air and once while breathing warm, humid air. The fall in nasal resistance was similar under both conditions (to 47% and 39% of baseline), through sRaw rose only after cold air inhalation (to 172% of baseline). The results indicate that the nasal response to exercise is not related to bronchial obstruction in asthmatic subjects after exercise or to the temperature or humidity of the air inspired through the mouth during exercise.     

Laser acupuncture in children and adolescents with exercise induced asthma

BACKGROUND: Laser acupuncture, a painless technique, is a widely used alternative treatment method for childhood asthma, although its efficacy has not been proved in controlled clinical studies. METHODS: A double blind, placebo controlled, crossover study was performed to investigate the possible protective effect of a single laser acupuncture treatment on cold dry air hyperventilation induced bronchoconstriction in 44 children and adolescents of mean age 11.9 years (range 7.5-16.7) with exercise induced asthma. Laser acupuncture was performed on real and placebo points in random order on two consecutive days. Lung function was measured before laser acupuncture, immediately after laser acupuncture (just before cold dry air challenge (CACh)), and 3 and 15 minutes after CACh. CACh consisted of a 4 minute isocapnic hyperventilation of -10 degrees C absolute dry air. RESULTS: Comparison of real acupuncture with placebo acupuncture showed no significant differences in the mean maximum CACh induced decrease in forced expiratory volume in 1 second (27.2 (18.2)% v 23.8 (16.2)%) and maximal expiratory flow at 25% remaining vital capacity (51.6 (20.8)% v 44.4 (22.3)%). CONCLUSIONS: A single laser acupuncture treatment offers no protection against exercise induced bronchoconstriction in paediatric and adolescent patients.     

Forced oscillation technique and spirometry in cold air provocation tests.

BACKGROUND: Impedance measurements by the forced pseudo random noise oscillation technique can be used to study the mechanical characteristics of the respiratory system. The objective of this study was to analyse the changes in impedance to a cold air provocation test in patients with asthma, and to correlate these changes with those in the forced expiratory volume in one second (FEV1). METHODS: The response to isocapnic hyperventilation with cold air was assessed by respiratory impedance measurements and spirometry in 60 patients with bronchial asthma in whom the provocative dose of histamine resulting in a 20% fall in FEV1 (PD20) was < or = 8 mumol. RESULTS: Cold air provocation resulted in a mean(SD) fall in FEV1 from 3.75(0.85) litres to 3.10(0.90) litres. The mean(SD) decrease in FEV1 as a percentage of predicted was 15.4(3.8)%. The oscillatory resistance at 8 Hz increased from a mean(SD) of 0.367(0.108) kPa/l/s to 0.613(0.213) kPa/l/s and at 28 Hz the resistance increased from 0.348(0.089) to 0.403(0.099) kPa/l/s. Frequency dependence of resistance became significantly more negative. The reactance at 8 Hz decreased from a mean(SD) of -0.035 (0.041) kPa/l/s to -0.234(0.199) kPa/l/s, and the resonant frequency increased from 12.5(4.9) Hz to 25.7(9.1) Hz. Significant correlations were calculated between the decrease in FEV1 and changes in the various impedance parameters, especially between the decrease in FEV1 and the increase in resistance at 8 Hz (r = -0.66), and the decrease in FEV1 and the increase in the resonant frequency (r = -0.63). CONCLUSION: Cold air provocation in asthmatic subjects results in changes in the impedance of the respiratory system that correlate well with the changes in FEV1. These changes in impedance reflect ventilatory inhomogeneities in the peripheral compartment of the bronchial tree. These observations show the value of this technique in the evaluation of induced bronchoconstriction, as both a quantitative and a qualitative analysis of the response is possible.     

Role of cooling and drying in hyperventilation induced asthma.

Respiratory heat loss has been proposed as a mechanism of exercise induced asthma. Whether the predominant stimulus is airway drying or cooling remains unclear. We have measured changes in FEV1 after isocapnic cold air hyperventilation (CAH) (-23.4 degrees (SD 0.43 degrees) C) and dry ambient air hyperventilation (AAH) (18.7 degrees (0.52 degrees)C) in seven asthmatic patients (mean age 31 (SD 9) years and baseline FEV1 3.2(0.9)1) and in seven normal subjects (age 28(6) years and FEV1 3.6(0.7)1). The inspired water content in both cases was 0.3 mg/l air. The rate of respiratory heat exchange per breath was calculated in watts (W) with microcomputer based equipment. Cold air hyperventilation caused a fall in FEV1 almost twice that of ambient air hyperventilation at each level of ventilation: CAH v AAH (% fall) 8.0 (5.1) v 3.9 (4.0) at 15 l/min, 11.6 (7.8) v 7.0 (4.4) at 30 l/min, and 20.7 (10.9) v 12.4 (6.3) at 60 l/min. Identical latent heat loss (evaporative drying) was imposed on the airway during the two challenges. Sensible heat loss (convective cooling) in cold air hyperventilation was 41 W at 15 l/min, 63 W at 30 l/min, and 114 W at 60 l/min; whereas in ambient air hyperventilation the loss was 6, 13, and 23 W respectively. It is concluded that the rate of cooling of the upper airway is the predominant stimulus in hyperventilation induced asthma.     

The airway microvasculature and exercise induced asthma.

It has been proposed that exercise induced asthma is a result of "rapid expansion of the blood volume of peribronchial plexi" (McFadden ER, Lancet 1990;335:880-3). This hypothesis proposes that the development of exercise induced asthma depends on the thermal gradient in the airways at the end of hyperpnoea. The events that result in exercise induced asthma are vasoconstriction and airway cooling followed by reactive hyperaemia. We agree that the airway microcirculation has the potential for contributing to the pathophysiology of exercise induced asthma. We do, however, question whether reactive hyperaemia, in response to airway cooling, is the mechanism whereby hyperpnoea provokes airways obstruction in asthmatic patients. Further, we question whether vasoconstriction accompanies dry air breathing and whether an abnormal temperature gradient and rapid rewarming of the airways are prerequisites for exercise induced asthma. From published experiments we conclude that dry air breathing is associated with vasodilation and increase in airway blood flow rather than vasoconstriction and a decrease in blood flow to the airways. We propose that the stimulus for the increase in airway blood flow is an increase in osmolarity of the airway submucosa. This osmotic change is caused by the movement of water to the airway lumen in response to evaporative water loss during hyperpnoea. The increase in airway blood flow may occur directly or indirectly by the osmotic release of mediators. Exercise induced asthma is most likely to be due to the contraction of bronchial smooth muscle caused by the same mediators. Whether it is enhanced or inhibited by alterations in airway blood flow is not yet established in man.     

Measurement of flow-resistive work of heat and moisture exchangers during simulated active breathing

The performance of seven commercially available heat and moisture exchangers (HME) was evaluated in the dynamic flow state. A piston pump was used as a simulator with operative conditions with stroke volume of 500 ml and with respiratory rates of 20 and 40 cycles.min-1. A pressure-volume loop was produced for each HME and the flow-resistive work calculated. Each HME was studied under both dry and wet conditions. It was found that the flow-resistive work of the Humid-Vent 2 was the smallest and that of the Breathaid the largest under both conditions. The results suggest that application of HME causes a significant increase in resistance and demands a greater breathing effort from patients. Its use should thus be avoided in marginal patients.     

Bronchial responsiveness to methacholine in chronic bronchitis: relationship to airflow obstruction and cold air responsiveness.

The response to inhaled methacholine is increased in patients with chronic airflow obstruction, but it is not known whether this is due to true hyperresponsiveness or is a result of the airflow obstruction. In asthmatics the response to methacholine correlates with the bronchoconstriction produced by hyperventilation of cold dry air. We studied 27 patients with a history of smoking and chronic bronchitis with a range of severity of airflow obstruction. Bronchial responses to methacholine (expressed as the provocation concentration causing a fall in FEV1 of 20%-PC20) and isocapnic hyperventilation of cold dry air were measured. In 19 patients the PC20 was less than 8 mg/ml (that is, in the asthmatic range) but only three developed bronchoconstriction in response to hyperventilation. There was a linear correlation between the log PC20 and the FEV1 (r = 0.86, p less than 0.001). The results suggest that in patients with chronic airflow obstruction the response to methacholine is determined by the degree of airflow obstruction, and cannot be used in the diagnosis of asthma in the absence of additional information.     

Differences in responsiveness to hyperventilation and methacholine in asthma and chronic bronchitis.

In a previous study on 27 patients with chronic bronchitis we found that only three developed bronchoconstriction in response to hyperventilation of cold, dry air despite an increased responsiveness to methacholine inhalation. We therefore investigated bronchial responsiveness to hyperventilation with cold, dry air and methacholine in 27 patients with stable asthma who had a similar range of baseline FEV1 values but who developed bronchoconstriction that could be reversed to give an FEV1 more than 70% of the predicted value. Baseline FEV1 was 0.88-3.98 l (37-114% predicted). All but one subject developed bronchoconstriction in response to hyperventilation. There was a linear relationship between baseline FEV1 and response to methacholine (r2 = 0.37, p less than 0.001) and the relationship was significantly different from that found in the bronchitic subjects (F2.50 = 24.94, p less than 0.001). In general, the response to methacholine was greater in the asthmatic than in the bronchitic subjects for any baseline FEV1. The results suggest that there are different mechanisms underlying the increased responsiveness to methacholine in asthma and chronic bronchitis.     

Laryngeal resistance immediately after panting in asthmatic subjects.

The panting manoeuvre may be used during the assessment of airway resistance and in asthmatic patients during bronchial provocation testing or spontaneous asthma. To study whether panting opens the larynx in patients with asthma, laryngeal resistance was examined in six patients with stable asthma before and after methacholine induced bronchoconstriction and in another six patients with spontaneous asthma. Subjects were asked to pant and then to hold their breath immediately afterwards. Laryngeal resistance after panting was compared to that during quiet tidal breathing. Change in laryngeal resistance was estimated by a method using low frequency sound and respiratory resistance by forced oscillation at 10 Hz. Mean baseline respiratory resistance during inspiration was 0.245 and 0.470 kPa/l.s before and after methacholine in the patients with stable asthma and 0.480 kPa/l.s in the patients with spontaneous asthma. In the patients with stable asthma mean laryngeal resistance was lower after panting than during the preceding quiet tidal breathing, both before and after methacholine induced bronchoconstriction (by 0.08 before and by 0.065 kPa/l.s after). In contrast, the patients with spontaneous asthma showed an increase in laryngeal resistance after panting of 0.089 kPa/l.s. The magnitude of change in laryngeal resistance after panting was similar to the change in respiratory resistance in the patients with spontaneous asthma and in the patients with stable asthma after methacholine, but was greater than the change in respiratory resistance in the patients with stable asthma before methacholine. These results suggest that panting may cause different effects on the laryngeal aperture in patients with stable and spontaneous asthma.     

Control of body temperature during abdominal aortic surgery

Background. Careful control of body temperature during anesthesia aims to prevent cardiocirculatory complications during the phase of recovery from anesthesia. Numerous studies have examined methods for warming the gases breathed by the patient, but the question of whether low flow anesthesia or heat and moisture exchangers can also influence the pattern of body temperature remains unresolved.
Methods. In this clinical trial we evaluated the mean body temperature profile measured at five points in 40 patients divided into two groups: Group I was ventilated with a non-rebreathing circuit and Group II was ventilated with a circle system using low flow rates of fresh gases. All patients were treated with a hydrophobic heat moisture exchanger.
Results. The results point out a significantly ( P < 0.01) lower decrease in mean body temperature 40 min after the start of mechanical ventilation with the use of low flow rate anesthesia.
Conclusions. This study shows that anesthesia carried out using low fresh gas flow rates and heat moisture exchanger is able to reduce the fall in mean body temperature, when compared with anesthesia carried out using high fresh gas flow rates and heat moisture exchanger.     

An evaluation of six disposable heat and moisture exchangers

Six disposable heat and moisture exchangers were tested on patients undergoing anaesthesia requiring mechanical ventilation. Inspiratory humidity and temperature were monitored to find the steady-state values reached with each device together with the times taken for these to be achieved. The exchangers were tested in a non-rebreathing T-piece circuit and in a conventional circle system with a fresh gas flow of 6 l/min: the Siemens 150 provided 28 and 32 mg of water/litre of inspired gas (at about 30 degrees C) in 10 and 5 min respectively, but is rather heavy and bulky. The Portex Humidvent provided 25 and 30 mg/l, and although taking longer to reach steady state (27 and 15 min respectively) is small, light and cheap. The Siemens 151 provided 25 and 27 mg/l in 18 and 10 min respectively, but is heavier than the Portex exchanger. The performances of these three devices were not significantly different from each other in either study (P less than 0.05). For the T-piece system the Pall and Engstrom exchangers were the next most efficient. The Pall device provided 18 and 23 mg/l (in 18 and 8 min respectively) and the Engstrom provided 20 and 23 mg/l (in 19 and 10 min respectively). In the circle system, there were no significant differences between the performances of the Portex, Siemens 151, Pall and Engstrom exchangers. The Pall is also a very effective bacterial filter and has been found to be satisfactory in the intensive care setting. The Terumo appeared to perform no better than a circle system with catheter mount (13 mg/l at 27 degrees C). It would seem that more complex humidification equipment is not necessary during anaesthesia if an efficient heat and moisture exchanger is used.     

Comparison of airway reactivity induced by histamine, methacholine, and isocapnic hyperventilation in normal and asthmatic subjects

In an investigation of a rapid screening test for airway reactivity using isocapnic hyperventilation with room air and cold air the results of this test were compared with the airway response to histamine and methacholine challenge. Twelve non-atopic, non-smoking normal subjects and 11 subjects with stable asthma who had an FEV1 above 74% of the predicted value were studied. In the normal subjects isocapnic hyperventilation with room air (75 l/min; 22 degrees C (SEM 0.2 degrees); 10 mg H2O/l air) and isocapnic hyperventilation with cold air (77 l/min; -10 degrees C (0.9 degrees); 2.4 mg H2O/l air) produced no significant change in FEV1. In the asthmatic subjects, hyperventilation with room air (71 l/min; 22 degrees C (0.8 degrees); 10 mg H2O/l air) caused a mean fall in FEV1 of 11.7%; cold air hyperventilation (70 l/min; -10 degrees C (0.9 degrees); 2.4 mg H2O/l air) caused a mean fall in FEV1 of 20.4%. Cold air hyperventilation produced greater separation between normal and asthmatic subjects than room air. The provocative concentration of histamine required to reduce the FEV1 by 20% (PC20) correlated closely with the PC20 for methacholine (r = 0.95; p less than 0.001). Both tests separated normal from asthmatic subjects. PC20 for both histamine and methacholine correlated with the fall in FEV1 after cold air hyperventilation (r = 0.93, p less than 0.001; r = 0.87, p less than 0.001 respectively). We conclude that the results of a rapid screening test based on hyperventilation with cold air correlate well with a standard pharmacological challenge.     

A mask to modify inspired air temperature and humidity and its effect on exercise induced asthma.

BACKGROUND: Heat and moisture loss from the respiratory tract during exercise are important triggers of exercise induced asthma. METHODS: A new heat and moisture exchange mask has been developed which both recovers exhaled heat and water and has a sufficiently low resistance for use during exercise. The effect of the mask on inspired air temperature was studied in four normal subjects. Eight asthmatic subjects performed identical exercise protocols on three separate days, breathing room air through a conventional mouthpiece, a dummy mask, and the new heat and moisture exchange mask. Seven different asthmatic subjects exercised while breathing cold air at -13 degrees C through a dummy or active mask. RESULTS: All subjects found the new mask comfortable to wear. The mean inspired temperature when the mask was used rose to 32.5 (1.4) degrees C when normal subjects breathed room air at 24 degrees C and to 19.1 (2.7) degrees C when they inhaled subfreezing air at -13 degrees C. The heat and moisture exchange mask significantly reduced the median fall in forced expiratory volume in one second (FEV1) after exercise to 13% (range 0-49%) when asthmatic subjects breathed room air compared with 33% (10-65%) with the dummy mask and 28% (21-70%) with the mouthpiece. The fall in FEV1 when the asthmatic subjects breathed cold air was 10% (0-26%) with the heat and moisture exchange mask compared with 22% (13-51%) with the dummy mask. CONCLUSION: Use of a heat and moisture exchange mask can raise the inspired temperature and humidity and ameliorate the severity of exercise induced asthma. The mask may be of practical value in non-contact sport or for people working in subzero temperatures.     

Assessment of a hygroscopic heat and moisture exchanger for paediatric use

A laboratory study of a widely available heat and moisture exchanger marketed for paediatric use was undertaken. The deadspace, measured by volume displacement, was 12 ml, similar to that of a standard catheter mount for paediatric use. Pressure drop across the device was measured at several different flows in five samples of the device in both the dry and wet state. Calculated resistance proved to be markedly lower when compared with that of other anaesthetic equipment such as tracheal tubes, and with similar humidification devices for paediatric use.     

Effect of sleep deprivation on overnight bronchoconstriction in nocturnal asthma.

Nocturnal cough and wheeze are common in asthma. The cause of nocturnal asthma is unknown and there is conflicting evidence on whether sleep is a factor. Twelve adult asthmatic subjects with nocturnal wheeze were studied on two occasions: on one night subjects were allowed to sleep and on the other they were kept awake all night, wakefulness being confirmed by electroencephalogram. Every patient developed bronchoconstriction overnight both on the asleep night, when peak expiratory flow (PEF) fell from a mean (SE) of 418 (40) 1 min-1 at 10 pm to 270 (46) 1 min-1 in the morning, and on the awake night (PEF 10 pm 465 (43), morning 371 (43) 1 min-1). The morning values of PEF were, however, higher (p less than 0.1) after the awake night and both the absolute and the percentage overnight falls in PEF were greater when the patients slept (asleep night 38% (6%), awake night 20% (4%); p less than 0.01). This study suggests that sleep is an important factor in determining overnight bronchoconstriction in patients with nocturnal asthma.     

A Comparative Evaluation of Disposable Humidifiers

The performance of four hygroscopic condenser humidifiers (HCH) and two conventional heat and moisture exchangers (HME), all commercially available, have been evaluated in laboratory tests. A clinical study was also made in order to confirm the test results. It was found that the new generation of heat and moisture exchangers, which have hygroscopic properties, are superior to conventional HME units. When dry inspired gases are used, the HCH units, but not the HME units, deliver a moisture level that is comparable to what is produced in the upper trachea during normal breathing the nose. Our clinical experience as well as the extensive body of literature on humidification requirements support the conclusion that in most cases they can be recommended to be used for extended time periods even with dry gases. The humidification efficiency decreases with increasing tidal volume for all units. However, the best units can be used up to tidal volumes of 1000-1500 ml. The dead space of these units varies from 90 ml to 100 ml and the resistances are only about 50 Pa at 0.5 l/s. The weight is in the range 18-40 g. One of the best HCH devices also has distinct antimicrobial properties. The others are found at least not to promote organism growth. This feature added to others such as safety and simplicity in use make an HCH device an attractive alternative to conventional humidification techniques.     

Inhibition by sodium cromoglycate of bronchoconstriction stimulated by respiratory heat loss: comparison of pressurised aerosol and powder

The protective effect was examined of three doses (2, 10, and 20 mg) of sodium cromoglycate inhaled from a pressurised metered dose inhaler on the response to isocapnic hyperventilation of cold dry air in 10 asthmatic subjects. This was compared with the effect of cromoglycate powder (20 mg) inhaled from a Spincap and with placebo given on two occasions. The medications were inhaled on separate days, in random order and with the use of a double blind double dummy technique, 20 minutes before isocapnic hyperventilation of two fold increasing volumes of air (-15 degrees C, 0% humidity) to produce a 20% fall in the post-treatment FEV1. The response was expressed as the provocative dose of respiratory heat loss required to cause a fall in FEV1 of 15% (PD15, kcal/min). The mean baseline spirometric indices exceeded 85% of predicted normal values on each test day; both placebo treatments reduced the baseline FEV1 by comparison with all active treatments (p less than 0.0001). Comparison of the PD15 on the two placebo days confirmed excellent reproducibility. All doses of cromoglycate shifted the respiratory heat loss dose-response curve to the right of the placebo curve; PD15 after all active treatments exceeded PD15 after placebo (p less than 0.0001). There was no cromoglycate dose-response relationship between the three doses of aerosol (p greater than 0.05), or between any dose of aerosol and powder (p greater than 0.05). It is concluded that cromoglycate aerosol inhaled from a pressurised inhaler in a dose of 2 mg gives the same magnitude of protection against bronchoconstriction stimulated by airway cooling as 20 mg of pressurised aerosol or powder from a Spincap.     

Evaluation of bronchial responsiveness to exercise in children as an objective measure of asthma in epidemiological surveys.

BACKGROUND: Exercise has been proposed as a useful challenge test for the measurement of bronchial responsiveness in community surveys of the prevalence of childhood asthma. This study aimed to develop a standardised exercise challenge in which the sensitivity to detect asthma was increased by inhalation of dry air. METHODS: Sixty four children aged 12-13 years who had reported wheeze in the past 12 months and 70 control subjects were invited to participate in an exercise challenge at school. Subjects performed eight minutes of cycle exercise while breathing dry air at a workload calculated to produce a minute ventilation of 60% maximum voluntary ventilation during the final three minutes. A fall in forced expiratory volume in one second (FEV1) of 10% or more from baseline was considered a positive test. Data on recent asthma symptoms, asthma morbidity, and use of medication were collected by parent completed questionnaires in those subjects who reported wheeze in the past 12 months. Repeatability of the exercise test was determined in a further 13 children with known asthma. RESULTS: Fifty five children (88%) who reported wheeze in the previous 12 months and 54 control subjects (77%) were studied. Nine subjects in whom baseline FEV1 was less than 75% predicted did not perform the exercise test. Technically unsatisfactory tests were obtained in five subjects. Twenty six (57%) subjects who reported wheeze and three controls (6%) had a positive exercise test, giving a sensitivity of 57% (26 of 46) and specificity of 94% (47 of 50). Estimates of the repeatability of the exercise test showed a mean difference in percentage fall in FEV1 for patients with asthma of 3.08% (95% limits of agreement -7.76% to 13.92%). CONCLUSIONS: Despite attempts to maximise the stimulus to bronchoconstriction in this exercise challenge test, its sensitivity and specificity were not improved in comparison with previous epidemiological studies of the prevalence of asthma.     

The effect of a heat and moisture exchanger on humidity in a low-flow anaesthesia system

The heat and humidity in a low-flow breathing system was measured in order to study the inherent humidifying properties of the system at low fresh gas flows (< 1 and 2 l.min⁻¹) and whether a heat and moisture exchanger could compensate for the loss of heat and humidification occurring at higher fresh gas flows (5 l.min⁻¹) in these systems. Sixty patients were randomly divided into three groups (< 1, 2 and 5 l.min⁻¹ fresh gas flows) with a heat and moisture exchanger and three groups without a heat and moisture exchanger in the breathing system. Thirty minutes after the start of anaesthesia a control measurement was performed, after which a heat and moisture exchanger was inserted into the breathing system of the three groups randomly allocated to have one. Three more measurements were performed at 10, 30 and 60 min after control. At low fresh gas flows the humidifying properties of the low-flow breathing system are adequate (i.e. provide an absolute humidity > 20 mg.l⁻¹) but at a fresh gas flow of 5 l.min⁻¹ there is a need for a heat and moisture exchanger for adequate humidification of the inspired gas.     

Comparison of four stainless steel heat exchangers for neonatal ECMO applications

Conventional neonatal extracorporeal membrane oxygenation (ECMO) circuits utilize a heat exchanger distal to the oxygenator to replace ambient heat loss and maintain patient normothermia. A secondary function of the ECMO heat exchanger is to act as an arterial line bubble trap to protect the patient against accidental air embolism. Using an asanguinous recirculating test circuit, we measured and compared heat transfer properties, pressure drop, air trapping capabilities, and priming characteristics of four commercially available stainless steel heat exchangers currently being used in neonatal ECMO circuits: Avecor ECMOtherm, Gish HE-3, Gish HE-4, and Electromedics D1079. Manufacturers' product specifications were also compared. The pressure drop across all four heat exchangers was less than 10 mmHg with flow rates up to 500 ml/min. The Gish HE-3 and HE-4 were the easiest to prime and de-air, while the Electromedics D1079 was the most difficult. The heat exchangers with integral bubble traps (D1079 and HE-4) have superior air trapping capabilities. The ECMOtherm provided moderate air trapping capabilities ( greater than 7.3 ml +/- 1.5 ml) at flow rates under 300 ml/min. The low prime HE-3 was the poorest at trapping air; less than 1 ml at a 400 ml/min pump flow rate. Thermal analysis indicated that the D1079 had the highest performance factor, though all four heat exchangers had similar heat transfer rates and were capable of warming perfusate from 34 degrees to 37 degrees C on a single pass at pump flow rates of 500 ml/min.(ABSTRACT TRUNCATED AT 250 WORDS)