Wheeze monitoring in children for assessment of nocturnal asthma and response to therapy.

The utilisation of nocturnal wheeze monitoring and quantification for assessment of asthma activity was studied in symptomatic school-aged children before and during treatment. Twelve children 6-14 yrs of age with mild or moderate untreated asthma were studied at home three times: before, 48 h and 6 weeks into treatment with 5 mg montelukast daily. Lung sounds were recorded overnight by an automatic wheeze detection device (PulmoTrack). Per cent wheezing within each respiratory cycle was calculated every 30 s throughout the night and a Nocturnal Wheeze Index (NWI) was calculated for the total night. The results were compared with spirometric indices (forced expiratory volume in one second (FEV1), forced vital capacity), bronchial reactivity (provocative concentration causing a 20% fall in FEV1 by adenosine 5'-monophosphate (PC20)) and daily symptom scores, performed in parallel at each stage of the study. The pretreatment NWI was 814+/-898 (mean+/-SD), which declined to 318+/-199 2 days after onset, and to 137+/-101 after 6 weeks of treatment. The NWI in seven healthy children was 47+/-43. The FEV1, PC20 and symptom scores improved in parallel. Wheeze monitoring provides quantitative and noninvasive information about the extent of nocturnal wheezing in children, correlates well with conventional indices of asthma activity and can assist in assessing efficacy of treatment.

Sublingual electrical stimulation of the tongue during wakefulness and sleep.

Pharyngeal obstruction in patients with obstructive sleep apnea (OSA) is thought to result from decreased upper airway muscle tone during sleep. The goal of the present study was to estimate the role of the tongue muscles in maintaining pharyngeal patency during sleep. Using non-invasive, sub-lingual surface electrical stimulation (ES), we measured tongue protrusion force during wakefulness and upper airway resistance during sleep in seven healthy subjects and six patients with OSA. During wakefulness, ES produced similar protrusion forces in healthy subjects and patients with OSA. ES of the anterior sublingual surface, causing preferential contraction of the genioglossus, resulted in smaller effects than combined ES of the anterior and lateral surface, which also stimulated tongue retractors. During sleep, trans-pharyngeal resistance decreased and peak inspiratory flow rate increased from 319+/-24 to 459+/-27 and from 58+/-16 to 270+/-35 ml/sec for healthy subjects and OSA patients, respectively (P<0.001). However, ES was usually unsuccessful in reopening the upper airway in the presence of complete apneas. We conclude that non-invasive ES of the tongue improves flow dynamics during sleep. Combined activation of tongue protrusors and retractors may have a beneficial mechanical effect. The magnitude of responses observed suggests that in addition to the stimulated muscles, other muscles and/or forces have a substantial impact on pharyngeal patency.

A rat lung model of instilled liquid transport in the pulmonary airways.

When a liquid is instilled in the pulmonary airways during medical therapy, the method of instillation affects the liquid distribution throughout the lung. To investigate the fluid transport dynamics, exogenous surfactant (Survanta) mixed with a radiopaque tracer is instilled into tracheae of vertical, excised rat lungs (ventilation 40 breaths/min, 4 ml tidal volume). Two methods are compared: For case A, the liquid drains by gravity into the upper airways followed by inspiration; for case B, the liquid initially forms a plug in the trachea, followed by inspiration. Experiments are continuously recorded using a microfocal X-ray source and an image-intensifier, charge-coupled device image train. Video images recorded at 30 images/s are digitized and analyzed. Transport dynamics during the first few breaths are quantified statistically and follow trends for liquid plug propagation theory. A plug of liquid driven by forced air can reach alveolar regions within the first few breaths. Homogeneity of distribution measured at end inspiration for several breaths demonstrates that case B is twice as homogeneous as case A. The formation of a liquid plug in the trachea, before inspiration, is important in creating a more uniform liquid distribution throughout the lungs.

Liquid plug flow in straight and bifurcating tubes.

A finite-length liquid plug may be present in an airway due to disease, airway closure, or by direct instillation for medical therapy. Air forced by ventilation propagates the plug through the airways, where it deposits fluid onto the airway walls. The plug may encounter single or bifurcating airways, an airway surface liquid, and other liquid plugs in nearby airways. In order to understand how these flow situations influence plug transport, benchtop experiments are performed for liquid plug flow in: Case (i) straight dry tubes, Case (ii) straight pre-wetted tubes, Case (iii) bifurcating dry tubes, and Case (iv) bifurcating tubes with a liquid blockage in one daughter. Data are obtainedfor the trailing film thickness and plug splitting ratio as a function of capillary number and plug volumes. For Case (i), the finite length plug in a dry tube has similar behavior to a semi-infinite plug. For Case (ii), the trailing film thickness is dependent upon the plug capillary number (Ca) and not the precursor film thickness, although the shortening or lengthening of the liquid plug is influenced by the precursor film. For Case (iii), the plug splits evenly between the two daughters and the deposited film thickness depends on the local plug Ca, except for a small discrepancy that may be due to an entrance effect or from curvature of the tubes. For Case (iv), a plug passing from the parent to daughters will deliver more liquid to the unblocked daughter (nearly double, consistently) and then the plug will then travel at greater Ca in the unblocked daughter as the blocked. The flow asymmetry is enhanced for a larger blockage volume and diminished for a larger parent plug volume and parent-Ca.

Influence of intravenous perfluorocarbon administration on the dynamic behavior of lung surfactant.

Intravenous administration of perfluorocarbon (PFC) compounds can lead to pulmonary hyperinflation and respiratory distress in some mammals. This study was designed to quantify the effects of two PFC emulsions on the dynamic behavior of lung surfactant and to demonstrate that PFC is retained in the liquid lining the lung. New Zealand White rabbits received isotonic saline (3 ml/kg), Fluosol (15 ml/kg) or Oxygent (90% perfluorooctyl-bromide emulsion, 3 ml/kg). After seven days we euthanized the animals and lavaged the lungs. Surface tension-surface area relationships (sigma-A loops) were measured with the lavage fluid placed in a Wilhelmy plate-oscillating bellows apparatus. Loop hysteresis area after Fluosol administration was 334 +/- 92 dyne-cm, significantly greater than after saline (203 +/- 36 dyne-cm) but not Oxygent (274 +/- 66 dyne-cm). Loop hysteresis slope was higher with Oxygent (0.8 +/- 0.4 dyne/cm3) than after saline (0.6 +/- 0.3 dyne/cm3) or Fluosol (0.5 +/- 0.1 dyne/cm3). 282 MHz 19F NMR spectral analysis demonstrates that both PFCs tested appear only in the extracellular fraction of the lavage fluid. These results show that pulmonary elimination of intravascular PFC leads to PFC presence in the liquid lining the airways where it alters surfactant dynamic mechanical behavior.

Perfluorocarbon induced alterations in pulmonary mechanics.

Perfluorocarbon (PFC) compounds induce pulmonary hyperinflation and respiratory distress in some animals following intravenous administration. This study was designed to quantify the effects of two PFC emulsions on lung volumes and compliance and to identify the mechanism of pulmonary hyperinflation. New Zealand White rabbits received isotonic saline (3 ml/kg), Fluosol (15 ml/kg) or Oxygent (90% perfluorooctyl-bromide emulsion, 3 ml/kg). After seven days we measured functional residual capacity, vital capacity, lung compliance and thoracic gas volume. Gross and microscopic histologic examination of the lungs was performed. Functional residual capacity after Fluosol administration was 16.0 +/- 4.0 ml/kg, significantly greater than after saline (3.4 +/- 1.0 ml/kg) or Oxygent (4.0 +/- 1.4 ml/kg). Vital capacity was lower with Fluosol (30 +/- 5.0 ml/kg) than after saline (37 +/- 3.0 ml/kg) or Oxygent (37 +/- 2.0 ml/kg). Thoracic gas volume increased from 9 +/- 1.0 ml/kg (saline) to 16 +/- 13 ml/kg (Oxygent) and 33 +/- 7.0 ml/kg (Fluosol). Lung compliance was the same after saline (1.6 +/- 0.5 ml.cm H2O-1.kg-1) and Oxygent (1.5 +/- 0.3 ml.cm H2O-1.kg-1) but lower after Fluosol (0.9 +/- 0.1 ml.cm H2O-1.kg-1). Gross pathology demonstrated foam exudation from airways of animals receiving PFCs and intra-alveolar foam was identified by light microscopy. These results show intra-airway foam formation causes gas trapping and shifts tidal breathing to a less compliant region of the pressure-volume curve.

Acoustic reflectometry and endotracheal intubation.

To determine whether acoustic reflection measurement of the upper airway can be used to identify tracheas that are difficult to intubate, we conducted a pilot study of adults with a documented history of unexpected failed endotracheal intubation (16 cases) and compared them with 16 controls with previous successful intubation. The two groups were matched by age, sex, height, and weight. Acoustic reflection measurements of airway cross-sectional area versus distance were made at six combinations of body (upright and supine) and neck (flexed, neutral, and extended) positions. Cumulative airway volumes were calculated from the incisors to the glottis, and these were subdivided into oral and pharyngeal volumes. For supine position with the neck extended, all patients who had been successfully intubated had pharyngeal volumes more than 43.4 mL (mean +/- SD, 56.9 +/- 8.3 mL), whereas pharyngeal volumes were less than 37.5 mL in all patients who had a history of unexpected failed intubation (mean +/- SD, 19.7 +/- 10.2 mL; P < 0.05). Using a cutoff of 40.2 mL, acoustic reflection enabled us to distinguish between patients with previous unexpected failed endotracheal intubation and those with previous successful intubation.

Chest vibration redistributes intra-airway CO2 during tracheal insufflation in ventilatory failure.

OBJECTIVE: To determine if high-frequency external chest wall vibration added to low flow intratracheal fresh gas insufflation alters the intra-airway CO2 distribution and the resistance to CO2 transport from the lungs. DESIGN: Prospective study. SETTING: Experimental laboratory. SUBJECTS: Six adult anesthesized and paralyzed mongrel dogs (mean weight 24.3+/- 4.4 kg). INTERVENTIONS: Dogs were ventilated by three methods: a) intermittent positive pressure ventilation; b) intermittent positive pressure ventilation with tracheal insufflation of fresh gas (FIO2 of 0.4) flowing at 0.15 L/kg/min through a catheter positioned at the carina; and c) intermittent positive pressure ventilation with tracheal insufflation and with external high-frequency chest wall vibration of the dependent hemithorax. MEASUREMENTS AND MAIN RESULTS: We measured arterial blood gas values as an index of global gas exchange, and intrapulmonary airway CO2 concentrations as an index of local gas exchange. Intra-airway CO2 concentrations along the axis of the airways were measured via a sampling catheter. Airway axial concentration profiles were constructed and resistances to gas transport were calculated from the measured data. Vibration increased intraluminal CO2 concentrations from 1.1% to 2.5% mouthward of the insufflation catheter tip. Peak resistance to CO2 transport decreased by 65% during vibration relative to the insufflation-only value. Vibration displaced peak transport resistance from second- to fourth-generation airways. CONCLUSIONS: Global gas exchange improves during ventilation by chest wall vibration with low flow insufflation. Local gas exchange in the central airways is also improved due to increased intraluminal mixing and CO2 elimination. This ventilation technique may confer therapeutic advantages over conventional mechanical ventilation in the treatment of ventilatory failure.

A rapid noninvasive blood pressure measurement method for discrete value and full waveform determination.

A new noninvasive measurement method providing rapid measurement of systemic arterial blood pressure (BP) and its validation is described. The method combines precisely timed electrocardiographic-gated rapid release of occluding counter-pressure (600 mmHg/s) with photoplethysmographic detection of radial artery filling to measure arterial opening pressure. A complete BP waveform is reconstructed from multiple repetitions of the measurement cycle at successively increasing time intervals relative to the electrocardiographic signal. Systolic and diastolic values can be measured within two to four cardiac cycles at the peak and trough of the BP wave. The new method was compared with sphygmomanometry in 26 randomly selected subjects over a sphygmomanometric pressure range of 53-110 (diastolic) and 100-190 mmHg (systolic). The mean pressure differences between the sphygmomanometric and new methods were -1.3 +/- 15.2 (SD) (systolic) and 0.7 +/- 9.9 mmHg (diastolic), and corresponding BP values measured by these methods were highly correlated [P < 0.001; R2 = 0.87 (systolic); R2 = 0.80 (diastolic)]. The new method was compared with sphygmomanometry and intra-arterial BP in six patients. These tests confirmed the method's validity compared with established methods. The new method was ostensibly immune to mechanical perturbations when tested during cycle ergometry at 60 W. The new method may facilitate the study of circulatory phenomena previously inaccessible by available noninvasive methods and minimizes patient discomfort and circulatory arrest at the measurement site.

Airflow effects on amplitude and spectral content of normal breath sounds.

Even though it is well known that breath-sound amplitude (BSA) increases with airflow, the exact quantitative relationships and their distribution within the relevant frequency range have not yet been determined. To evaluate these relationships, the spectral content of tracheal and chest wall breath sounds was measured during breath hold, inspiration, and expiration in six normal men. Average spectra were measured at six flow rates from 0.5 to 3.0 l/s. The areas under the spectral curves of the breath sounds minus the corresponding areas under the breath-hold spectra (BSA) were found to have power relationships with flow (F), best modeled as BSA = k.F alpha, where k and alpha are constants. The overall mean +/- SD value of the power (alpha) was 1.66 +/- 0.35, significantly less than the previously reported second power. Isoflow inspiratory chest wall sound amplitudes were 1.99 +/- 0.70- to 2.43 +/- 0.65-fold larger than the amplitudes of the corresponding expiratory sounds, whereas tracheal sound amplitudes were not dependent on respiratory phase. Isoflow breath sounds from the left posterior base were 32% louder than those from the right lung base (P < 0.01). BSA-F relationships were not frequency dependent during expiration but were significantly stronger in higher than in lower frequencies during inspiration over both posterior bases. These data are compatible with sound generation by turbulent flow in a bifurcating network with 1) flow separation, 2) downstream movement of eddies, and 3) collision of fast-moving cores of the inflowing air with carinas, all occurring during inspiration but not during expiration.

Spectral characteristics of chest wall breath sounds in normal subjects.

BACKGROUND: This study was carried out to establish a reliable bank of information on the spectral characteristics of chest wall breath sounds from healthy men and women, both non-smokers and smokers. METHODS: Chest wall breath sounds from 272 men and 81 women were measured using contact acoustic sensors, amplifiers, and fast Fourier transform (FFT) based spectral analysis software. Inspiratory and expiratory sounds were picked up at three standard locations on the chest wall during breathing at flows of 1-2 l/s and analysed breath by breath in real time. RESULTS: The amplitude spectrum of normal chest wall breath sounds has two linear parts in the log-log plane--low and high frequency segments--that are best characterised by their corresponding regression lines. Four parameters are needed and are sufficient for complete quantitative representation of each of the spectra: the slopes of the two regression lines plus the amplitude and frequency coordinates of their intersection. The range of slopes of the high frequency lines was -12.7 to -15.2 dB/oct during inspiration and -13.4 to -20.3 dB/oct during expiration. The frequency at which this line crossed the zero dB level--that is, the amplitude resolution threshold of the system--was designated as the maximal frequency (Fmax) which varied from 736 to 999 Hz during inspiration and from 426 to 796 Hz during expiration with higher values in women than in men. The mean (SD) regression coefficient of the high frequency line was 0.89 (0.05). CONCLUSIONS: These data define the boundaries of normal chest wall breath sounds and may be used as reference for comparison with abnormal sounds.

The acoustic properties of snores.

This study was undertaken in an attempt to characterize the acoustic properties of snoring sounds in the time and frequency domains, and to correlate between these properties and the mechanical events underlying their production. Three experimental set-ups were used: 1) Dog model--six mongrel dogs, in which partial upper airway obstruction was created by an implanted supraglottic balloon. Flow, supraglottic pressure, and snoring sounds were recorded during different degrees of obstruction. Fifteen to 20 snores from each dog (total 100 snores) were analysed. 2) Simulated human snores--Six simulated snores from each of four subjects were recorded in two locations (trachea and ambient) with simultaneous airflow, and their correlations examined. 3) Snoring patients--snores were recorded with an ambient microphone from nine subjects with "heavy" snoring and no obstructive sleep apnoea (OSA). Forty to 50 snores from each subject were analysed (total of 400 snores). The snoring sound was analysed in the time (time-expanded waveform) and frequency (power spectrum) domains. After analysing these snores, we were able to identify two dominant patterns which are distinctly different from each other: the "simple-waveform" and the "complex-waveform". The complex-waveform snore is characterized by repetitive, equally-spaced, train of sound structures, starting with a large deflection followed by a decaying amplitude wave. In the frequency domain, it is characterized by multiple, equally-spaced peaks of power (comb-like spectrum). Simple-waveform snores have a quasi-sinusoidal waveform, with a range of variants, and almost no secondary internal oscillations. Their power spectrum contains only 1-3 peaks, of which the first is the most prominent. We developed a mathematical representation of these waveforms, which is presented along with its implications. The complex-waveform snores result from colliding of the airway walls and represent actual brief airway closure. Simple-waveform snores are of higher frequency and probably result from oscillation around a neutral position without actual closure of the lumen.

Comparative study of intra-airway gas transport by alternative modes of ventilation.

The effectiveness of three alternative modes of ventilation [high-frequency ventilation (HFV), constant-flow ventilation (CFV), and high-frequency external vibration ventilation (HFVV)] was compared. Local intra-airway gas transport was measured with catheters placed in the distal trachea and in bronchi located 5.5, 9, and 11 cm from the carina. A new bolus dispersion method was devised to measure the local effective diffusivities (Deff) induced by these modes of ventilation and by cardiogenic oscillations relative to molecular diffusivity (Dmol). Mixing induced by cardiogenic oscillations was 7 +/- 2- to 26 +/- 4-fold greater than by molecular diffusion alone. Intra-airway transport by CFV, applied at three flow rates (0.3, 1.0, and 3.0 l.min-1.kg-1), was most effective in the trachea but fell sharply in the more peripheral airways. Local transport by HFVV, at a frequency of 22 Hz and a vertical amplitude of 0.4 cm, was most effective in the periphery (Deff = 793 x Dmol), whereas the effectiveness of transport by HFV, applied with 10 and 20 ml at 22 Hz, was evenly distributed. Doubling the HFV oscillatory volume caused a 4.5 +/- 2.7-fold increase in Deff/Dmol. Combining HFVV with CFV at 0.3 l.min-1.kg-1 induced transport rates that were 187- to 2,034-fold greater than by molecular diffusion alone in the bronchi and a higher relative transport (due to convection) in the trachea. We conclude that the combination of HFVV with low-flow CFV provides a high rate of intra-airway transport with minimal mechanical perturbations to the pulmonary system.

Electrically-activated dilator muscles reduce pharyngeal resistance in anaesthetized dogs with upper airway obstruction.

There is current controversy as to whether electrical stimulation of upper airway musculature can be used us a beneficial treatment modality in patients with obstructive sleep apnoea syndrome. Increased upper airway (UAW) muscle activity decreases UAW resistance (Ruaw) in isolated UAW of dogs. In the present study, we evaluated the effect of UAW muscle contraction on UAW patency in anaesthetized dogs in vivo breathing spontaneously through partially and completely obstructed UAW. Airflow and supraglottic pressure were measured to obtain Ruaw. Ruaw could be regulated by inhalation of a rubber balloon implanted transcutaneously in the pharyngeal submucosa to produce partial or complete obstruction. Wire electrodes were implanted bilaterally into the genioglossus (GG), geniohyoid (GH), sternothyroid (ST), and sternohyoid (SH) muscles for electrical stimulation (ES), and into the alae nasi for electromyographic (EMG) recording. Three levels of electrical stimulation were delivered to each muscle before and during partial or complete UAW obstruction. Genioglossus and geniohyoid stimulation both resulted in a significant reduction in Ruaw, which was most pronounced during partial obstruction, reducing Ruaw from 54 +/- 11 to 14 +/- 3 and from 74 +/- 12 to 31 +/- 5 cmH2O.L-1.s, respectively. At low voltage, stimulation of the genioglossus was more effective than stimulation of the geniohyoid in reducing Ruaw. Furthermore, electrical stimulation of the genioglossus but not of the geniohyoid released total obstruction. In contrast, electrical stimulation of the sternohyoid and sternothyroid produced no significant change in Ruaw. These findings demonstrate that selective UAW dilatory muscle contraction in spontaneously breathing anaesthetized dogs reduces Ruaw in the presence of UAW obstruction and releases UAW occlusion, with the genioglossus being the most effective muscle. This favours further attempts to investigate the benefits of electrical stimulation of selected upper airway muscles in the treatment of obstructive sleep apnoea syndrome.

Intra-airway gas transport during high-frequency chest vibration with tracheal insufflation in dogs.

High-frequency external chest vibration with tracheal insufflation (high-frequency vibration ventilation) has previously been shown to be an effective mode of artificial ventilation in experimental animals. To investigate the intra-airway gas mixing during high-frequency vibration ventilation (frequency 30 Hz, amplitude 0.4 cm), we used an analysis of the single-breath washout curve that gives the vibration-induced mixing coefficient distribution relative to the no-vibration situation. Data from four anesthetized dogs were collected during constant-flow insufflation at six rates (0.05-0.4 l.min-1.kg-1), at three insufflation durations (2, 4, and 7 s), and with the insufflation catheter outlet at three positions (carina, trachea, and a bronchus) while the vibration was on and off. Vibration enhanced intra-airway gas mixing 14.1 +/- 3.9-fold, with the peak of the enhancement distribution located 125 +/- 29 ml from the airway opening and a distribution width of 121 +/- 29 ml. As insufflation flow increased, the position of the peak enhancement shifted toward the alveolar zone and diminished in peak amplitude. Changing the insufflation duration and the catheter position did not affect the intra-airway mixing induced by vibration. External chest vibration causes a substantial increase of intra-airway gas mixing, bringing alveolar gas to central airways. This leads to overall increased pulmonary gas transport when fresh gas is insufflating the tracheal carina area.

Dependency of upper airway patency on head position: the effect of muscle contraction.

In the present study we examined the effect of flexion and extension of the head on upper airway (UAW) patency in anesthetized dogs, and compared the dilatory and stabilizing effects of electrically stimulated UAW muscles at the different head positions. Flexion of the head increased UAW resistance (Ruaw) and reduced maximal flow (Vmax), but had little effect on the negative pressure at which UAW collapse occurred (Pcrit). Extension of the head, on the other hand, resulted in more negative Pcrit values and increased Vmax without significantly affecting Ruaw. Electrically induced UAW muscle contraction affected the pressure-flow curve and Ruaw, as well as Pcrit. Changing head position had a substantial effect on the dilatory and stabilizing effect of the various UAW muscles. However, independent of head position, genioglossus stimulation was most effective in reducing Ruaw and increasing Pcrit. We conclude that in the anesthetized, supine dog, head position affects the mechanical properties of the UAW and the effects of UAW muscle contraction.

Gas density does not affect pulmonary acoustic transmission in normal men.

Fremitus, the transmission of sound and vibration from the mouth to the chest wall, has long been used clinically to examine the pulmonary system. Recently, modern technology has become available to measure the acoustic transfer function (TF) and transit times (TT) of the pulmonary system. Because sound speed is inversely proportional to the square root of gas density in free gas, but not in porous media, we measured the effect of air and Heliox (80% He-20% O2) breathing on pulmonary sound transmission in six healthy subjects to investigate the mechanism of sound transmission. Wide-band noise (75-2,000 Hz) was "injected" into the mouth and picked up over the trachea and chest wall. The averaged power spectra, TF, phase, and coherence were calculated using a fast Fourier transform-based algorithm. The phase data were used to calculate TT as a function of frequency. TF was found to consist of a low-pass filter property with essentially flat transmitted energy to 300 Hz and exponential decline to 600 Hz at the anterior right upper lobe (CR) and flat transmission to 100 Hz with exponential decline to 150 Hz at the right posterior base (BR). TF was not affected by breathing Heliox. The average TT values, calculated from the slopes of the averaged phase, were 1.5 +/- 0.5 ms for trachea to CR and 5.2 +/- 0.5 ms for trachea to BR transmission during air breathing. During Heliox breathing, the values of TT were 1.5 +/- 0.5 ms and 4.9 +/- 0.5 ms from the trachea to CR and from the trachea to BR locations, respectively. These results suggest that sound transmission in the respiratory system is dominated by wave propagation through the parenchymal porous structure.

Intra-airway CO2 distribution during airway insufflation in ventilatory failure.

Low-flow intratracheal gas insufflation is known to be an effective means of providing partial ventilatory support in respiratory failure. We studied the effects of catheter position on intraluminal CO2 concentration profiles and gas transport resistance during intra-airway insufflation at 0.15 l.kg-1.min-1 in six anesthetized paralyzed mechanically hypoventilated dogs. The two positions of the distal tip of the insufflation catheter were 0.5 cm proximal to and 4.0 cm distal to the carina. Local airway CO2 concentrations were measured via a sampling catheter passed through the tracheobronchial tree. Resistance to gas transport was calculated from the measured data. Arterial PO2 and arterial PCO2 remained constant with carinal and bronchial insufflation. Distal positioning of the insufflation tip resulted in a redistribution of ventilation between the lungs, with the ipsilateral lung being relatively hyperventilated and the contralateral lung being relatively hypoventilated. Intraluminal CO2 concentrations were markedly reduced in the ipsilateral lung compared with the profile in the contralateral lung during fresh gas delivery into the main-stem bronchus. The region of peak transport resistance was found to be in the second-generation airways during carinal insufflation and in the fourth-generation airways with intrabronchial insufflation. We conclude that gas exchange during low-flow insufflation occurs by the same mechanisms responsible for CO2 elimination in constant-flow ventilation. Overall gas exchange is not affected by position of the jet catheter.

The effect of hypothermia on gas exchange in anaesthetized rats in a confined atmosphere.

Both hypothermia and anaesthesia are known to prolong hypoxic survival. The combined effect of anaesthesia and hypothermia on survival in a confined atmosphere was studied in rats whose O2 consumption (VO2) produced hypoxia. Blood gases and respiratory parameters were measured in either ambient temperature TA = 30 degrees C (final body temperature TB = 36 degrees C) or initial TA = 30 degrees C followed by TA = 0 degree C below 100 torr inspired O2 (final TB = 19 degrees C). Oxygen consumption, breathing frequency and heart frequency decreased in hypoxic hypothermia compared to hypoxic normothermia. Hypothermia did not affect blood pressure, blood gas tension of pH. Hypothermia in hypoxia: 1) reduced the demand for oxygen with respect to O2 transport; 2) abolished the normothermic elevation of lactate in the blood, and 3) maintained high arterial saturation of oxygen. In contrast with awake rats (our previous study), there was no difference in the terminal inspired PO2 and survival time for normothermia and hypothermia in the anaesthetized rat, but survival time was doubled by anaesthesia as compared to awake rats due to reduced VO2.