Janie M. Fouke

Proximal, Tracheal, and Alveolar Pressures during High-Frequency Oscillatory Ventilation in a Normal Rabbit Model

ABSTRACT: To study the effect of different high-frequency oscillatory ventilation parameters on airway pressure we measured oscillatory pressure amplitude (| Paw |) and mean airway pressure (Paw)at three sites in open-chested normal rabbits: proximal, trachea, and alveolus. Five animals were studied to test a new pleural capsule design, which was then used in two groups of animals to measure right upper (n = 4)or middle (n = 5) lobe alveolar pressures. Animals were randomly sequenced through combinations of frequency (10, 15, and 20 Hz) and fractional inspiratory time (T) (0.3 and 0.5) while normoxic and encapnic. During capsule testing, we noted that alveolar pressures increased (p < 0.05) with increasing capsule mass. suggesting that compressive forces from the capsule may alter the capsule measurement. We thus used a lowmass (430 mg) transducer system in the rabbit high-frequency oscillatory ventilation experiments. Using multifactorial analysis of variance, we found significant main effects of Ti on Paw, and of measurement site on both |Paw| and Paw (p < 0.009). Frequency did not influence variations in either |Paw| Paw. For both Ti settings, alveolar upper lobe Paw was lower compared with that of the middle lobe (p < 0.0005). Lengthening Ti (03 to 0.5) increased tracheal Paw in each capsule group (p < 0.0005). At Ti = 0.5, tracheal Paw exceeded Paw measured proximally (p < 0.05). Our data support in vivo alveolar Paw inhomogeneity and demonstrate significant changes in pressures within the lung related to Ti during high-frequency oscillatory ventilation.

Oral breathing in newborn infants.

Newborn infants are considered obligate nasal breathers, hence dependent on a patent nasal airway for ventilation. The conditions under which oral breathing could occur and the contribution of oral ventilation to total ventilation were studied in 30 healthy term infants (aged 1 to 3 days). Nasal and oral airflow were measured using two resistance-matched pneumotachometers, and heart rate, tcPO2, etCO2, and sleep state were continuously recorded. In three of 10 infants studied in undisturbed sleep, spontaneous oronasal breathing was noted during both active and quiet sleep (mean duration 19 +/- 25 minutes), the distribution of tidal volume being 70% +/- 12% nasal and 30% +/- 12% oral. Episodes of oronasal breathing were also observed after crying in six infants (mean duration 21 +/- 19 seconds). In an additional 20 infants, multiple 15-second end-expiratory nasal occlusions were performed; eight (40%) of these infants initiated and sustained oral breathing in response to nasal occlusion. Respiratory rate, tidal volume, heart rate, and tcPO2 did not change when oral breathing occurred in response to nasal occlusion, although minute ventilation decreased from 265 to 199 ml/min/kg (P less than 0.05). These results demonstrate that newborn infants may use the oral airway for ventilation, both spontaneously and in response to complete nasal occlusion.

Measurement of Tidal Volume during High- Frequency Jet Ventilation1

ABSTRACT. The measurement of tidal volume during high-frequency jet ventilation is difficult due to the high-frequency components of the inspiratory flow. To validate tidal volume measured with a screen pneumotachograph placed on the expiratory limb, we simultaneously determined tidal volume with a body plethysmograph in seven anesthetized normal adult New Zealand rabbits before and after saline lung lavage. Four to six comparisons of tidal volume were obtained by varying peak inspiratory pressures at each combination of frequency (120, 240, and 480/min) and inspiratory to expiratory time ratio (1:1,1:3,1:5, 1:9). Overall, 90% of the tidal volumes measured with the pneumotachograph were within 10% of 1 ml of the volumes determined with the plethysmograph, independent of frequency, inspiratory to expiratory time ratio, and lung compliance. There was unidirectional outward flow at the pneumotachograph during inspiration when both normal and saline lavaged lungs were being ventilated, suggesting a lack of gas entrainment. We conclude that a pneumotachograph on the expiratory limb may be used to measure tidal volume and gas entrainment in vivo during high-frequency jet ventilation. Determination of tidal volume may serve to optimize ventilator settings during high-frequency jet ventilation and facilitate an understanding of the mechanisms involved in gas exchange.

Effects of sodium cyanide and nicotine on upper airway resistance in anesthetized dogs

The purpose of this study was to determine whether pharmacologic interventions which increase respiratory drive could also reduce flow resistance in the upper airway. Studies were performed in twelve anesthetized supine dogs. In six animals breathing spontaneously through the intact upper airway, intravenous administration of respiratory stimulants (sodium cyanide and nicotine) produced a dose-related decrease in upper airway. In nine animals, upper airway resistance was measured across the isolated upper airway. The stimulants produced a dose-related decrease in upper airway resistance. In both preparations inspiratory resistance fell at lower doses than expiratory resistance. Eventually a dose could be given which resulted in comparable, minimal values of resistance during both inspiration and expiration. Mechanisms for changes in resistance were clarified using lateral radiographs of the neck and transbronchoscopic views of the upper airway. Pharmacologic challenge resulted in a change in the route of airflow (from nose only to nose-and-mouth breathing) as well as a change in caliber of the airway at the level of the naso-pharynx and hyoid apparatus. In anesthetized dogs, respiratory stimulants will decrease upper airway resistance by increasing activation of upper airway muscles which may enlarge the airway, change the route of flow, and thus overcoming collapsing forces produced by increased chest wall muscle activation.

Phasic changes in upper airway impedance

To identify within breath variations in the mechanical properties of the isolated upper airway, we examined changes in impedance spectra over the course of the respiratory cycle. Changes were evaluated with a modified forced oscillation technique applied to the isolated, sealed upper airways of nine anesthetized mongrel dogs. Upper airway impedance spectra were studied during sequential 350 msec epochs. We found spectral changes which were reproducible within the respiratory cycle. Impedance spectra revealed that during mid-inspiration at the point of peak upper airway muscle activity, the low frequency real part decreased and the imaginary part was less negative and less steep. During late inspiration and early expiration the impedance values returned to their end-expiratory values. The only significant change in parameter estimates from a three-parameter model indicated an increase in compliance. Since these changes correlated not only with tidal flow through the lower trachea and lung but also with upper airway muscle activation, we reasoned that changes in impedance could have resulted from an increase in upper airway size. Therefore, we used a sealed speaker system and, while the animal was apnoeic, evaluated impedance at two different airway pressures and the resultant volumes. The changes in impedance spectra with a volume increase were similar to those seen during spontaneous breathing efforts. We conclude that the mechanical properties of the upper airway change during the respiratory cycle and that these changes correlate with the respiratory activation of upper airway muscles. We suspect that these changes in input impedance could reflect a change in the size of the airway rather than a true increase in elasticity.

Noninvasive detection of water and blood content in soft tissue from the optical reflectance spectrum

The diffusive reflectance intensity of skin in the near-infrared range is shown to be greatly influenced by its water content, similar to the way in which hemoglobin content affects the skin color in the visible range. The different behavior of water and hemoglobin at selected wavelengths might allow us to distinguish changes in fluid balance. The simplicity and high sensitivity of an optical method show a promising way of identifying a slight change in water content in soft tissue. The understanding of how the skin reflectance spectrum responds to the change in blood and water content will be the basis of a simplified multi-wavelength monitoring instrument.

Tissue composition discrimination by NIR spectroscopy

In the NIR spectral region below 1300 nm, the light absorption property of biological soft tissue is strongly influenced by its constituents, especially water and fat. One may be able to estimate the NIR absorption property based on the composition of tissue.

ALVEOLAR PRESSURES DURING HIGH-FREQUENCY JET VENTILATION(HFJV) IN NORMAL AND SURFACTANT-DEFICIENT LUNGS

HFJV has been reported to reduce airway pressures during assisted ventilation in preterm infants. Nonetheless, augmented pressure transmission to alveoli has been observed at frequencies near resonance. We determined the effect of surfactant deficiency on airway to alveolar pressure transmission in 6 postmortem adult rabbit lungs (3 normal and 3 surfactant-deficient by multiple saline lavage) during both conventional ventilation (CV, f=30/min, rectangular wave) and HFJV (f=200/min, triangular wave). Alveolar pressure was measured via a piezoelectric transducer on the visceral pleura and communicating with the alveoli. Airway pressure was measured with a similar transducer and/or catheter, both inside the distal endotracheal tube. With each ventilator, measurements were obtained maintaining either peak inspiratory pressure(PIP 20-30cmH2O), positive end expiratory pressure(PEEP 3-13cmH2O) or ΔP(PIP-PEEP 7-27cmH2O) constant. During CV the ratios of alveolar to airway PIP, PEEP and ΔP were always 1.0, while during HFJV they ranged from 0.9 to 1.1. Alveolar to airway pressure ratios were not affected by saline lavage and were independent of magnitude or combination of pressures used. We conclude that in this rabbit model of normal and surfactant-deficient lungs, airway pressures are transmitted to the alveoli with little distortion during HFJV at the frequencies employed. Thus, the previously reported reduction in airway pressures during HFJV may in fact portend decreased barotrauma at the alveolar level. Supported by ALA-Ohio

Reflectance spectra of a multi-layered skin model

The visible and near-infrared diffuse reflmtance spectrum of a layered skin model was studied using a bifurcated optid fiber bundle. The contribution by subcutaneous fatty tissue to the reflectance intensity is strongly wavelength dependent and ranged from 0% to more than 15%.

Heat and water transport across the airway wall

The thermal profiles in the airways of healthy human volunteers and patients with asthma differ following the cessation of hyperpnea. The asthmatics appear to have a greater ability to rewarm their airways. A model is developed that describes the radial transport of heat and water across the trachea which included a time-varying blood supply. Simulation results indicate that changes in tissue perfusion coupled with changes in mucosal thickness could account for the difference between the two groups.<<ETX>>