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Biomedical subjects

R D Hubmayr

Publications and source records attributed to R D Hubmayr.

At least 55 records · Page 3Linked to original sources
PubMed1991

Oxygen-induced hypercarbia in obstructive pulmonary disease.

We investigated the mechanisms responsible for oxygen-induced hypercarbia in ventilator-dependent patients with advanced chronic obstructive pulmonary disease (COPD). To quantitate the effects of oxygen (O2) on respiratory drive, we determined the CO2 recruitment threshold (PCO2 RT) in 10 mechanically ventilated patients under normoxic (PaO2 = 67 +/- 7 mm Hg) and hyperoxic (PaO2 = 370 +/- 67 mm Hg) conditions. PCO2 RT is a measure of the CO2 responsiveness of the mechanically unloaded respiratory system and, as such, is independent of mechanical impedance and respiratory muscle strength. After O2 supplementation, PCO2 RT increased from 42 +/- 6 to 45 +/- 6 mm Hg (p less than or equal to 0.05), indicating a suppression of so-called hypoxic respiratory drive. The effect of hyperoxia on the dead space to tidal volume ratio (VD/VT) and CO2 elimination (VCO2) was studied in 6 patients. Measurements were made at identical ventilator settings, thus eliminating breathing pattern- and respiratory work-related effects on these variables. VD/VT rose from 0.49 +/- 0.09 to 0.55 +/- 0.06 (p less than or equal to 0.05), but VCO2 remained constant at 0.21 L/min. We discuss why measuring O2-induced changes in minute ventilation, VCO2, PaO2, and VD/VT in spontaneously breathing patients is insufficient to distinguish between gas exchange- and respiratory drive-related mechanisms for hypercarbia. Based on the O2-induced increase in PCO2 RT, we conclude that so-called suppression of hypoxic drive plays an important role in the pathogenesis of this disorder.

Aged↗
PubMed1991

Influence of sleep state on CO2 responsiveness. A study of the unloaded respiratory pump in humans.

We measured the ventilatory recruitment threshold for CO2 (RT) during wakefulness, nonrapid eye movement sleep (NREM), and rapid eye movement sleep (REM) in eight patients with respiratory failure. Because the lungs were mechanically ventilated during the RT measurements, we were able to define the effects of arousal state on the chemoresponsiveness of the unloaded respiratory system. Ventilator settings were held constant in each patient, assuring that mechanoreceptor input to the respiratory controller remained the same during all measurements. RT increased from 38 +/- 6 mm Hg during relaxed wakefulness to 42 +/- 8 mm Hg during NREM sleep (p less than or equal to 0.01), consistent with a blunting of chemoresponsiveness during sleep. In five subjects we were able to measure RT during REM sleep also. In four of them, REM RT exceeded the wakefulness measurement by at least 3 mm Hg. The remaining patient showed no demonstrable changes in RT during either sleep state. We conclude that the loss of a wakefulness stimulus contributes to sleep-induced hypercarbia in humans.

Adult↗
PubMed1991

The control of breathing during weaning from mechanical ventilation.

Using the recruitment threshold technique, we measured the CO2 responsiveness of the unloaded respiratory pump in 14 mechanically ventilated patients prior to weaning. The CO2 recruitment threshold (CO2RT) was compared with the arterial CO2 tension during unassisted breathing (CO2SB) and with the PaCO2 during mechanical ventilation (CO2MV) at machine settings determined by the primary physician. Based on these comparisons, we tested the hypotheses that (1) patients without weaning-induced respiratory distress (group 1) maintain CO2SB near CO2RT, (2) patients with weaning-induced respiratory distress (group 2) retain CO2SB above CO2RT, thereby manifesting incomplete load compensation, and (3) CO2MV is ventilator setting dependent and provides insufficient information about the ventilatory requirement during weaning. Respiratory distress was prospectively defined as sustained tachypnea (rate greater than or equal to 30) or intense dyspnea (Borg scale rating) and limited weaning in nine of 14 patients. The average CO2RT was 40 mm Hg in both groups. All patients in group 1 maintained CO2SB near CO2RT (p greater than 0.1). Seven of nine patients in group 2 retained CO2 by greater than or equal to 3 mm Hg above CO2RT (p less than 0.01). There was no significant difference between CO2MV and CO2SB in either group. We conclude that CO2RT provides a better reference of the adequacy of ventilatory load compensation during weather than CO2MV.

Aged↗
PubMed1991

Metered dose inhalers for bronchodilator delivery in intubated, mechanically ventilated patients.

We determined the relative efficacy of two bronchodilator aerosol delivery methods in 18 intubated mechanically ventilated patients with airways obstruction. Two treatment arms, consisting of albuterol 270 micrograms (three puffs) from a metered dose inhaler and albuterol 2.5 mg from a saline solution nebulized with an updraft inhaler, were compared in a single blind, randomized crossover design. Pulmonary function was evaluated using an interrupter technique. Changes in passive expiratory flow at respiratory system recoil pressures between 6 and 10 cm H2O provided the therapeutic endpoints. Paired measurements were made before and 30 minutes after drug delivery. The MDI and NEB resulted in similar improvements in iso-recoil flow (mean increase for both groups = 0.1 L/s). Treatment sequence, severity of obstruction, and bronchodilator responsiveness had no effect on relative efficacy. Albuterol caused a small but significant increase in heart rate that was similar following both delivery methods. We conclude that bronchodilator aerosol delivery with metered dose inhalers provides a viable alternative to nebulizer therapy in intubated mechanically ventilated patients and may result in a cost savings to hospitals and patients.

Aerosols↗
PubMed1990

Physiologic approach to mechanical ventilation.

An understanding of ventilator management must be predicated on physiologic principles. There is a considerable array of equipment, permitting an almost limitless number of permutations in ventilator therapy, to support patients with respiratory failure. The physician who understands patient-ventilator interactions and their effects on cardiopulmonary function will be best equipped to individualize therapy. We believe that further refinements in supportive equipment probably will not improve outcome significantly. Thus, more emphasis should be placed on elucidating the underlying disease process and its effects on respiratory structure and function.

Airway Obstruction↗
PubMed1990

Determinants of transdiaphragmatic pressure in dogs.

We measured the transdiaphragmatic pressure (Pdi) during bilateral phrenic nerve stimulation and evaluated the determinants of its change with lung volume, chest wall geometry, and respiratory system impedance in supine dogs. Four rows of radiopaque markers were sewn onto muscle bundles of the costal and crural diaphragm between their origin on the central tendon and their insertion on the rib cage and spine. The length of the diaphragm (L) was determined from the projection images of marker rows using biplane fluoroscopy. Measurements were made at lung volumes between total lung capacity and functional residual capacity before and after the infusion of Ringer lactate solution into the abdominal cavity. In contrast to relaxation, during tetanic stimulation the active lengths of the muscle bundles were similar at all volumes, but the diaphragm assumed different shapes. Although the small differences in active muscle length with volume and liquid loads are consistent with only small changes in muscle force output, Pdi varied by a factor of greater than or equal to 5. There was no single L/Pdi curve that fitted all data during 50-Hz stimulations. We conclude that under these experimental conditions Pdi is not a unique measure of the force produced by the diaphragm and that lung volume, chest wall geometry, and respiratory system impedance are important determinants of the mechanical efficiency of the diaphragm as a pressure generator.

Action Potentials↗
PubMed1990

Effect of body position on regional diaphragm function in dogs.

The in situ lengths of muscle bundles of the crural and three regions of the costal diaphragm between origin and insertion were determined with a video roentgenographic technique in dogs. At total lung capacity (TLC) in both the prone and supine positions, the length of the diaphragm is not significantly different from the unstressed excised length, suggesting that the diaphragm is not under tension at TLC and that there is a hydrostatic gradient of pleural pressure on the diaphragmatic surface. Except for the ventral region of the costal diaphragm, which does not change length at lung volumes greater than 70% TLC, all other regions are stretched during passive deflations from TLC. Therefore below TLC the diaphragm is under passive tension and supports a transdiaphragmatic pressure (Pdi). The length of the diaphragm relative to its unstressed length is not uniform at functional residual capacity (FRC) and does not follow a strict vertical gradient that reverses when the animal is changed from the supine to the prone position. By inference, the length of muscle bundles is determined by factors other than the vertical gradient of Pdi. During mechanical ventilation, regional shortening is identical to the passive deflation length-volume relationship near FRC. Prone and supine FRC is the same, but the diaphragm is slightly shorter in the prone position. In both positions, during spontaneous ventilation there are no consistent differences in regional fractional shortening, despite regional differences in initial length relative to unstressed length.

Animals↗
PubMed1990

The ventilatory recruitment threshold for carbon dioxide.

We report our initial experience with a technique with which the chemoresponsiveness of the respiratory controller can be characterized in terms of an inspiratory on-switch threshold to CO2. After suppression of phasic respiratory muscle activity by mechanical ventilation, a CO2 recruitment threshold (PCO2RT) was defined as the lowest alveolar CO2 tension at which CO2 supplementation to inspired gas caused a reappearance of inspiratory efforts. Because PCO2RT can be determined in the absence of a mechanical load on the ventilatory pump, respiratory system mechanics and inspiratory muscle function should not influence the measurement itself. Thus, this technique may be helpful to study ventilatory requirements and load responses in critically ill patients with respiratory failure. We have shown that inspiratory muscle recruitment can be equally well-inferred from changes in the airway pressure and flow tracings during mechanical ventilation, from the pattern of chest wall displacement, and from the integrated diaphragm electromyogram. Within a subject, PCO2RT is a reproducible measurement that is not influenced by ventilator settings and end-expiratory lung volume, provided that phasic respiratory muscle has been suppressed prior to CO2 supplementation. Details of the methodology, the likely determinants of PCO2RT, and the clinical utility of this technique are discussed.

Adult↗
PubMed1990

The assessment of major airway function in a ventilator-dependent patient with tracheomalacia.

A 60-pack-year smoker presented with cough, dyspnea and orthopnea of three months' duration. Spirometry revealed severe reduction in maximal expiratory flow; CT of the chest and bronchoscopy demonstrated expiratory collapse of a mid-tracheal segment, and a presumptive diagnosis of tracheomalacia was made. A right lateral thoracotomy was performed to resect the unstable segment and improve maximal expiratory flow. Diffuse major airway disease with absence of cartilaginous rings from the thoracic inlet to the mainstem bronchi was encountered. The trachea and mainstem bronchi were stented externally. A high resistance to airflow and absence of expiratory flow limitation were present, suggesting a fixed rather than variable intrathoracic obstruction of major airways. This case illustrates some potential pitfalls in preoperative assessment of patients with tracheomalacia. Recordings of airway pressure and flow during mechanical ventilation are useful in distinguishing between fixed and variable intrathoracic obstruction and may complement tests of airway anatomy.

Airway Resistance↗
PubMed1989

In vivo regional diaphragm function in dogs.

A biplane videofluorographic system was used to track the position of metallic markers affixed to the abdominal surface of the left hemidiaphragm in supine anesthetized dogs. Regional shortening was determined from intermarker distances of rows of markers placed along muscle bundles in the ventral, middle, and dorsal regions of the costal diaphragm and of one row on the crural diaphragm. Considerable variability of regional shortening was seen in a given row, which was reproducible on repeat study in individual dogs but which differed between mechanical ventilation and spontaneous breathing. There were no consistent patterns among dogs. Regional shortening obtained from the change in length of rows extending from chest wall to central tendon showed no consistent differences among dogs during spontaneous breathing. At equal tidal volumes, all regions (except the ventral costal diaphragm) shortened more during spontaneous breathing than during mechanical ventilation.

Animals↗
PubMed1989

The effects of positive expiratory pressure on isovolume flow and dynamic hyperinflation in patients receiving mechanical ventilation.

The use of continuous positive airway pressure (CPAP) or positive end-expiratory pressure (PEEP) has been advocated by some to assist in the weaning process of patients receiving mechanical ventilation for respiratory failure. The efficacy of this technique and its effect on respiratory system mechanics are not well understood. The theoretical advantage of CPAP or PEEP during the weaning process can be obliterated if excessive dynamic hyperinflation is induced. A key determinant of the individual response to this proposed weaning technique is the recognition of the presence or absence of expiratory flow limitation. We studied the effect of progressively increased levels of applied PEEP on isovolume expiratory flow and end-expiratory lung volume in seven patients during controlled mechanical ventilation. In the absence of expiratory flow limitation, passive expiratory flow decreased and end-expiratory lung volume increased when any level of PEEP was applied. In contrast, flow-limited patients did not demonstrate a change in isovolume expiratory flow or end-expiratory lung volume until the applied PEEP reduced the driving pressure for expiratory flow below a critical value. All patients demonstrated dynamic hyperinflation during controlled ventilation as evident by the existence of intrinsic PEEP. The nominal value of applied PEEP that caused a reduction in isovolume expiratory flow was unrelated to the initial level of intrinsic PEEP. The clinical implications of these findings with respect to CPAP therapy during weaning from mechanical ventilation are discussed.

Humans↗
PubMed1989

Transdiaphragmatic twitch pressure. Effects of lung volume and chest wall shape.

The effect of lung volume and thoracoabdominal shape on the transdiaphragmatic twitch pressure (Pdit) amplitude was evaluated in six volunteers during airway occlusion. Twitch stimulation was applied through fine wire electrodes implanted near both phrenic nerves. Stimulations were tolerated with little discomfort and constant phrenic nerve responses were maintained for hours. At FRC the group mean Pdit was 31.4 cm H2O (range, 19 to 36 cm H2O), and its coefficient of variation ranged between 2 and 5% in individual subjects. At 1 L above FRC, the Pdit decreased a mean of 7.8 cm H2O (range, 2.8 to 11.9 cm H2O). This change was caused primarily by a decrease in esophageal pressure amplitude. The shape of the relaxed chest wall was altered by loading the rib cage with a force of 5 to 9 kg. Load and shape had little effect on Pdit independently of lung volume. Our modified technique of phrenic nerve stimulation through small wire electrodes is ideally suited for longitudinal intervention studies in patients. We conclude that the variability of Pdit with shape is small compared with its expected decrease with lung volume.

Action Potentials↗
PubMed1988

Mediastinal mass and tracheal obstruction during general anesthesia.

A 24-year-old man had a large anterior mediastinal mass and a nonproductive cough of 6 weeks' duration. With the patient under general anesthesia, a diagnostic mediastinoscopy was performed with endotracheal intubation. During the procedure, acute respiratory failure developed as a result of tracheal obstruction. Fiberoptic bronchoscopic examination of the patient in the supine position revealed almost total extrinsic compression of the trachea and no evidence of intraluminal disease. Reexamination of the trachea with the patient in sitting and semiprone positions showed resolution of the extrinsic compression and respiratory distress. Flow-volume curves obtained before treatment of the mediastinal mass (histologically diagnosed as Hodgkin's lymphoma) disclosed major airway compression with the patient in the supine position; the abnormality disappeared after chemotherapy. The mechanisms responsible for tracheal compression by mediastinal masses during general anesthesia may include the following: (1) the effect of anesthesia on pulmonary mechanics, (2) the supine body position, (3) the elimination of glottic regulation of airflow by endotracheal intubation, (4) changes related to the surgical manipulation of the tumor itself, (5) the size and location of the mediastinal mass, (6) the young age of the patient, and (7) preexisting airways disease. Anticipation and prevention of potential respiratory complications and preparedness to treat them appropriately are important aspects of the management of these patients.

Adult↗
PubMed1988

Oxygen uptake during weaning from mechanical ventilation.

Total body oxygen uptake (VO2) increases during the transition from machine-assisted ventilation to spontaneous breathing. Since the volume of oxygen consumed by the respiratory muscles must contribute to the increase in VO2 (delta VO2), we explored whether delta VO2 and/or measurements of respiratory power output (Wresp) provide clinically useful information in the evaluation of disease state and weaning decisions in patients with respiratory failure. We determined the metabolic, ventilatory, and hemodynamic responses of ten patients during weaning from controlled mechanical ventilation, and compared delta VO2 and Wresp of patients without overt heart-lung disease (group 1) to that of patients with significant cardiopulmonary dysfunction and ventilator-dependent respiratory failure (group 2). We reasoned that for delta VO2 to be clinically useful, individual values must either clearly differ between groups, must be higher in patients with heart-lung disease, and/or correlate with weaning outcome and independent measurements of respiratory work. The VO2 increased in nine of ten patients. The differences between the groups in the values of delta VO2 (27 ml/min and 49 ml/min) and respiratory power (9.38 J/min and 11.99 J/min) were not significant. delta VO2 and Wresp were not correlated (r = 0.2), and neither predicted weaning outcome. We conclude that the sensitivity and specificity of delta VO2 and Wresp appear insufficient for evaluation of disease state and weaning decisions in individual patients.

Aged↗
PubMed1987

Respiratory system mechanics in ventilated patients: techniques and indications.

For the past 50 years, basic research has provided valuable insights into the concepts of respiratory system mechanics, but clinical application in the critical-care arena remains in its infancy. On the basis of the limited information that is available on critically ill patients, we believe that physicians who are responsible for the care of mechanically ventilated patients must understand the mechanical interactions between humans and machines. With measurements of flow, volume, and pressure, a more precise quantitative evaluation of the respiratory system can be obtained than with clinical assessment alone. In this article, we discuss the principles, techniques, and clinical applications of measurements of respiratory system mechanics in ventilated patients and suggest directions for further research that may prove to be clinically relevant.

Humans↗