Biomedical subjects
N R MacIntyre
Publications and source records attributed to N R MacIntyre.
Pulmonary rehabilitation of the patient with nonobstructive lung disease.
Disability from chronic lung disease is usually associated with COPD and pulmonary rehabilitation programs are designed to address this population. There are a number of chronic nonobstructive lung diseases, however, that can produce disability and that also may benefit from pulmonary rehabilitation. This article discusses how to classify nonobstructive lung disease patients and examines their evaluation, rehabilitation, and outcomes.
Ventilatory muscles and mechanical ventilatory support.
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Patient-ventilator flow dyssynchrony: flow-limited versus pressure-limited breaths.
OBJECTIVES: Patient-ventilator flow dyssynchrony occurs when ventilator flow delivery is insufficient to meet patient demands. If sufficiently severe, flow dyssynchrony can produce significant imposed loads on ventilatory muscles. Flow dyssynchrony can be improved by increasing ventilator flow delivery. We hypothesized that the variable flow pressure-limited breath would be a better approach for matching patient flow demands than adjusting a set flow on a conventional volume-cycled breath. DESIGN: Clinical interventional study. SETTING: Medical intensive care unit. PATIENTS: Sixteen stable, mechanically ventilated patients receiving volume-cycled assist-control ventilation. INTERVENTIONS: Flow dyssynchrony was produced by reducing the set flow by 50%. Dyssynchrony was quantified by measuring the esophageal pressure time product during the assisted breath. Two strategies were then employed in an attempt to reduce the dyssynchrony. One strategy was to increase flow back to the initial set flow and then further increase flow by an additional 25% (VI strategy). The other strategy was to use a pressure-limited breath feature coupled to a volume assist breath (the P strategy). With the P strategy, the pressure limit was set at 75% and 100% of the static elastic recoil pressure at end-inspiration. MEASUREMENTS AND MAIN RESULTS: Pressure time product, intrinsic positive end-expiratory pressure, and the ventilatory pattern were measured with each strategy and were analyzed by analysis of variance. Induced baseline flow dyssynchrony, as measured by the pressure time product, was > 5 cm H2O/sec in ten of 16 patients. This dyssynchrony was significantly reduced by both the VI strategy and the P strategy, although the P strategy appeared to be more effective in those patients with the greatest baseline dyssynchrony. Baseline inspiratory time was also shortened by both the VI strategy and the P strategy; the VI strategy shortened baseline inspiratory time more than the P strategy. Baseline tidal volume, frequency, and intrinsic positive end-expiratory pressure were only minimally affected by either strategy. CONCLUSION: The pressure-limited, variable-flow approach to ventilator gas delivery appears to be more responsive to a vigorous patient effort than a fixed-flow approach.
Applied PEEP during pressure support reduces the inspiratory threshold load of intrinsic PEEP.
OBJECTIVES: Mechanical ventilation in patients with obstructive airway disease (OAD) is associated with the development of dynamic hyperinflation and intrinsic positive end-expiratory pressure (PEEPi). One of the effects of this form of PEEPi is to act as an inspiratory threshold load that can produce ineffective breath triggering, dyspnea, and muscle fatigue. Recently it has been shown that applying PEEP in the ventilator circuit can reduce this imposed triggering load. We wished to investigate this further by studying patients with OAD being weaned with pressure support (PS) ventilation. Our first objective was to determine the prevalence and magnitude of this form of PEEPi in OAD patients who were clinically judged to be capable of triggering mechanical ventilatory breaths. Our second objective was to attempt to reduce the triggering load by applying circuit PEEP and then observe the response of patient-ventilator interactions during the patient-triggered, pressure-limited PS breath. DESIGN: Thirteen random patients with OAD who were receiving PS ventilation were studied by measuring airway pressures, airway gas flow, baseline esophageal pressure, esophageal pressure time products (PTP), and esophageal pressure changes before ventilator gas delivery began (delta Pes taken to represent PEEPi). Measurements were made at baseline and after stepwise increases in circuit PEEP up to the PEEPi. RESULTS: We found measurable PEEPi in all patients (average +/- SD of 9.54 +/- 4.3 cm H2O) and it was > 10 cm H2O in seven patients. As would be predicted, we observed progressive reductions in PEEPi as applied PEEP was given. We also observed that the component of patient effort (PTP) related to overcoming PEEPi also decreased, but the PTP related to tidal volume (VT) did not. The VT associated with the set PS thus did not change with application of PEEP, nor did the breathing frequency. CONCLUSION: PEEPi is common in OAD patients receiving mechanical ventilatory support. The imposed triggering load from PEEPi can be offset to large extent by circuit PEEP approaching the baseline PEEPi. Although total patient effort substantially falls with applied PEEP, the patient effort that combine with PS to effect VT does not.
Diffusing capacity of the lung for carbon monoxide.
The measurement of co uptake (VCO and DLCO) from alveolar gas is a unique way to noninvasively assess pulmonary vascular function, specifically the functional volume of the pulmonary capillary bed. Proper interpretation of results, however, needs to account for inherent assumptions regarding co distribution and timing procedures. Moreover, reasonable airway mechanics, lung volumes, and patient cooperation are required for accurate measurements. Potential clinical utility may be increased if measurements are made in different positions or under exercise conditions.
New modes of mechanical ventilation.
Many new approaches to mechanical ventilation have been developed. This article discusses these new strategies and modes. These include lung protection conventional ventilation strategies, long inspiratory time strategies, pressure-targeted breath enhancements, airway pressure-related release ventilation, and proportional-assist ventilation.
Pressure-controlled, inverse ratio ventilation that avoids air trapping in the adult respiratory distress syndrome.
OBJECTIVES: To investigate physiologic and outcome data in patients switched from volume-cycled conventional ratio ventilation to pressure-controlled inverse ratio ventilation that did not produce air trapping and intrinsic positive end-expiratory pressure (PEEP). SETTING: Medical intensive care unit. DESIGN: Retrospective analysis of crossover data and outcome. PATIENTS: Fourteen patients with the adult respiratory distress syndrome who were receiving mechanical ventilation with volume-cycled, conventional ratio ventilation followed by pressure-controlled, inverse ratio ventilation. INTERVENTIONS: Our approach to pressure-controlled, inverse ratio ventilation was to use tidal volumes and applied PEEP values comparable to those volumes and values used on volume-cycled, conventional ratio ventilation, use inspiratory times to increase mean airway pressure instead of additional applied PEEP, and avoid air trapping (intrinsic PEEP). MEASUREMENTS AND MAIN RESULTS: With this approach, there was a reduction in peak airway pressure from 53 +/- 8.5 (SD) to 40 +/- 5.9 cm H2O (p < .01), and an increase in mean airway pressure from 20 +/- 3.9 to 30 +/- 5.2 cm H2O (p < .01). Tidal volume, mean inflation pressure, and compliance did not change. Oxygenation (PaO2) improved from 57 +/- 11.3 torr (7.6 +/- 1.5 kPa) to 94 +/- 40.2 torr (12.5 +/- 5.4 kPa) (p = .01) but the oxygenation index (mean airway pressure x FIO2 x 100/PaO2) did not change significantly (25.9 +/- 10.3 to 27.2 +/- 12.2). There was no significant change in PaCO2 or pH even though delivered minute ventilation decreased from 17.4 +/- 4.3 to 14.8 +/- 5.8 L/min (p = .02). Cardiac index slightly decreased, but hemodynamic values were otherwise stable. Only three of the 14 study patients survived. CONCLUSIONS: These data demonstrate that oxygenation is primarily a function of mean airway pressure, and that longer inspiratory times can be used as an alternative to applied PEEP to increase this oxygenation. If no air trapping develops, lung inflation pressures and delivered volumes remain constant with this approach. Because the technique was used only in patients refractory to conventional techniques, the poor outcome is not surprising.
Breathing comfort during weaning with two ventilatory modes.
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Psychological factors in weaning from mechanical ventilatory support.
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Controversies in mechanical ventilation.
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Efficacy assessment criteria based on risk and cost.
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Combining pressure-limiting and volume-cycling features in a patient-interactive mechanical breath.
OBJECTIVES: To combine the patient synchrony effects of pressure-limited breath delivery strategies with the volume guarantee of volume-cycled breath delivery strategies, we designed a positive-pressure breath that incorporates both features. This breath is patient triggered and can be pressure limited. Breath termination (i.e., cycling) can either be flow- or volume-cycled, depending on whether a target volume has been attained. The pressure-limiting features are further enhanced by the capability to adjust demand-valve responsiveness at breath initiation. DESIGN: Prospective, mechanical simulation studies. SETTING: Engineering laboratory. INTERVENTIONS: A mechanical lung model that could simulate patient ventilatory efforts was used. Vigorous and weak patient efforts were studied in conjunction with use of volume assist, pressure support, and a combination pressure-limited and volume-cycled breath. MEASUREMENTS AND MAIN RESULTS: Two clinical situations were simulated by the model: a) a patient requiring total mechanical ventilatory support who had a very active ventilatory drive; and b) a patient receiving only partial ventilatory support who had an unstable ventilatory drive. A range of pressure-limiting and volume-cycling settings were given. Volume, pressure, and flow delivery were measured. Synchrony with patient effort was assessed by visual inspection of airway pressure graphics as well as by calculations of patient work during these support strategies. CONCLUSIONS: Flow dyssynchrony during fixed-flow, volume-cycled assisted breaths in patients with active ventilatory drives can be improved with this breath design while a guaranteed tidal volume is maintained. In addition, this combination breath can provide a volume "safety net" for patients in whom partial support with pressure-support ventilation is desired.
Normal values for single exhalation diffusing capacity and pulmonary capillary blood flow in sitting, supine positions, and during mild exercise.
Previous approaches to the measurements of pulmonary diffusing capacity (DL) and pulmonary capillary blood flow (QC) utilized either the rebreathing or the single inhalation technique in conjunction with radioisotope gas and mass spectrometry. In the present study, we utilized a newly developed rapid infrared analyzer in conjunction with the slow single exhalation technique on 100 healthy volunteers to establish normal values for DL and QC under sitting, supine, and exercise conditions. The exercise level was determined by a target heart rate: HRex = ([HRmax - HRrest]/3) + HRrest. Prediction equations based on regressions on age, sex, height, or weight were then computed for sitting, supine, and exercise values. We found that mean DL and QC increased by approximately 12 percent and 8 percent, respectively, from sitting to supine posture, and by approximately 30 percent and 100 percent, respectively, from sitting (rest) to mild exercise. These results provided a database for further studies in the single exhalation method in various clinical settings.
Pressure-limited versus volume-cycled breath delivery strategies.
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Building consensus on the use of mechanical ventilation.
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Clinically available new strategies for mechanical ventilatory support.
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Analysis of an endotracheal intubation service provided by respiratory care practitioners.
UNLABELLED: Our Respiratory Care Services Department provides an endotracheal intubation service that responds to all intubation requests. Intubation is performed by registered respiratory therapists who complete an 8-hour training program, advanced cardiac life support (ACLS) training and certification, and clinical performance of intubation with supervision. The goals of this service are (1) to provide competent persons for performing this service, (2) to assure a rapid response time, and (3) to be cost-effective. EVALUATION METHODS: A retrospective analysis of our service was conducted over a 1-year period (7/90 to 6/91), and calculations were made of the intubation success rate and complication rate. RESULTS: Of the 833 total intubations, 791 were successfully performed by respiratory care practitioners; 730 of those successful intubations (92.3%) were accomplished in fewer than 3 attempts. Recognized complications occurred in 96 intubations (12.1%) and included oral bleeding, vomiting, and short periods of oxygen desaturation. In the 5.1% (42) of the patients not intubated by our service, 22 required heavy sedation, and an anesthesiologist was consulted; 17 patients were intubated by other physicians; and 3 tracheotomies were performed. Multiple intubation attempts were a result of secretions, induced bradycardia, blade-light malfunction, damaged cuff, and esophageal intubations. CONCLUSION: Respiratory Care Services can provide an effective intubation service. Cost savings were realized by centralizing equipment.