Paul B. Blanch

Decreasing imposed work of the breathing apparatus to zero using pressure-support ventilation

ObjectivesTo apply pressure-support ventilation with the goal of decreasing the imposed work of the breathing apparatus (endotracheal tube, breathing circuit tubing, and the ventilators demand-flow system) to zero and to evaluate a clinical method of measuring the imposed work of breathing. DesignA prospective evaluation of adult and pediatric patients receiving mechanical ventilatory support. SettingA surgical and a pediatric intensive care unit in a university hospital. PatientsFifteen patients (11 adult and four pediatric), who were diagnosed with acute respiratory failure from various etiologies, and who were intubated and spontaneously breathing, received continous positive airway pressure and pressure-support ventilation. Measurements and Main ResultsImposed work of the breathing apparatus was calculated by integrating pressure measured at the tracheal end of the endotracheal tube from a narrow air-filled catheter and the change in volume from a miniature pneumotachograph (flow sensor) positioned between the “Y” piece of the breathing circuit and the endotracheal tube. Pressure and volume singnals were directed to a computerized, portable respiratory monitor (Bicore Monitoring Systems) that provides real-time display of the pressure-volume (work) loops and calculation of the imposed work. Imposed work was measured at 0 cm H2O pressure-support ventilation, and then incremental levels of pressure-support ventilation were applied until the imposed work decreased to zero. Imposed work decreased in a quadratic fashion after incremental levels of pressure-support ventilation (r = -.83 [r2 = .69]; p<.001). At pressure-support ventilation level of 0 cm H2O, the imposed work was 0.60 ± 0.17 joule/L. At mean pressure-support ventilation levels of 13.5 ± 4.8 cm H2O, imposed work decreased to O joule/L. ConclusionsIdeally, the imposed work of the breathing apparatus should be zero to decrease the afterload on the ventilatory muscles and, thus, the patients work of breathing. Eliminating the imposed work is achieved using appropriate levels of pressure-support ventilation. We describe an easily applied, practical method of measuring imposed work using a commercially available, portable, bedside respiratory monitor. We recommened that all patients diagnosed with respiratory failure and compromised pulmonary mechanics and who are intubated and breathing spontaneously, recieve at least a minimal level of pressure-support ventilation that results in zero breathing apparatus-imposed work of breathing. (Crit Care Med 1993;21:1333–1338)

Imposed work of breathing and methods of triggering a demand-flow, continuous positive airway pressure system

ObjectivesTo compare the inspiratory imposed work of breathing during spontaneous ventilation with continuous positive airway pressure using three methods of triggering “ON” the demand-flow system of a ventilator: a) conventional pressure triggering with the pressure measuring/triggering site inside the ventilator on the exhalation limb of the breathing circuit; b) tracheal pressure triggering from the tracheal or carinal end of the endotracheal tube; and c) flow-by (flow triggered) triggering. DesignMultitrial tests under simulated clinical conditions using a mechanical lung model. SettingA research laboratory at a university medical center. InterventionsSpontaneous breathing with continuous positive airway pressure, at peak sinusoidal inspiratory flow rate demands of 30, 60, and 90 L/min with sizes 6, 7, 8, and 9 mm internal diameter endotracheal tubes at each flow rate during conventional pressure triggering, tracheal pressure triggering, and flow-by. Measurements and Main ResultsPressures were measured at the tracheal end of the endotracheal tube, “Y” piece of the breathing circuit, and inside the ventilator on the exhalation limb of the breathing circuit. Volume measured between the endotracheal tube and lung model and pressure measured at the tracheal end of the endotracheal tube were integrated to generate pressure-volume (work) loops to calculate the inspiratory imposed work of the total breathing apparatus (i.e., endotracheal tube, breathing circuit, and ventilator). Significantly (p < .05) greater decreases in pressure during spontaneous inhalation were measured for all methods of triggering at the tracheal end of the endotracheal tube than at the Y piece or inside the ventilator. Inspiratory-imposed work was significantly lower during tracheal pressure triggering compared with conventional pressure triggering and flow-by under most conditions. For example, with a 7-nrm internal diameter endotracheal tube at a peak inspiratory flow rate demand of 60 L/min, imposed work was 382% and 315% lower, respectively, during tracheal pressure triggering compared with the conventional pressure triggering and flow-by triggering methods. Under all conditions, inspiratory imposed work was lower during flow-by triggering compared with conventional pressure triggering. The smaller the internal diameter of the endotracheal tube and the greater the peak inspiratory flow rate demand, the greater the inspiratory imposed work of breathing for all methods of triggering. Under all conditions, inspiratory-imposed work was significantly greater at a peak inspiratory flow rate demand of 90 L/min than at 60 L/min, and at a peak inspiratory flow rate demand of 60 L/min than at 30 L/min. ConclusionsAn endotracheal tube is a resistor in the breathing apparatus over which a pressure decrease must be developed by the patient in order to inhale spontaneously. An endotracheal tube, therefore, imposes substantial resistance and work. The results indicate that the pressure measuring/triggering site for a ventilators demand-flow system should be at the tracheal or carinal end of an endotracheal

Site of pressure measurement during spontaneous breathing with continuous positive airway pressure: effect on calculating imposed work of breathing.

ObjectiveTo describe the importance of measuring pressure at the tracheal end of the endotrachealtube during spontaneous breathing with continuous positive airway pressure in order to correctly assess: a) the changes in airway pressure and b) the work imposed by the breathing apparatus. DesignMultitrial tests under simulated clinical conditions using a mechanical lung model. SettingA research laboratory at a university medical center. InterventionsSpontaneous breathing with continuous positive airway pressure, at peak sinusoidal inspiratory flow-rate demands of 30 and then 60 L/min with sizes 6, 7, 8, and 9 mm internal diameter endotracheal tubes at each flow rate. Measurements and Main ResultsPressure, flow rate, and inhaled and exhaled volumes, during simulated spontaneous ventilation with continuous positive airway pressure were measured. Pressure was measured alternately at the “Y” piece of the breathing tubing of the continuous positive airway pressure system and at the tracheal end of the endotracheal tube to calculate the work imposed by the breathing circuit, endotracheal tube, and the total breathing apparatus. Greater changes in pressure and work were measured at the tracheal end of the endotracheal tube than at the “Y” piece of the breathing tubing for all test conditions. For example, at a peak inspiratory flow-rate demand of 30 L/min when pressures measured at the tracheal end of endotracheal tubes were compared with pressures measured at the “Y” piece, the total work imposed by the breathing apparatus increased by approximately 145% with a 6-mm tube, 95% with a 7-mm tube, 50% with an 8-mm tube, and 40% with a 9-mm tube (p <.05). Measuring pressure at the “Y” piece of the tubing results in significant underestimations of the changes in pressure and the work imposed, especially when the endotracheal tube has a small internal diameter and/or when the peak inspiratory flow-rate demand is high. ConclusionsThe results indicate that pressure should be measured as close to the patients airway as possible, i.e., at the tracheal end of the endotracheal tube, rather than using the traditional approach of measuring pressure and assessing work at the inspiratory or expiratory limbs, or “Y” piece of the breathing tubing. (Crit Care Med 1992; 20:528–533)

Evaluation of a fiberoptic system for airway pressure monitoring

Objective..Our objective was to evaluate the accuracy of a novel fiberoptic system for airway pressure measurement at the carinal end of the endotracheal tube in an in vitro pediatric lung model.Methods. A fiberoptic pressure measuring system was compared to the conventional method of measuring airway pressure with a pneumatic transducer using a test lung model. Pressure measurements were obtained using four endotracheal tubes of various internal diameters (ID) (3 to 6 mm) during simulated spontaneous and mechanical ventilation. Airway pressure was measured using both methods simultaneously and the results were compared by statistical analysis.Results. Airway pressure measured by the fiberoptic system was not significantly different from measurements obtained by the pneumatic transducer except when using the 3-mm and 4-mm ID endotracheal tubes during mechanical ventilation.Conclusions. We conclude that the fiberoptic system provides accurate and precise measurement of airway pressure during spontaneous and mechanical ventilation. Additionally, the statistically significant differences obtained for 3- and 4-mm tubes are not large enough to be clinically significant. The fiberoptic system offers advantages over the pneumatic system for measuring the airway pressure. These advantages include decreased chance of false pressure measurement secondary to occlusion with water or mucous, less chance of kinking, and, possibly, more rapid response to pressure changes due to the mechanical ventilator.RésuméObjectifs. Evaluer in vitro, à l’aide d’un modèle de poumon-test pédiatrique, l’exactitude d’un système innovant à fibre optique de la mesure de pressions des voles aériennes à l’extrémité distale d’une sonde endotrachéale.Méthodes. Une pression mesurée par un système à fibre optique a été comparée à la méthode conventionnelle de mesure des pressions des voies aériennes avec un capteur pneumatique, sur un mo`ele de poumon-test. Les mesures de pression ont été obtenues en utilisant 4 sondes endotrac’eales d’un diamètre interne (DI) de 3 á 6 mm, pendant la simulation d’une ventila tion spontanee et mecanique. La pression des voies aériennes a été mesurée en utilisant simultanément les 2 méthodes et les résultats ont été analysés statistiquement.Résultats. La pression des voies aeriennes mesurée par le système à fibre optique n’a pas été significativement différente des mesures obtenues par le capteur pneumatique, sauf en ventilation mécanique lors de l’utilisation des sondes endotrachéales de DI de 3 et 4 mm.Conclusions. Nous concluons que pendant les ventilations spontanee et mecanique, le système à fibre optique donne des mesures exactes et précises des pressions des voies aeriennes. De plus, les différences statistiquement significatives obtenues pour les sondes de DI de 3 et 4 mm ne sont pas assez grandes pour être cliniquement significatives. Le système à fibre optique offre des avantages par rapport au systeme pneumatique pour la mesure de pression des voies aériennes. Ces avantages comprennent: la diminution du risque d’avoir de fausses mesures secondaires a une occlusion par de l’eau ou du mucus, moins de risque de torsion et peut être une reponse plus rapide aux changements de pression dus à la ventilation mécanique.AbstraktFragestellung. Die Beurteilung der Genauigkeit eines neuen fiberoptischen Systems zur in-vitro-Messung des Atemwegsdruckes am kranialen Ende eines Endotrachealtubus anhand des Modells einer Kinderlunge.Methodik. Ein fiberoptisches Druckmessystems wurde mit der konventionellen Methode der Atemwegsdruckmessung mittels pneumatischem Transducer verglichen, wobei dazu eine Testlunge verwendet wurde. Die Druckmessung erfolgte mit 4 verschiedenen Endotrachealtuben unterschiedlichen Innendurchmessers (ID) (3-6 mm) wahrend simulierter spontaner und mechanischer Ventilation. Der Atemwegsdruck wurde bei gleichzeitiger Anwendung beider Methoden gemessen und die Ergebnisse durch statistische Analysen verglichen.Ergebnisse. Der Atemwegsdruck, der mittels fiberoptischem System gemessen worden war, unterschied sich nicht signifikant von den Werten, die mit dem pneumatischen Transducer erhalten wurden, auβPer wenn man die 3 mm und 4 mm IDEndotrachealtuben während der mechanischen Ventilation verwendete.Schluβfifolgerung. Wir schliefien daraus, dap das fiberoptische System genaue und prazise Werte des Atemwegsdrucks wahrend spontaner und mechanischer Ventilation liefert. Zusätzlich allerdings sind die statistisch signifikanten Unterschiede zwischen 3 mm und 4 mm Tuben nicht so groβP, um klinische Signifikanz zu erhalten. Das fiberoptische System bietet zur Atemwegsdruck-Messung einige Vorteile gegeniiber dem pneumatischen System. Diese Vorteile beinhalten: vermindertes Risiko falscher Druckmessung sekundar zu VerschluP mit Wasser oder Schleim; geringeres Risiko von Knotenbildung und wahrscheinlich eine raschere Reaktion auf Druckänderungen, die durch das Beatmungsgerät verursacht sind.ResumenObjetivos. Evaluar la exactitud de un nuevo sistema fibrooptico para medir presión de vía aérea en el extremo carinal del tubo endotraqueal, en un modeloin vitro de pulmón pediatrico.Métodos. Un sistema fibrooptico para medir presión fue comparado con un transductor neumático, que es el metodo convencional usado para medir presión de vía aérea, o sea, mediante un modeloin vitro del pulmón. Las mediciones de presión fueron obtenidas usando cuatro tubos endotraqueales de diferentes diámetros internos (ID) (3- a 6-mm) durante ventilatión simulada espontánea y mecánica. La presioń de vía aérea se midió usando ambos métodos simultaneamente y los resultados se compararon mediante análisis estatístico.Resultados. La presion de vía aérea medida por el sistema fibrooptico no fue significativamente diferente de las mediciones obtenidas con el transductor neumático, excepto al usar los tubos endotraqueales 3-mm y 4-mm ID durante ventilatión mecánica.Conclusiones. Concluimos que el sistema fibrooptico proporciona mediciones exactas y precisas de la presion de vía aérea durante ventilat’on mecanica y espontánea. Adicionalmente, las diferencias estatisticamente significativas obtenidas para los tubos de 3- y 4-mm no fueron suficientemente grandes como para alcanzar significatión clínica. El sistema fibrooptico posée ventajas sobre el sistema neumatico para medir presión de vía ’erea. Estas ventajas incluyen: menor probabilidad de falsas mediciones secundarias a oclusión con agua o mucus, menos probabilidad de acodadura, y posiblemente respuesta más rápida a los cambios de presión debidos al ventilador mecánico.

A new pediatric respiratory monitor that accurately measures imposed work of breathing: a validation study.

Objective. A new, microprocessor-controlled respiratory monitor (model CP-100 Pediatric, Bicore Monitoring Systems, Irvine, CA) that measures imposed work of breathing and a variety of respiratory parameters for pediatric patients receiving ventilatory support has recently been developed. To validate its accuracy, measurements obtained using this monitor were compared with those obtained using conventional laboratory equipment.Methods. An in vitro lung model was used to simulate spontaneously breathing pediatric patients ranging from infancy to 10 years of age. Tidal volume, respiratory rate, and peak inspiratory flow rates were simulated in a stepwise manner. Values for imposed work, tidal volume, peak inspiratory flow rate, and change in airway pressure for both methods were compared using regression analysis.Results. The coefficients of determination (r2) describing the relationships of both methods of measuring imposed work, tidal volume, peak inspiratory flow rate, and the change in airway pressure ranged from 0.99 to 1.00, and were highly significant (p<0.001). For all measurements, bias was minimal and precision was calculated.Conclusions. Our data reveal that this pediatric respiratory monitor accurately measures imposed work of breathing, as well as tidal volume, flow rate, and airway pressure. Imposed work of breathing measurements obtained from the monitor may be used to adjust pressure support ventilation, so that the imposed work of the breathing apparatus is reduced to zero and the patients total work of breathing is thus decreased.

Tracheal pressure control provides automatic and variable inspiratory pressure assist to decrease the imposed resistive work of breathing.

ObjectiveTo evaluate the operation of a continuous positive airway pressure system by using tracheal airway pressure (PT) as the control signal for system operation (i.e., tracheal pressure control). DesignRepeated measures. SettingUniversity research laboratory. SubjectsTwelve anesthetized, spontaneously breathing swine. InterventionsSubjects were intubated and connected to a tracheal pressure control system (5 cm H2O continuous positive airway pressure). Varying inspiratory flow demands and degrees of partial endotracheal tube occlusion (25%, 50%, and 75%) were studied. Tracheal pressure control was compared with a conventionally controlled system (pressure from breathing circuit Y-piece [PY] used as control signal) during endotracheal tube occlusion. Measurements and ResultsImposed resistive work of breathing (work to spontaneously inhale through endotracheal tube and ventilator circuit), work by ventilation system assisting inhalation, PT, PY, tidal volume, and inspiratory flow demands were measured. As inspiratory flow demands increased (range, 0.2–2.3 L/sec), pressure assist increased automatically (range, 5–40 cm H2O) as well as work of breathing by ventilation system assisting inhalation (range, 0.2–2.5 J/L). Imposed resistive work of breathing was nullified at the lower and was negligible at the higher flow demands. During endotracheal tube occlusion with a conventionally controlled system, PY was unchanged, whereas PT decreased (up to −15 cm H2O) and imposed resistive work of breathing increased (up to 1.05 J/L). With tracheal pressure control, PY increased automatically (range, 8–52 cm H2O), whereas PT varied slightly (range, 2 to −4.6 cm H2O). Imposed resistive work of breathing was negligible (range, 0–0.2 J/L). Breathing circuit pressure (PY), not pulmonary airway pressure (PT), increased significantly during tracheal pressure control. ConclusionsTracheal pressure control results in automatic and variable levels of pressure assist to decrease imposed resistive work of breathing under conditions of varying spontaneous inspiratory flow demands and endotracheal tube occlusion. Conventional systems are potentially flawed when PY is used as the control signal because they do not function in this manner and do not accurately assess pulmonary airway pressure.

Behavior of nitric oxide infused at constant flow rates directly into a breathing circuit during controlled mechanical ventilation

OBJECTIVES This study was designed to test the hypothesis that the practice of infusing nitric oxide at constant flow rates directly into breathing circuits with intermittent (pulsatile) flow can lead to streaming and tidal pooling of the nitric oxide. This study was also designed to show the extent to which streaming and tidal pooling of nitric oxide affect nitric oxide delivery. DESIGN A series of five in vitro experiments was performed. For each experiment, either one or two features of the nitric oxide delivery/sampling system were varied, and the effects of these variations were evaluated with regard to measured nitric oxide concentration changes. The results from each experiment were analyzed using either one- or two-factor analysis of variance. SETTING University research laboratory. SUBJECTS Breaths were provided by a mechanical ventilator that was connected to a lung model. A standard, corrugated, adult breathing circuit was used. Gas samples were obtained from either the lung models bellows or selected sites within the breathing circuit. Nitric oxide concentrations were measured, using an electrochemical gas analyzer. INTERVENTIONS The system features that were varied included the cross-sectional position of the sampling site within the breathing circuit, the distance between the infusion port and the sampling site, the breathing frequency, the distance between the Y-piece and the infusion port, and the airway (deadspace) volume. MEASUREMENTS AND MAIN RESULTS Streaming of nitric oxide within the breathing circuit was detected as far as 25 cm downstream of the infusion site (p < .0001). Pooling of nitric oxide was detected both near and downstream of the infusion site (p < .0001). Increasing the breathing frequency from 5 to 30 breaths/min increased mixing thoroughness (p < .005). Increasing the distance between the Y-piece and the infusion port from 15 to 180 cm decreased nitric oxide delivery to our lung model (p < .0001). Interestingly, increasing airway (deadspace) volume from 150 to 450 mL decreased nitric oxide delivery to our lung model (p < .0001). CONCLUSIONS Estimates of nitric oxide delivery using a constant flow rate of nitric oxide infused directly into a breathing circuit during controlled mechanical ventilation can be confounded by streaming and tidal propagation of nitric oxide pools. Improved reproducibility of reported dose-response relationships is likely to be achieved through further study of nitric oxide behavior within the breathing circuits. Reduced toxicity associated with nitric oxide inhalation may also be achieved through a better understanding of this nitric oxide behavior.

Propagation of Nitric Oxide Pools During Controlled Mechanical Ventilation

Objective. Infusing nitric oxide at a constant rate into a breathing circuit with intermittent mainstream flow causes formation of nitric oxide pools between successive breaths. We hypothesized that incomplete mixing of these pools can confound estimates of delivered nitric oxide concentrations. Methods. Nitric oxide flowed at a constant rate into the upstream end of a standard adult breathing circuit connected to a lung model. One-milliliter gas samples were obtained from various sites within the breathing system and during various phases of the breathing cycle. These samples were aspirated periodically by a microprocessor controlled apparatus and analyzed using an electrochemical sensor. Results. The pools of nitric oxide distorted into hollow parabolic cone shapes and remained unmixed during their propagation into the lungs. In our preparation, time-averaged nitric oxide concentrations were minimal 60 cm downstream of the infusion site (18 ppm) and maximal 15 cm upstream of the Y-piece (36 ppm). The concentrations were mid-range within the lung (23 ppm), yet were substantially less than predicted by assuming homogeneity of the gases (31 ppm). Generally, nitric oxide concentrations within the lung were different from all other sites tested. Conclusion. Incomplete mixing of nitric oxide confounds estimates of delivered nitric oxide concentrations. When nitric oxide is infused at a constant rate into a breathing circuit, we doubt that any sampling site outside the patient’s lungs can reliably predict delivered nitric oxide concentrations. Strategies to ensure complete mixing and representative sampling of nitric oxide should be considered carefully when designing nitric oxide delivery systems.

Mechanical ventilation of a patient with decreased lung compliance and tracheal dilatation.

Tracheal injury resulting from tracheal intubation is common. Injuries vary in type and severity, from mucosal sloughing to tracheal stenosis and fistula formation. We report a patient with poor lung compliance and massive tracheal dilatation as a result of prolonged mechanical ventilation with high inflation pressure despite the use of a high-volume, low-pressure cuff. To reduce the tracheal dilatation but maintain adequate ventilation and continuous positive airway pressure, we substituted a longer double-cuff tracheotomy appliance and used an automatic intermittent cuff inflator. The problems related to the design of modern tracheal tube cuffs are discussed.