Robert R. Kirby

Cardiovascular and pulmonary responses following etomidate induction of anesthesia in patients with demonstrated cardiac disease.

Cardiovascular and pulmonary effects following the administration of 0.3 mg/kg of etomidate were studied in patients with documented cardiac disease. The only significant change was a slight elevation (2 torr) in arterial carbon dioxide tension.

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)

Terminology update: optimal PEEP.

: The term, optimal PEEP, requires redefinition in the light of new clinical data. With the onset of acute respiratory failure heralded by blood gas evidence of decreased oxygenation, PEEP is supplied in quantities sufficient to restore intrapulmonary shunt (Qsp/Qt) to a preselected goal of 15%. This is compatible with published criteria defining adequate blood gas exchange. Now rather than permitting reduction of cardiac output to be the end point of PEEP application, selective cardiovascular interventions to support preload, contractility, or afterload are made as appropriate so that cardiac function may be maintained until the preselected endpoint of shunt reduction of 15% can be made.

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)

Continuous positive airway pressure (CPAP) by face mask.

The authors studied the use of CPAP by face mask in 44 patients with mild to moderate acute respiratory insufficiency (mean PaO2/FIO2 = 171 ± 42 (SD). All patients improved their oxygenation (mean PaO2/FIO2 = 300 ± 68 (SD)) after the application of CPAP (mean 9 ± 3 (SD) cm H2O). One patient developed a respiratory addenda and was intubated. No other significant complications or adverse side effects occurred. The authors conclude that CPAP by face mask in a select group of patients will improve lung function and arterial oxygenation without the use of tracheal intubation and mechanical ventilation.

Respiratory complications in patients with traumatic cervical spine injuries: case report and review of the literature.

Spinal cord injuries continue to be a devastating medical problem. By impairing voluntary and involuntary nervous system function, virtually every body system function is affected. However, pulmonary function alteration and respiratory complications continue to be the major causes of morbidity and mortality in patients with spinal cord injuries. The current understanding of respiratory problems faced by patients with loss of innervation from cervical spinal cord injuries are reviewed.

A new mechanical method to influence pulmonary perfusion in critically ill patients.

A new device, the Roto-Rest bed, has been used in critically ill patients to effect lateral to lateral position changes and continuous rotation to influence distribution of pulmonary blood flow. Selection of extreme lateral position can be made particularly in cases of respiratory failure with primarily unilateral involvement. Dependent positioning of the uninvolved lung to match ventilation and perfusion can be accomplished simply. Rapid resolution of a large pulmonary contusion serves to illustrate the clinical use of the apparatus.

Intermittent inspiratory chest tube occlusion to limit bronchopleural cutaneous airleaks.

A significant bronchopleural cutaneous fistula (BPCF) developed in a 36-year-old female who required mechanical ventilation for acute respiratory failure. Progressive increase in arterial PCO2 to 75 torr occurred because of inability to effect satisfactory alveolar ventilation. Insertion of unidirectional values into the chest tube drainage apparatus, which were closed synchronously each time the ventilator cycled to the inspiratory phase, allowed effective alveolar ventilation to be achieved with subsequent reduction of arterial CO2 to previous levels. Both high inspiratory (120 torr) and expiratory (23 torr) positive pressures were employed with intermittent mandatory ventilation (IMV). Deleterious effects on cardiopulmonary function were not observed, and the patient was weaned successfully from mechanical support with spontaneous closure of the BPCFs.

Intermittent mandatory ventilation in the neonate.

Intermittent Mandatory Ventilation (IMV) provides an alternative method of support to assisted and controlled mechanical ventilation in neonates with acute respiratory failure. Specific advantages include the use of only the level of mechanical support which is required on an individual basis. Because intrapleural pressure is reduced with IMV, as opposed to intermittent positive pressure ventilation (IPPV), venous return and cardiac output are maintained at more nearly normal levels, with or without positive end-expiratory pressure (PEEP). Duration of mechanical ventilatory support is reduced, weaning enhanced, and the incidence of pulmonary barotrauma decreased with IMV. Physiological homeostasis in terms of acid-base changes is more readily maintained as a result of precise regulation of alveolar ventilation. Newer concepts in mechanical support, such as “reversed” inspiratory:expiratory ratio (I:E ratio) ventilation, are possible with IMV because of the lower ventilator rates employed compared to IPPV.