Michael J. Banner

Influence of tidal volume on the distribution of gas between the lungs and stomach in the nonintubated patient receiving positive-pressure ventilation.

Abstract Objectives: When ventilating a nonintubated patient in cardiac arrest, the European Resuscitation Council has recently recommended a decrease in the tidal volume from 0.8 to 1.2 L to 0.5 L, partly in an effort to decrease peak flow rate, and therefore, to minimize stomach inflation. The purpose of the present study was to examine the validity of the European Resuscitation Councils recommendation in terms of gas distribution between lungs and stomach in a bench model that simulates ventilation of a nonintubated patient with a self‐inflatable bag representing tidal volumes of 0.5 and 0.75 L. Design: A bench model of a patient with a nonintubated airway was used consisting of face mask, manikin head, training lung (lung compliance, 50 mL/cm H2 O; airway resistance, 5 cm H2 O/L/sec), adjustable lower esophageal sphincter pressure (LESP) and simulated stomach. Setting: University hospital laboratory. Subjects: Thirty healthcare professionals. Interventions: Healthcare professionals performed 1‐min bag‐mask ventilation at each LESP level of 5, 10, and 15 cm H2 O at a rate of 12 breaths/min, using an adult and pediatric self‐inflating bag, respectively. Volunteers were blinded to the LESP, which was randomly varied. Measurements and Main Results: Both types of self‐inflating bags induced stomach inflation, with higher stomach and lower lung tidal volumes when the LESP was decreased. Lung tidal volume with the pediatric bag was significantly (p < .05) lower at all LESP levels when compared with the adult bag, and ranged between 240 mL at an LESP of 15 cm H2 O and 120 mL at an LESP of 5 cm H2 O. Stomach tidal volume with the adult bag ranged between 250 mL at an LESP of 15 cm H2 O and increased to 550 mL at an LESP of 5 cm H2 O. Stomach tidal volume with the pediatric bag was significantly lower (p < .05) at all LESP levels when compared with the adult bag and ranged between 70 mL at an LESP of 15 cm H2 O and 300 mL at an LESP of 5 cm H2 O. Conclusions: Our data support the recommendation of the European Resuscitation Council to decrease tidal volumes to 0.5 L when ventilating a cardiac arrest victim with an unprotected airway. A small tidal volume may be a better trade‐off in the basic life support phase, as this may provide reasonable ventilation while avoiding massive stomach inflation. (Crit Care Med 1998; 26:364‐368) When ventilating a nonintubated cardiac arrest patient, the European Resuscitation Council has recently recommended a decrease in the tidal volume from 0.8 to 1.2 L [1] to 0.5 L [2], partly in an effort to decrease peak flow rate, and therefore, to minimize stomach inflation. Ventilation volume has an effect on pH, CO2 elimination, and oxygenation when pulmonary blood flow is extremely low, such as during cardiopulmonary resuscitation (CPR) or shock [3]. The Airway and Ventilation Management Working Group of the European Resuscitation Council [2] stated that ventilating a nonintubated cardiac arrest patient with a smaller tidal volume may be a better trade‐off in order to provide reasonable ventilation, while avoiding massive stomach inflation that may result in life‐threatening pulmonary complications. The gas distribution between lungs and stomach during positive‐pressure ventilation in a nonintubated airway depends on patient characteristics (lower esophageal sphincter pressure [LESP], airway resistance, and respiratory system compliance) and performance variables of the rescuer applying positive‐pressure ventilation (tidal volume, peak flow rate, and upper airway pressure). Thus, assessing the above‐mentioned recommendation in a clinical study is difficult to perform due to many confounding variables that are difficult to control and to evaluate when emergently managing therapy during CPR. As an example of the usefulness of laboratory models of ventilation, the American Heart Association [1] recommended increasing inspiratory time when ventilating a nonintubated cardiac arrest patient [1,4]. For example, such a model has the advantage that each variable of respiratory mechanics can be controlled and adjusted to investigate a certain hypothesis. We [4,5] previously studied the effect of tidal volume and peak flow rate on gas distribution between lungs and stomach during mechanical positive‐pressure ventilation, using a modification of the earlier described bench model of an unprotected airway. Although a mechanical ventilator is a valuable tool to study gas distribution in an unprotected airway, a self‐inflatable bag is the device usually used by the emergency medical service and in the hospital during the initial care of a cardiac arrest victim. Thus, the purpose of the present study was to examine the validity of the European Resuscitation Council recommendation [2] in terms of gas distribution between lungs and stomach in a bench model that simulates positive‐pressure ventilation of a nonintubated patient with self‐inflatable bags representing tidal volumes of 0.5 L and 0.75 L. Since observations in an animal model [6] showed that the LESP decreases rapidly after an untreated cardiac arrest, we further evaluated the effect of a decreased LESP on gas distribution between lungs and stomach in this model.

Inspiratory muscle strength training improves weaning outcome in failure to wean patients: a randomized trial

IntroductionMost patients are readily liberated from mechanical ventilation (MV) support, however, 10% - 15% of patients experience failure to wean (FTW). FTW patients account for approximately 40% of all MV days and have significantly worse clinical outcomes. MV induced inspiratory muscle weakness has been implicated as a contributor to FTW and recent work has documented inspiratory muscle weakness in humans supported with MV.MethodsWe conducted a single center, single-blind, randomized controlled trial to test whether inspiratory muscle strength training (IMST) would improve weaning outcome in FTW patients. Of 129 patients evaluated for participation, 69 were enrolled and studied. 35 subjects were randomly assigned to the IMST condition and 34 to the SHAM treatment. IMST was performed with a threshold inspiratory device, set at the highest pressure tolerated and progressed daily. SHAM training provided a constant, low inspiratory pressure load. Subjects completed 4 sets of 6-10 training breaths, 5 days per week. Subjects also performed progressively longer breathing trials daily per protocol. The weaning criterion was 72 consecutive hours without MV support. Subjects were blinded to group assignment, and were treated until weaned or 28 days.ResultsGroups were comparable on demographic and clinical variables at baseline. The IMST and SHAM groups respectively received 41.9 ± 25.5 vs. 47.3 ± 33.0 days of MV support prior to starting intervention, P = 0.36. The IMST and SHAM groups participated in 9.7 ± 4.0 and 11.0 ± 4.8 training sessions, respectively, P = 0.09. The SHAM groups pre to post-training maximal inspiratory pressure (MIP) change was not significant (-43.5 ± 17.8 vs. -45.1 ± 19.5 cm H2O, P = 0.39), while the IMST groups MIP increased (-44.4 ± 18.4 vs. -54.1 ± 17.8 cm H2O, P < 0.0001). There were no adverse events observed during IMST or SHAM treatments. Twenty-five of 35 IMST subjects weaned (71%, 95% confidence interval (CI) = 55% to 84%), while 16 of 34 (47%, 95% CI = 31% to 63%) SHAM subjects weaned, P = .039. The number of patients needed to be treated for effect was 4 (95% CI = 2 to 80).ConclusionsAn IMST program can lead to increased MIP and improved weaning outcome in FTW patients compared to SHAM treatment.Trial RegistrationClinicalTrials.gov: NCT00419458

Respiratory system compliance decreases after cardiopulmonary resuscitation and stomach inflation: impact of large and small tidal volumes on calculated peak airway pressure

The purpose of the present study was to evaluate respiratory system compliance after cardiopulmonary resuscitation (CPR) and subsequent stomach inflation. Further, we calculated peak airway pressure according to the different tidal volume recommendations of the European Resuscitation Council (7.5 ml/kg) and the American Heart Association (15 ml/kg) for ventilation of an unintubated cardiac arrest victim. After 4 min of ventricular fibrillation, and 6 min of CPR, return of spontaneous circulation (ROSC) after defibrillation occurred in seven pigs. Respiratory system compliance was measured at prearrest, after ROSC, and after 2 and 4 l of stomach inflation in the postresuscitation phase; peak airway pressure was subsequently calculated. Before cardiac arrest the mean (+/- S.D.) respiratory system compliance was 30 +/- 3 ml/cm H2O, and decreased significantly (P < 0.05) after ROSC to 24 +/- 5 ml/cm H2O, and further declined significantly to 18 +/- 4 ml/cm H2O after 2 l, and to 13 +/- 3 ml/cm H2O after 4 l of stomach inflation. At prearrest, the mean +/- S.D. calculated peak airway pressure according to European versus American guidelines was 9 +/- 1 versus 18 +/- 3 cm H2O, after ROSC 12 +/- 2 versus 23 +/- 4 cm H2O, and 15 +/- 2 versus 30 +/- 5 cm H2O after 2 l, and 22 +/- 6 versus 44 +/- 12 cm H2O after 4 l of stomach inflation. In conclusion, respiratory system compliance decreased significantly after CPR and subsequent induction of stomach inflation in an animal model with a wide open airway. This may have a significant impact on peak airway pressure and distribution of gas during ventilation of an unintubated patient with cardiac arrest.

Ventilation caused by external chest compression is unable to sustain effective gas exchange during CPR: a comparison with mechanical ventilation

OBJECTIVE To compare the tidal volume, minute ventilation, and gas exchange caused by mechanical chest compression with and without mechanical ventilatory support during cardiopulmonary resuscitation (CPR) in a laboratory model of cardiac arrest. DESIGN A laboratory swine model of CPR was used. Eight animals with and eight animals without mechanical ventilation received chest compression (100/min) for 10 min. Coronary perfusion pressure, tidal volume, and minute ventilation were recorded continuously. INTERVENTIONS Ventricular fibrillation for 6 min without CPR, then mechanical chest compression for 10 min. RESULTS During the first minute of chest compression, mean (+/- S.D.) minute ventilation was 11.2 +/- 5.9 l/min in the mechanically ventilated group and 4.5 +/- 2.8 l/min in the group without mechanical ventilation (P = 0.01). Minute ventilation gradually declined to 5.8 +/- 1.4 l/min and 1.7 +/- 1.6 l/min, respectively, during the last minute of chest compression (P < 0.0001). After 10 min of chest compression, mean arterial pH was significantly more acidemic in the group without mechanical ventilation (7.16 +/- 0.13 compared with 7.30 +/- 0.07 units) and PCO2 was higher (62 +/- 19 compared with 35 +/- 9 mmHg). Mixed venous PCO2 was also higher (76 +/- 15 compared with 61 +/- 8 mmHg). CONCLUSION Standard chest compression alone produced measurable tidal volume and minute ventilation. However, after 10 min of chest compression following 6 min of untreated ventricular fibrillation, it failed to sustain pulmonary gas exchange as indicated by significantly greater arterial and mixed venous hypercarbic acidosis when compared with a group receiving mechanical ventilation.

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

Portable devices used to detect endotracheal intubation during emergency situations : A review

OBJECTIVES To review the operational characteristics of commercial devices used to detect endotracheal intubation; and to identify an ideal device for detecting endotracheal intubation in emergency situations, especially in the prehospital setting during cardiac arrest. DATA SOURCES Relevant articles from the medical literature are referenced. STUDY SELECTION The authors identified the need for understanding the basic operation principles of portable devices used to detect endotracheal intubation and to correctly use them in unpredictable clinical situations. DATA EXTRACTION Data from published literature. DATA SYNTHESIS Recently, a number of new portable devices have been marketed for detecting endotracheal intubation, each having advantages and disadvantages, especially when used during emergency situations. The devices are classified based on their principle of operation. Some rely on CO2 detection (STATCAP, Easy Cap, and Pedi-Cap), others utilize the transmission of light (Trachlight, SURCH-LITE), one operates based on reflection of sound energy (SCOTI), and some depend on aspiration of air (TubeChek and TubeChek-B). A brief description of each device and its operational characteristics are reviewed. A comparative analysis among the devices is made based on size, portability, cost, ease of operation, need for calibration or regular maintenance, reliability for patients with and without cardiac arrest, and the possibility of use for adult and pediatric patients. False-negative and false-positive results for each device are also discussed. False-negative results mean that although the endotracheal tube is in the trachea, the device indicates it is not. False-positive results mean that although the endotracheal tube is in the esophagus, the device indicates it is in the trachea. CONCLUSIONS Although no clinical comparative study of commercial devices to detect endotracheal intubation exists, the syringe device (TubeChek) has most of the characteristics necessary for a device to be considered ideal in emergency situations in the prehospital setting. It is simple, inexpensive, easy to handle and operate, disposable, does not require maintenance, gives reliable results for patients with and without cardiac arrest, and can be used for almost all age groups. The device may yield false-negative results, most commonly in the presence of copious secretions and in cases of accidental endobronchial intubation. Regardless of the device used, clinical judgment and direct visualization of the endotracheal tube in the trachea are required to unequivocally confirm proper endotracheal tube placement.

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)

Ventilation during CPR: Two-rescuer standards reappraised

Current American Heart Association standards for ventilation during two-rescuer CPR recommend that a 0.8- to 1.2-L breath be delivered in 0.5 second after every fifth chest compression. Delivering a high-volume breath over a brief inspiratory time (TI) may lead to hypoventilation and gastric insufflation in victims with an unprotected airway. We reasoned that lengthening TI would lower peak inspiratory pressure and peak inspiratory flow rate, and thus improve lung inflation. To study this possibility, a mechanical model of the airway and upper gastrointestinal tract was designed. A ventilator delivering a sinusoidal wave form was used to simulate artificial ventilation. A 0.8-L breath was delivered at 0.5, 1.0, or 1.5 seconds at three lung compliances (CLs). Also, the effect of lengthening TI was studied with increased airway resistance. Lengthening TI improved lung inflation and decreased gastric insufflation at all CLs, but more so with normal CL than with decreased CL. This study demonstrates the need for evaluating alternative ventilatory patterns with longer TI during CPR.

Hypertonic saline as a resuscitation solution in hemorrhagic shock: effects on extravascular lung water and cardiopulmonary function

To determine the effect of resuscitation with hypertonic saline on extravascular lung water, seven adult sheep were endotracheally intubated; mean arterial pressure (MAP), pulmonary artery pressure (PAP), pulmonary capillary wedge pressure (PCWP), and central venous pressure (CVP) were monitored. A 5-French, thermistor-tipped catheter was used to measure extravascular lung water (EVLW). Colloid oncotic pressure (COP), serum electrolytes and osmolality, and arterial and mixed venous blood gas tensions were measured. The COP-PCWP gradient and the shunt fraction (Qsp/Qt) were calculated. After baseline measurements, the animals were bled to an MAP of 50 mm Hg (blood volume removed, 16.2 +/- 3.6 ml/kg), which was maintained for 30 min, measurements then being repeated. Three percent sodium chloride solution was infused at 500 ml/15 min until two of three parameters--cardiac output (CO), PCWP, or MAP--were restored to baseline values. Data were recorded again and then 60 min later. No shed blood was reinfused. The total volume of hypertonic saline infused was 39 +/- 19 ml/kg. Pulmonary artery pressure did not vary throughout the study. PCWP, MAP, and CO were significantly lower than baseline (P less than 0.05) 30 min after bleeding but all except MAP returned to baseline with resuscitation. Throughout the study, EVLW did not vary despite a COP-PCWP gradient less than 4 mm Hg. Serum sodium levels and serum osmolality were significantly above baseline values after resuscitation. In this animal model of hemorrhagic shock, infusion of hypertonic saline effected resuscitation without compromising cardiopulmonary function or increasing EVLW.