Prehospital Protocols for Post-Return of Spontaneous Circulation Are Highly Variable.

BACKGROUND: Up to 44% of out-of-hospital cardiac arrest (OHCA) patients will rearrest in the immediate post-return of spontaneous circulation (post-ROSC) period, and rearrest is associated with decreased survival. Cardiac arrest guidelines are often equivocal regarding what post-ROSC care should be provided in the prehospital setting and when hospital transport should be initiated. Prehospital protocols must balance the benefit of time-dependent hospital-based care with the risk of early rearrest. We sought to describe current prehospital protocols for post-ROSC care in the treatment of OHCA. METHODS: A single trained abstractor systematically reviewed a purposeful sample of prehospital protocols for adult non-traumatic cardiac arrest from the United States using an a priori standardized data abstraction form. Protocols were either stand-alone or integrated into intra-arrest care. Exclusion criteria were non-911 ground transport agencies and protocols not revised since the 2015 American Heart Association guideline update. All protocols were publicly available via the Internet. Data abstraction was conducted in May 2019. Measures of interest were counted and summarized. Proportions and 95% confidence intervals were calculated. RESULTS: We identified and reviewed 82 prehospital protocols from 46 states and the District of Columbia. Seven protocols were excluded due to the revision date, leaving 75 protocols included in the study. Six protocols (8%; CI 3.7-16%) provide no guidance on prehospital post-ROSC care. 12-lead electrocardiogram (ECG) acquisition (63/75 [84%; CI 73-91%]) and transport to percutaneous coronary intervention-capable hospitals (55/75 [73%; CI 62-83%]) are common, although not ubiquitous. Of those that do require a 12-lead ECG, 40% [CI 27-54%] required the presence of an ST-elevation myocardial infarction to inform their transport decision. Only 9 (12%; CI 6.4-22%) provide any guidance on when to initiate transport post-ROSC, with 4 (5%; CI 2-13%) requiring a post-ROSC stabilization period prior to transport. CONCLUSION: Prehospital treatment and transport protocols for post-ROSC care are highly variable across the United States.

Use of a cardiocerebral-protective drug cocktail prior to countershock in a porcine model of prolonged ventricular fibrillation.

OBJECTIVES: This was the third study in a series exploring whether the use of combination pharmacotherapy with delayed countershock would produce higher rates of return of spontaneous circulation (ROSC) and one-hour survival when compared with standard advanced cardiac life support (ACLS) therapy in prolonged ventricular fibrillation (VF). METHODS: Twenty-four female, mixed-breed, domestic swine (ranging in mass from 22 to 25 kg) were used in this prospective, blinded, randomized, experimental trial. Animals were sedated (ketamine/xylazine), anesthetized (alpha-chloralose), paralyzed (pancuronium), mechanically ventilated with room air, and monitored with electrocardiography, arterial pressure, and Swan-Ganz catheters. VF was induced with a 3 s, 60 Hz, 100 mA transthoracic shock, and remained untreated for 8 min. One minute of basic life support followed (standardized by use of a mechanical device). At 9 min, animals were treated with one of three regimes: Group 1, cardiocerebral-protective cocktail (antioxidant U-74389G (3.0 mg/kg), epinephrine (0.2 mg/kg), lidocaine (1.0 mg/kg), bretylium (5.0 mg/kg), magnesium (2.0 g), and propranolol (1.0 mg)); Group 2, magnesium (2.0 g); and Group 3, standard ACLS. Groups 1 and 2 received drugs at minute nine (first countershock at minute 11), while Group 3 received first countershock at minute nine. Data were analyzed with two-tailed Fisher's exact tests. RESULTS: ROSC was achieved in Group 1, 7/7 (100%); Group 2, 3/9 (33%, P versus Group 1=0.01); and Group 3, 3/8 (38%; P versus Group 1=0.02). One-hour survival was attained in Group 1, 7/7 (100%); Group 2, 3/9 (33%; P versus Group 1=0.01), and Group 3, 1/8 (13%; P versus Group 1=0.001). CONCLUSIONS: Combination pharmacotherapy with a cardiocerebral-protective drug cocktail prior to countershock produced superior rates of ROSC and one-hour survival when compared with singular drug therapy (Group 2) and standard ACLS (Group 3) in this porcine model of prolonged VF.

Bench to bedside: resuscitation from prolonged ventricular fibrillation.

Ventricular fibrillation (VF) remains the most common cardiac arrest heart rhythm. Defibrillation is the primary treatment and is very effective if delivered early within a few minutes of onset of VF. However, successful treatment of VF becomes increasingly more difficult when the duration of VF exceeds 4 minutes. Classically, successful cardiac arrest resuscitation has been thought of as simply achieving restoration of spontaneous circulation (ROSC). However, this traditional approach fails to consider the high early post-cardiac arrest mortality and morbidity and ignores the reperfusion injuries, which are manifest in the heart and brain. More recently, resuscitation from cardiac arrest has been divided into two phases; phase I, achieving ROSC, and phase II, treatment of reperfusion injury. The focus in both phases of resuscitation remains the heart and brain, as prolonged VF remains primarily a two-organ disease. These two organs are most sensitive to oxygen and substrate deprivation and account for the vast majority of early post-resuscitation mortality and morbidity. This review focuses first on the initial resuscitation (achieving ROSC) and then on the reperfusion issues affecting the heart and brain.

Effects of biphasic vs monophasic defibrillation on the scaling exponent in a swine model of prolonged ventricular fibrillation.

OBJECTIVE: Mathematical analyses of ventricular fibrillation (VF) have resulted in the derivation of a measure termed the scaling exponent (ScE) that characterizes the duration of VF and probability of defibrillation success. The purpose of this study was to compare the effects of biphasic defibrillation waveform (BDW) and monophasic defibrillation waveform (MDW) rescue shocks on ScE in a swine model of prolonged VF. METHODS: Utstein guidelines for the laboratory study of cardiopulmonary resuscitation were followed. Twenty mixed-breed domestic swine (mass range 20.5-26.8 kg) were instrumented and randomized to receive either MDW or BDW rescue shocks. Ventricular fibrillation was induced and untreated for a nonintervention interval of 8 minutes. Rescue shocks were delivered at 8, 10, and 12 minutes of elapsed VF time. The energy sequence for the three MDW shocks was 70, 100, and 150 J (approximately 3, 4, and 6 J/kg). All BDW shocks were delivered at 50 J (approximately 2.5 J/kg). Only VF was shocked. Chest compressions and drugs were not provided. Rhythm analysis and ScE calculation were performed offline. Continuous and discontinuous linear regression models were fit to plots of ScE vs time. Defibrillation success and progression of ScE slope were analyzed using Fisher's exact test, paired t-tests, and repeated-measures analysis of variance (ANOVA). RESULTS: Baseline characteristics were similar for both groups. Successful termination of VF occurred on the first rescue shock in 1 of 10 (10%) in the MDW group and 3 of 10 (30%) in the BDW group; this difference was not statistically significant (p = 0.58). No other defibrillation successes were observed. No animals achieved return of spontaneous circulation. The ScE values during the protocol progressed from 1.330 (95% CI = 1.287 to 1.373) to 1.724 (95% CI = 1.603 to 1.845) for MDW and 1.338 (95% CI = 1.261 to 1.415) to 1.639 (95% CI = 1.530 to 1.745) for BDW. Both groups showed a trend toward increasing ScE values with successive rescue shocks. Repeated-measures ANOVA using both continuous and discontinuous models demonstrated no difference in overall ScE slope progression between study groups. CONCLUSIONS: Mode of defibrillation waveform (BDW vs MDW) does not appear to impact ScE trends. Additional studies must be performed to better evaluate the clinical implications of this finding.

The locations of nonresidential out-of-hospital cardiac arrests in the City of Pittsburgh over a three-year period: implications for automated external defibrillator placement.

OBJECTIVES: To determine the locations of nonresidential out-of-hospital cardiac arrests (OHCAs) in the City of Pittsburgh and to determine whether there are "high-risk" locations that might benefit from placement of automated external defibrillators (AEDs). METHODS: This was a retrospective case review of paramedic calls for OHCA over a three-year period, in a mid-sized northeastern city. Cardiac arrests that were traumatic or for which the patients were dead on arrival (DOA) or had advanced directives for no resuscitation were excluded. Cardiac arrests that occurred in a public location (i.e., not a private residence) were categorized. RESULTS: A total of 971 OHCAs occurred in the City of Pittsburgh from January 1, 1997, to December 31, 1999. Of these, 575 (59%) occurred in private residences, and 396 (41%) occurred in nonresidential locations. Fifteen locations had at least one cardiac arrest per year for three years, accounting for 166 (43%) of the total nonresidential OHCAs. Twelve locations had two arrests during the three-year period, accounting for 24 (6%) of the total nonresidential OHCAs. One hundred ninety-four locations had a single episode of cardiac arrest, accounting for 51% of the OHCAs. Nursing homes and dialysis centers accounted for 178 (94%) OHCAs in the 27 locations that had two or more cardiac arrests. A local sports/events complex (Three Rivers Stadium) was the only other single location to have more than two cardiac arrests in the three-year study period, with a total of three. However, events at this complex are routinely staffed by paramedics equipped with defibrillators. CONCLUSION: The majority of nonresidential OHCAs occur as singular, isolated events. Other than nursing homes and dialysis centers, there were no identifiable high-risk locations for nonresidential OHCA within the City of Pittsburgh.

Physician interpretation and quantitative measures of electrocardiographic ventricular fibrillation waveform.

OBJECTIVES: The characteristics of the ventricular fibrillation (VF) waveform may influence treatment decisions and the likelihood of therapeutic success. However, assessment of VF as being fine or coarse and the distinction between fine VF and asystole are largely subjective. The authors sought to determine the level of agreement among physicians for interpretation of varying VF waveforms, and to compare these subjective interpretations with quantitative measures. METHODS: Six-second segments of waveform from LIFEPAK 300 units were collected. Fifty segments, including 45 VF and five ventricular tachycardia (VT) distracters, were graphed to simulate rhythm strips. These waveforms were quantitatively described using scaling exponent, root-mean-squared amplitude, and centroid frequency. Thirty-two emergency medicine residents were asked to interpret the arrhythmias as VT, "coarse" VF, "fine" VF, or asystole. Their responses were compared with the qantitative measures. Interphysician agreement was assessed with the kappa statistic. RESULTS: One thousand four hundred forty interpretations were analyzed. There was fair agreement between physicians about the classification of arrhythmias (kappa = 0.39). Mean values associated with coarse VF, fine VF, and asystole differed in all three quantitative measure categories. The decision whether to defibrillate was highly correlated with the distinction between VF and asystole (Pearson chi-square = 1,170.40, df = 1, p[two-sided] < 0.001). CONCLUSIONS: With only fair agreement on the threshold of fine VF and asystole, defibrillation decisions are largely subjective and caregiver-specific. These data suggest that quantitative measures of the VF waveform could augment the current standard of subjective classification of VF by emergency care providers.

Two-thumb vs. two-finger chest compression in an infant model of prolonged cardiopulmonary resuscitation.

OBJECTIVE: Previous experiments in the authors' swine lab have shown that cardiopulmonary resuscitation (CPR) using two-thumb chest compression with a thoracic squeeze (TT) produces higher blood and perfusion pressures when compared with the American Heart Association (AHA)-recommended two-finger (TF) technique. Previous studies were of short duration (1-2 minutes). The hypothesis was that TT would be superior to TF during prolonged CPR in an infant model. METHODS: This was a prospective, randomized crossover experiment in a laboratory setting. Twenty-one AHA-certified rescuers performed basic CPR for two 10-minute periods, one with TT and the other with TF. Trials were separated by 2-14 days, and the order was randomly assigned. The experimental circuit consisted of a modified manikin with a fixed-volume arterial system attached to a neonatal monitor via an arterial pressure transducer. The arterial circuit was composed of a 50-mL bag of normal saline solution (air removed) attached to the manikin chest plate and connected to the transducer with a 20-gauge intravenous catheter and tubing. Rescuers were blinded to the arterial pressure tracing. Systolic blood pressure (SBP), diastolic blood pressure (DBP), and mean arterial pressure (MAP) were recorded in mm Hg, and pulse pressures (PPs) were calculated. Data were analyzed with two-way repeated-measures analysis of variance. Sphericity assumed modeling, with Greenhouse-Geisser and Huynh-Feldt adjustments, was applied. RESULTS: Marginal means for TT SBP (68.9), DBP (17.6), MAP (35.3), and PP (51.4) were higher than for TF SBP (44.8), DBP (12.5), MAP (23.3), and PP (32.2). All four pressures were significantly different between the two techniques (p< or =0.001). CONCLUSION: In this infant CPR model, TT chest compression produced higher MAP, SBP, DBP, and PP when compared with TF chest compression during a clinically relevant duration of prolonged CPR.

Ventricular fibrillation exhibits dynamical properties and self-similarity.

Electrocardiographic recordings of ventricular fibrillation (VF) appear chaotic. Previous attempts to characterize the chaotic nature of VF have relied on peak-to-peak intervals [Witkowski et al., Phys. Rev. Lett. 1995;75(6):1230-3; Garfinkel et al., J. Clin. Investig. 1997;99(2):305-314; Hastings et al., Proc. Natl. Acad. Sci. USA 1996;93:10495-9], the frequency spectrum [Goldberger et al., 1986;19:282-289] or other derived measures [Kaplan and Cohen, Circ. Res. 1990;67:886-92], with results that demonstrate some characteristics of chaos. We have sought to determine whether VF is chaotic rather than random and whether the waveform can be described quantitatively using the tools of fractal geometry. We have constructed an attractor, measured the correlation dimensions, estimated the embedding dimension and measured Lyapunov exponents. When the digitized waveform is analyzed directly, VF exhibits nonrandom, chaotic behavior over a decade of sampling frequency. Within the scaling range we have estimated the Hurst exponent, and the self-similarity dimension of the VF waveform, supporting the presence of chaotic dynamics. Furthermore, these characteristics are measurable in a porcine model of VF under different recording conditions, and in VF recordings taken from human subjects immediately prior to defibrillation. Analyses of the Hurst exponents and self-similarity dimensions are correlated with the duration of VF, which may have clinical applications.

Scaling structure of electrocardiographic waveform during prolonged ventricular fibrillation in swine.

Ventricular fibrillation (VF) is the most common arrhythmia causing sudden cardiac death. However, the likelihood of successful defibrillation declines with increasing duration of VF. Because the morphology of the electrocardiogram (ECG) waveform during VF also changes with time, this study examined a new measure that describes the VF waveform and distinguishes between early and late VF. Surface ECG recordings were digitized at 200 samples/s from nine swine with induced VF. A new measure called the scaling exponent was calculated by examining the power-law relationship between the summation of amplitudes of a 1,024-point (5.12 second) waveform segment and the time scale of measurement. The scaling exponent is a local estimate of the fractal dimension of the ECG waveform. A consistent power-law relationship was observed for measurement time scales of 0.005-0.040 seconds. Calculation of the scaling exponent produced similar results between subjects, and distinguished early VF (< 4-minute duration) from late VF (> or = 4-minute duration). The scaling exponent was dependent on the order of the data, supporting the hypothesis that the surface ECG during VF is a deterministic rather than a random signal. The waveform of VF results from the interaction of multiple fronts of depolarization within the heart, and may be described using the tools of nonlinear dynamics. As a quantitative descriptor of waveform structure, the scaling exponent characterizes the time dependent organization of VF.

Combination pharmacotherapy with delayed countershock vs standard advanced cardiac life support after prolonged ventricular fibrillation.

OBJECTIVE: To test the hypothesis that combination pharmacotherapy with delayed countershock would produce higher rates of return of spontaneous circulation (ROSC) and one-hour survival when compared with standard Advanced Cardiac Life Support (ACLS) therapy. METHODS: A prospective, block-randomized, blinded, laboratory experiment was conducted in an established swine model of prolonged ventricular fibrillation (VF). Fifty-six female domestic swine were anesthetized, instrumented, and shocked into VF with a bipolar pacing catheter. The VF was untreated for 8 minutes, then basic CPR was done mechanically for 1 minute. At 9 minutes of VF, the animals were randomized to treatment with one of seven therapies: group 1, combination pharmacotherapy with epinephrine (0.20 mg/kg), lidocaine (1.0 mg/kg), bretylium (5.0 mg/kg), propranolol (1.0 mg), and U-74389G (3.0 mg/kg); group 2, epinephrine (0.20 mg/kg); group 3, lidocaine (1.0 mg/kg) and bretylium (5.0 mg/kg); group 4, propranolol (1.0 mg); group 5, U-74389G (3.0 mg/kg); group 6, normal saline solution (volume equal to that for group 1); and group 7, standard ACLS (first countershock at 9 minutes of VF). Initial countershocks for groups 1-6 were given after 11 minutes of VF. Data were analyzed with two-tailed Fisher's exact test, with alpha set at 0.05. RESULTS: Return of spontaneous circulation occurred in group 1 = 8/8 (100%); group 2 = 7/8 (88%); group 3 = 3/8 (38%); group 4 = 3/8 (38%); group 5 = 5/8 (63%); group 6 = 4/8 (50%); and group 7 = 3/8 (38%). One-hour survival occurred in group 1 = 8/8 (100%); group 2 = 5/8 (63%); group 3 = 2/8 (25%); group 4 = 2/8 (25%); group 5 = 3/8 (38%); group 6 = 2/8 (25%); and group 7 = 1/8 (13%). CONCLUSIONS: Combination pharmacotherapy with delayed countershock (group 1) produced significantly higher rates of ROSC (p = 0.03) and one-hour survival (p = 0.001) when compared with standard ACLS in this porcine model of prolonged VF.

Evaluation of biphasic transthoracic defibrillation in an animal model of prolonged ventricular fibrillation.

OBJECTIVES: To determine whether a biphasic defibrillation waveform (BDW) would produce a superior rate of converting prolonged ventricular fibrillation (VF) into a perfusing rhythm and delay the occurrence of asystole and/or pulseless electrical activity (PEA) during the resuscitation attempt, when compared with a monophasic defibrillation waveform (MDW). METHODS: The authors performed a prospective, randomized, blinded experiment using an established swine model of prolonged VF. Thirty-four mixed-breed domestic swine (mean mass 22.9 kg) were sedated (ketamine/xylazine), anesthetized (isoflurane), and intubated. Aortic and femoral venous catheters were placed. ECG was monitored continuously. The animals were shocked into VF (3-s, 100-mA, 60-Hz shock), and were untreated for 8 minutes. Advanced Cardiac Life Support (ACLS) began with 1 minute of standardized (Thumper) chest compressions and ventilation. The animals were randomized to treatment with either BDW or MDW. Standard ACLS protocols were followed. The energy sequence was 2.5 J/kg first, 3.5 J/kg second, and 4.5 J/kg for all subsequent shocks. Outcome variables were time to event of asystole/PEA, return of spontaneous circulation (ROSC), and one-hour survival. Data were analyzed with two-tailed Fisher's exact test and Kaplan-Meier survival plots (alpha = 0.05). RESULTS: ROSC occurred more frequently in the BDW group (7/17) compared with the MDW group (1/17); p = 0.04. Survival analysis showed that the BDW significantly delayed the occurrence of asystole/PEA during the resuscitation attempt when compared with the MDW; log-ranked p = 0.02. One-hour survival rates (5/17 BDW and 1/17 MDW, p = 0.17) did not differ. CONCLUSIONS: BDW resulted in a superior rate of ROSC and delay in the occurrence of asystole/ PEA during the resuscitation attempt when compared with MDW.

Statistical methodology: IX. survival analysis.

Survival analysis is a group of statistical methods used to analyze data representing the time to an event of interest, e.g., the duration of survival after an out-of-hospital cardiac arrest or the length of time a patient stays in the ED. Survival analysis properly accounts for patients who are lost to follow-up and for patients who have not yet experienced the event of interest at the end of the study's observation period (censored data). This article acquaints the reader with the terminology, methodology, and limitations of survival analysis. Specific methods discussed include life tables, the Kaplan-Meier product limit estimate, the log-rank test, and the multivariate Cox proportional hazards model.

Experimental tricyclic antidepressant toxicity: a randomized, controlled comparison of hypertonic saline solution, sodium bicarbonate, and hyperventilation.

STUDY OBJECTIVE: We sought to compare the effects of hypertonic sodium chloride solution (HTS), sodium bicarbonate solution, and hyperventilation (HV) on severe tricyclic antidepressant (TCA) toxicity in a swine model. METHODS: Twenty-four mixed-breed, domestic swine of either sex were given an intravenous infusion of nortriptyline (NT) until development of both a QRS duration longer than 120 ms and a systolic blood pressure (SBP) less than or equal to 50 mm Hg. Animals were randomly assigned to 1 of 4 groups. On reaching toxicity, the control group received 10 mL/kg of 5% dextrose in water (D5W); the HTS group received 10 mL/kg of 7.5% NaCl solution (15 mEq Na+/kg); the NaHCO3 group received 3 mEq/kg of 8.4% sodium bicarbonate solution followed by enough D5W solution to equal 10 mL/kg of total volume; and the HV group was mechanically hyperventilated to maintain arterial pH between 7.50 and 7.60 and given 10 mL/kg of D5W. RESULTS: The mean SBP 10 minutes after treatment was 54+/-18 mm Hg in the control group, 134+/-21 mm Hg in the HTS group, 85+/-19 mm Hg in the NaHCO3 group, and 60+/-12 mm Hg in the HV group (P<.05). Mean QRS duration 10 minutes after treatment was 144+/-38 ms in the control group, 80+/-14 ms in the HTS group, 105+/-38 ms in the NaHCO3 group, and 125+/-46 ms in the HV group (P<.05). CONCLUSION: In this model of TCA, toxicity HTS was more effective than sodium bicarbonate. Hyperventilation had little effect. Sodium loading may be the most important factor in reversing TCA toxicity.

Comparison of automated and manual ventilation in a prehospital pediatric model.

OBJECTIVE: Portable transport ventilators (TV) and demand valves (DV) may be effective and easy-to-use alternatives to bag-valve (BV) for prehospital ventilation of adults. The purpose of the study was to determine whether such devices maintain arterial blood gases and airway pressures similar to those for BV in a pediatric swine model. METHOD: This study was a prospective, randomized, crossover design using immature swine (9.6 +/- 0.9 kg) to model ventilation in small children. Anesthetized, intubated, paralyzed, and cannulated animals were ventilated initially on standard mechanical hospital ventilation (HV). They were then assigned in random order to 10-minute intervals of ventilation using BV, TV, low-frequency jet ventilation (JV), and DV. Data were analyzed using repeated-measures ANOVA and Tukey multiple comparisons (alpha = 0.05). RESULTS: The PaO2 exceeded 90 mm Hg for all animal/ventilation combinations. Blood PaCO2 was lower for BV and DV than it was for TV, JV, or HV. In contrast, blood pH was higher for BV and DV than it was for TV, JV, or HV. Peak airway pressure was higher for BV than it was for HV, TV, or JV; it was lower for JV than it was for HV, TV, or BV. CONCLUSION: This animal model suggests that automated TV and JV may provide more effective ventilation of children than do manual BV or DV devices. Although promising, these findings require application in children under prehospital emergent conditions.

A comparison of paramedic didactic training hours and NREMT-P examination performance.

OBJECTIVE: The didactic hours required by paramedic training programs differ tremendously throughout the country. The authors hypothesized that a correlation exists between paramedic didactic training hours and pass/fail performance on the National Registry Examination. METHODS: The authors retrospectively examined the performance of every applicant for the National Registry Paramedic Examination in June 1996. Applicants were identified by their National Registry Identification Number. A database was established linking the applicants to whether they passed or failed the NREMT-P exam. The authors then searched through the National Registry's archives and pulled each applicant's original application papers. On each application is a space to indicate the number of didactic hours of training that the particular applicant underwent. This information was also included in the database. The numbers of didactic hours of training and pass/fail performance on the NREMT-P exam were compared by point biserial correlation. RESULTS: One thousand six hundred applicants applied for the National Registry Examination in June 1996. For the 1,553 applicants included in the study, the mean number of didactic training hours was 377.5, with a standard deviation of 167.0 and a range of 63 to 1,594. The correlation between paramedic didactic training hours and pass/fail performance on the National Registry Examination was rpb = 0.0493, p = 0.0522. CONCLUSION: There is no correlation between paramedic didactic training hours and pass/fail performance on the NREMT-P examination.

A randomized, crossover comparison of various ventilation devices in a swine pneumothorax model.

OBJECTIVE: Bag-valve-mask devices (BVMs), manually triggered ventilators (MTVs), and automatic transport ventilators (ATVs) are three available options for out-of-hospital artificial ventilation. Use of the MTV and the ATVs has been limited by concern for causing or worsening a pneumothorax (PTX). The purpose of this study was to compare the effects of ventilation with a BVM, ATV, and MTV in a swine PTX model. METHODS: This was a randomized, crossover study. Six fasted swine (21.2-24.5 kg, mean = 23.5 kg) were sedated and paralyzed. The swine were intubated with a cuffed endotracheal tube. A lung injury was created with a hemostat under direct visualization after the pleural cavity was entered by sharp dissection. A 16-Fr Foley catheter was inserted and sealed in the pleural space at the injury site. Prior to each trial, a PTX was created by instilling 300 mL of air through the Foley catheter. Each swine underwent 10-minute ventilation trials, at a rate of 12-14 breaths/min, with BVM, MTV, and ATV by the same investigator following each trial; PTX size was determined by withdrawing air through the Foley catheter. Analysis of variance for repeated measures (alpha = 0.05) was used for statistical analysis. RESULTS: There was no difference in the extent of PTX expansion produced by ventilation with BVM (339.8 +/- 35.9 mL), MTV 327.8 +/- 28.9 mL), or ATV (321.8 +/- 22.2 ml). CONCLUSION: BVMs, MTVs, and ATVs do not differ in their effects on PTX volumes. Concern for excessive expansion of PTX by MTVs and ATVs is not supported. Prospective evaluation is warranted to determine the efficacy of these ventilation techniques.

Noninvasive cerebral cooling in a swine model of cardiac arrest.

OBJECTIVE: Mild cerebral hypothermia improves neurologic outcome in animals resuscitated from cardiac arrest. This study examined whether one practical external cooling method, i.e., local application of ice to the heads and necks of swine, during resuscitation induces cerebral cooling. METHODS: Local external cerebral cooling was examined in a prospective laboratory investigation using 24 female swine in a model of cardiac arrest. The swine were randomized into hypothermia and normothermia groups. Intracerebral temperature was measured in the parietal cortex. Eight minutes after induction of ventricular fibrillation, chest compressions and mechanical ventilation were initiated. The hypothermia group was treated with 1,500 mL of ice in plastic bags applied to the head and neck, while the normothermia group received no extra interventions. Data were analyzed using repeated-measures ANOVA. RESULTS: In the normothermia group, there was no significant change in nasopharyngeal (-0.8 +/- 0.6 degree C), intracerebral (-0.6 +/- 0.8 degree C), or esophageal (-0.2 +/- 0.6 degree C) temperatures during 20 minutes of resuscitation. However, in the hypothermia group, application of ice during resuscitation significantly reduced nasopharyngeal (-2.9 +/- 1.4 degrees C), intracerebral (-2.1 +/- 0.6 degrees C), and esophageal (-1.4 +/- 0.8 degrees C) temperatures. CONCLUSIONS: External application of ice packs during resuscitation effectively reduced intracerebral temperatures in swine by an amount that improved neurologic outcomes in previous large animal studies. These data suggest that clinically significant cerebral cooling could be accomplished with a noninvasive, inexpensive, and universally available intervention. Further studies are required to assess the clinical feasibility and therapeutic efficacy of this intervention.