Effects of stomach inflation on haemodynamic and pulmonary function during spontaneous circulation in pigs.

AIM: Stomach inflation during mask ventilation is frequent, but the effects on haemodynamic and pulmonary function are unclear. We evaluated the effects of stomach inflation on haemodynamic and pulmonary function during spontaneous circulation in a porcine model. METHODS: Randomised prospective animal study. After randomisation, in 23 domestic pigs the stomach was inflated every 90s with 0L (control; n=8), 0.5L (n=7) or 1L (n=8) ambient air. RESULTS: After 22.5min, i.e. 8.5L in the 0.5L and 17L in the 1L stomach inflation group, stomach inflation increased central venous pressure (median) (control: 10mmHg vs. 1L: 23mmHg, P<0.05) and mean pulmonary artery pressure (control: 24mmHg vs. 1L: 45mmHg, P<0.05). As a result stroke volume index decreased (control: 135mL/kg vs. 0.5L: 90mL/kg, P<0.05; vs. 1L: 72mL/kg, P<0.05). Stomach inflation also decreased static pulmonary compliance (control: 24mL/cmH(2)O vs. 0.5L: 8mL/cmH(2)O, P<0.05; vs. 1L: 3mL/cmH(2)O, P<0.05), which increased peak airway pressure (control: 28cmH(2)O vs. 0.5L: 69cmH(2)O, P<0.05; vs. 1L: 73cmH(2)O, P<0.05). Additionally, arterial oxygen partial pressure (control: 305mmHg vs. 0.5L: 140mmHg, P<0.05; vs. 1L: 21mmHg, P<0.05) and systemic oxygen delivery (control: 53mLO(2)/min vs. 1L: 19mLO(2)/min, P<0.05) decreased. Stomach inflation increased mortality (control: 0/8 vs. 1L: 5/8, P<0.05). CONCLUSIONS: Stomach inflation with 1L when compared to 0.5L increments resulted in faster haemodynamic and pulmonary failure and increased mortality. Stomach inflation may cause a hyper-acute abdominal compartment syndrome.

Comparison of mechanical characteristics of the human and porcine chest during cardiopulmonary resuscitation.

BACKGROUND: Most studies investigating cardiopulmonary resuscitation (CPR) interventions or functionality of mechanical CPR devices have been performed using porcine models. The purpose of this study was to identify differences between mechanical characteristics of the human and porcine chest during CPR. MATERIAL AND METHODS: CPR data of 90 cardiac arrest patients was compared to data of 14 porcine from two animal studies. Chest stiffness k and viscosity mu were calculated from acceleration and pressure data recorded using a Laerdal Heartstart 4000SP defibrillator during CPR. K and mu were calculated at chest compression depths of 15, 30 and 50mm for three different time periods. RESULTS: At a depth of 15mm porcine chest stiffness was comparable to human chest stiffness at the beginning of resuscitation (4.8 vs. 4.5N/mm) and clearly lower after 200 chest compressions (2.9 vs. 4.5N/mm) (p<0.05). At 30 and 50mm porcine chest stiffness was higher at the beginning and comparable to human chest stiffness after 200 chest compressions. After 200 chest compressions porcine chest viscosity was similar to human chest viscosity at 15mm (108 vs. 110Ns/m), higher for 30mm (240 vs. 188Ns/m) and clearly higher for 50mm chest compression depth (672 vs. 339Ns/m) (p<0.05). CONCLUSION: In conclusion, human and porcine chest behave relatively similarly during CPR with respect to chest stiffness, but differences in chest viscosity at medium and deep chest compression depth should at least be kept in mind when extrapolating porcine results to humans.

The effects of multiple infusion line extensions on occlusion alarm function of an infusion pump.

BACKGROUND: For anesthesia or conscious sedation of patients undergoing diagnostic or therapeutic procedures in computed tomography or magnetic resonance imaging scans, an extension of infusion lines for continuous drug delivery of anesthetics or vasopressors is often necessary. In this study, we tried to determine if the length of the infusion line influenced the time until an alarm sounded after occlusion at the end of the infusion line. METHODS: We connected 2 infusion pump systems of the same model with 1, 2 or 3 infusion lines in series or with a spiral nonkinking low compliance infusion line, and started the infusion for 60 s. The end of the infusion line was then occluded by turning a stopcock to occlude the fluid flow. A pressure sensor was connected to the infusion line to record the actual pressure change in the line. The time until the pressure occlusion alarm sounded was measured 5 consecutive times at flow rates of 5, 20, and 50 mL/h. RESULTS: When using a single infusion line, pressure occlusion alarms were triggered after 2.4 +/- 0.1 min for infusion pump 1 and 2.6 +/- 0.2 min for infusion pump 2 at 50 mL/h, after 6.6 +/- 0.4 min and 5.6 +/- 0.5 min at 20 mL/h, and after 23.0 +/- 2.8 min and 20.9 +/- 3.6 min at 5 mL/h, respectively. When adding a second infusion line, a pressure occlusion alarm was triggered after 27.1 +/- 1.8 min for infusion pump 1 (P = 0.1) and after 29.2 +/- 1.4 min for infusion pump 2 (P = 0.07) at 5 mL/h. With 3 infusion lines, the pressure occlusion alarm of infusion pumps 1 and 2 were significantly prolonged when compared with 1 infusion line and were released at 31.6 +/- 3.0 min (P = 0.01) and 35.1 +/- 1.1 min (P = 0.001) at 5 mL/h, respectively. The pressure level triggering an alarm ranged in both infusion pumps between about 900 and 1100 Mbar. CONCLUSIONS: When simulating low flow rate infusions (5 mL/h) as for vasopressor support, occlusion alarm time was critically prolonged, especially with an increased length of infusion lines.

Effects of stomach inflation on haemodynamic and pulmonary function during cardiopulmonary resuscitation in pigs.

AIM: Stomach inflation during cardiopulmonary resuscitation (CPR) is frequent, but the effect on haemodynamic and pulmonary function is unclear. The purpose of this study was to evaluate the effect of clinically realistic stomach inflation on haemodynamic and pulmonary function during CPR in a porcine model. METHODS: After baseline measurements ventricular fibrillation was induced in 21 pigs, and the stomach was inflated with 0L (n=7), 5L (n=7) or 10L air (n=7) before initiating CPR. RESULTS: During CPR, 0, 5, and 10L stomach inflation resulted in higher mean pulmonary artery pressure [median (min-max)] [35 (28-40), 47 (25-50), and 51 (49-75) mmHg; P<0.05], but comparable coronary perfusion pressure [10 (2-20), 8 (4-35) and 5 (2-13) mmHg; P=0.54]. Increasing (0, 5, and 10L) stomach inflation decreased static pulmonary compliance [52 (38-98), 19 (8-32), and 12 (7-15) mL/cmH(2)O; P<0.05], and increased peak airway pressure [33 (27-36), 53 (45-104), and 103 (96-110) cmH(2)O; P<0.05). Arterial oxygen partial pressure was higher with 0L when compared with 5 and 10L stomach inflation [378 (88-440), 58 (47-113), and 54 (43-126) mmHg; P<0.05). Arterial carbon dioxide partial pressure was lower with 0L when compared with 5 and 10L stomach inflation [30 (24-36), 41(34-51), and 56 (45-68) mmHg; P<0.05]. Return of spontaneous circulation was comparable between groups (5/7 in 0L, 4/7 in 5L, and 3/7 in 10L stomach inflation; P=0.56). CONCLUSIONS: Increasing levels of stomach inflation had adverse effects on haemodynamic and pulmonary function, indicating an acute abdominal compartment syndrome in this CPR model.

Improving countershock success prediction during cardiopulmonary resuscitation using ventricular fibrillation features from higher ECG frequency bands.

BACKGROUND: Countershock outcome prediction using ventricular fibrillation (VF) feature analysis needs undisturbed electrocardiogram (ECG) signals and therefore requires interruption of cardiopulmonary resuscitation (CPR). Features that originate from higher frequency bands of the VF power spectrum may be less affected by CPR artefacts and as such reduce cumulative hands-off intervals. MATERIALS AND METHODS: From 192 patients with in-hospital and out-of-hospital cardiac arrest, four countershock outcome prediction features (peak-peak amplitude, mean slope, median slope, power spectrum analysis) were analysed in 550 short time ECG records, each including a CPR corrupted and a subsequent undisturbed sequence. ECG features calculated from the main frequency band (0-26Hz) and from bandpass-filtered subbands (>10-26Hz) were compared using the similarity level method and differences in shock advice numbers. RESULTS: The feature similarity between ECG periods with and without CPR artefacts was higher in bandpass-filtered (Sim=0.79, 0.8, 0.78, 0.66) than in unfiltered ECG traces (Sim=0.58, 0.69, 0.68, 0.47). For the features evaluated, the difference in number of shock advices between subsequent traces with and without CPR artefact was significantly reduced using VF analysis from higher frequency bands. CONCLUSION: The accuracy of shock outcome prediction during CPR could be increased by using filtered ECG features from higher ECG subbands instead of features derived from the main ECG spectrum.

Spatial-temporal filter effect in a computer model study of ventricular fibrillation.

Prediction of countershock success from ventricular fibrillation (VF) ECG is a major challenge in critical care medicine. Recent findings indicate that stable, high frequency mother rotors are one possible mechanism maintaining VF. A computer model study was performed to investigate how epicardiac sources are reflected in the ECG. In the cardiac tissues of two computer models - a model with cubic geometry and a simplified torso model with a left ventricle - a mother rotor was induced by increasing the potassium rectifier current. On the epicardium, the dominant frequency (DF) map revealed a constant DF of 23 Hz (cubic model) and 24.4 Hz (torso model) in the region of the mother rotor, respectively. A sharp drop of frequency (3-18 Hz in the cubic model and 12.4-18 Hz in the torso model) occurred in the surrounding epicardial tissue of chaotic fibrillatory conduction. While no organized pattern was observable on the body surface of the cubic model, the mother rotor frequency can be identified in the anterior surface of the torso model because of the chosen position of the mother rotor in the ventricle (shortest distance to the body surface). Nevertheless, the DFs were damped on the body surfaces of both models (4.6-8.5 Hz in the cubic model and 14.4-16.4 Hz in the torso model). Thus, it was shown in this computer model study that wave propagation transforms the spatial low pass filtering of the thorax into a temporal low pass. In contrast to the resistive-capacitive low pass filter formed by the tissue, this spatial-temporal low pass filter becomes effective at low frequencies (tens of Hertz). This effect damps the high frequency components arising from the heart and it hampers a direct observation of rapid, organized sources of VF in the ECGs, when in an emergency case an artifact-free recording is not possible.

A comparison of the combination of epinephrine and vasopressin with lipid emulsion in a porcine model of asphyxial cardiac arrest after intravenous injection of bupivacaine.

BACKGROUND: In a porcine model, we compared the effect of the combination of vasopressin/epinephrine with that of a lipid emulsion on survival after bupivacaine-induced cardiac arrest. METHODS: After administration of 5 mg/kg of a 0.5% bupivacaine solution i.v., ventilation was interrupted for 2 +/- 0.5 (mean +/- SD) min until asystole occurred. Cardiopulmonary resuscitation (CPR) was initiated after 1 min of untreated cardiac arrest. After 2 min of CPR, 10 animals received, every 5 min, either vasopressin combined with epinephrine or 4 mL/kg of a 20% lipid emulsion. Three minutes after each drug administration, up to three countershocks (4, 4, and 6 J/kg) were administered; all subsequent shocks with 6 J/kg. Blood for determination of the plasma bupivacaine concentration was drawn throughout the experiment. RESULTS: In the vasopressor group, all five pigs survived, whereas none of five pigs in the lipid group had restoration of spontaneous circulation (P < 0.01). There was no significant difference between groups in the plasma concentration of total bupivacaine. CONCLUSION: In this model of a bupivacaine-induced cardiac arrest, the vasopressor combination of vasopressin and epinephrine compared with lipid emulsion resulted in higher coronary perfusion pressure during CPR and survival rates.

Estimation of the duration of ventricular fibrillation using ECG single feature analysis.

The duration of untreated ventricular fibrillation (VF) is of paramount importance for CPR success. Moreover, therapeutic interventions taking into account the interval between cardiac arrest onset and initiation of CPR improve outcome. This study was performed to investigate whether VF feature analysis could be used to estimate the duration of VF in patients with out-of-hospital cardiac arrest. Demographic data recorded according to the Utstein guidelines and ECG recordings of 376 cardiac arrest patients from three European areas were analysed. Ten features in the time and frequency domain derived from different sub-bands of the initial VF ECG (n=127) were evaluated. The correlation between VF ECG features and cardiac arrest times was investigated using Pearson's correlation coefficient in a subset of 40 patients with reliably estimated downtimes and artefact-free initial VF tracings. No significant correlation (p<.05) between any of the VF ECG features and downtime could be found. The duration of cardiac arrest could not be estimated reliably from human VF ECG single feature analysis.

Prediction of countershock success using single features from multiple ventricular fibrillation frequency bands and feature combinations using neural networks.

Targeted defibrillation therapy is needed to optimise survival chances of ventricular fibrillation (VF) patients, but at present VF analysis strategies to optimise defibrillation timing have insufficient predictive power. From 197 patients with in-hospital and out-of-hospital cardiac arrest, 770 electrocardiogram (ECG) recordings of countershock attempts were analysed. Preshock VF ECG features in the time and frequency domain were tested retrospectively for outcome prediction. Using band pass filters, the ECG spectrum was split into various frequency bands of 2-26 Hz bandwidth in the range of 0-26 Hz. Neural networks were used for single feature combinations to optimise prediction of countershock success. Areas under curves (AUC) of receiver operating characteristics (ROC) were used to estimate prediction power of single and combined features. The highest ROC AUC of 0.863 was reached by the median slope in the interval 10-22 Hz resulting in a sensitivity of 95% and a specificity of 50%. The best specificity of 55% at the 95% sensitivity level was reached by power spectrum analysis (PSA) in the 6-26 Hz interval. Neural networks combining single predictive features were unable to increase outcome prediction. Using frequency band segmentation of human VF ECG, several single predictive features with high ROC AUC>0.840 were identified. Combining these single predictive features using neural networks did not further improve outcome prediction in human VF data. This may indicate that various simple VF features, such as median slope already reach the maximum prediction power extractable from VF ECG.