Clinical paperAlteration in transthoracic impedance following cardiac surgery
Introduction
A cardiac arrest following cardiac surgery is not an uncommon event, with an incidence of approximately 0.5–2% in the immediate post-operative period.1, 2, 3 Electrical cardioversion is necessary to terminate ventricular fibrillation/pulse less ventricular tachycardia. Successful electrical cardioversion by transthoracic defibrillation is dependent upon delivery of sufficient current through the heart to depolarize a critical mass of myocardium.4 While weak defibrillation shocks are likely to fail, sufficiently strong defibrillation shocks can cause temporary or permanent damage to the heart.5, 6 Therefore, any factor affecting trans-myocardial current may have an adverse effect on the success of defibrillation. The current resuscitation guidelines from the European Resuscitation Council have increased the energy settings for defibrillation following cardiac surgery. According to these guidelines, the energy level for the first shock has been increased from 200 to 360 J monophasic shock (150–200 J biphasic).7
Current flow during cardioversion is determined by the operator-selected electrical energy (J) and the impedance or resistance (Ω). Current is related to these two factors by the equation: Im α√E/Z, where Im is the peak discharge current; Z is the impedance and E the electrical energy selected.8 It is documented that transthoracic impedance (TTI) is an important factor determining trans-myocardial current.9, 10 While high impedance alters monophasic defibrillation waveforms, impedance compensatory mechanisms have been incorporated in the newer biphasic defibrillators.11
Cardiac surgery is routinely performed with cardiopulmonary bypass (CPB), which initiates an inflammatory response.12 It results in capillary fluid leak secondary to haemodilution, inflammation and hypothermia.13 There is evidence that increases in tissue blood volume contributes to a decline in TTI.14 However, it is not known whether CPB has any immediate impact on TTI. A previous study that compared pre-operative TTI to the values taken 3–5 days following median sternotomy showed a decline in TTI. They, however, did not perform TTI measurements within the first 24 h following cardiac surgery.15 We hypothesize that TTI will reduce within the first 24 h after cardiac surgery using CPB.
Section snippets
Study patients
Following Local Research Ethics Committee approval, 44 patients undergoing first time cardiac surgery were recruited. The power of the study was calculated using the specialist sample size statistical software package nQuery Advisor® Version 6.0. According to the power calculation, a sample size of 44 would have 90% power to detect difference of 0.5 S.D. or more in the change of TTI before and after bypass (assuming a simple paired t-test is used with the conventional 5% significance level).
Results
The data from 40 patients was included in the study. The demographic and operative data are as in Table 1.
TTI values showed a statistically significant drop from pre-operative to post-operative values (with or without PEEP) at all intervals (p < 0.001). (Table 2).
The continuing linear downward trend in TTI from pre-operative to post-operative values was also of high significance (p < 0.001) (repeated measures ANOVA) (Figure 1).
Operation time was not significantly related to TTI at 1 h (p = 0.66), 4 h (p
Discussion
Cardiac arrest following major cardiac surgery is relatively common with a reported incidence of 0.7% in the first 24 h.3 A successful defibrillation of the shockable rhythms needs delivery of an adequate defibrillation dose. While too low currents are likely to fail to defibrillate, too high currents can lead to cardiac damage.6 The new guidelines from the European Resuscitation Council have revised energy settings for defibrillation in patients following cardiac surgery.7 The recommended
Conclusion
Our study confirms that TTI reduces significantly (up to 30%) in the immediate post-operative period after cardiac surgery. This state may favour defibrillation by lower energy shocks. However, further studies may be required to elaborate optimum energy settings for defibrillation after cardiac surgery.
Limitations of the study
This study failed to show any exact cause for a reduction in TTI following cardiac surgery. The inflammation associated with sternotomy and pericardium as well as the pulmonary changes may be the most probable causes for a reduced TTI. It may be that sample size was not sufficient to get any significant correlation with the fluid balance or other variables. Also, that in the short time period for the study, it was not possible to obtain sufficient numbers of off pump cardiac surgical cases as a
Conflict of interest
None.
Acknowledgment
We are thankful to Mrs Julie Morris for her kind support during statistical analysis.
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