Factors affecting transthoracic impedance during electrical cardioversion

Successful cardioversion is dependent on the delivery of sufficient current. Current is determined by energy and transthoracic impedance (TTI). Our purpose was to assess factors affecting TTI in humans. Twenty-eight patients undergoing elective cardioversion were monitored up to 48 hours after shock delivery. We also studied 10 normal subjects, who did not receive shocks, for comparison. TTI declined 8% in the first hour in patients who received shocks, but also 6% in the normal subjects who wore the same pads for 1 hour but received no shocks. Twenty-four hours later, TTI returned to baseline in the nonshocked subjects but remained reduced (93% of control, p < 0.05) in the shocked patients. There was a correlation between the total energy delivered and the decline in TTI (r = 0.69). TTI was 9% lower at end-expiration than at end-inspiration (55 ± 3 vs 50 ± 3 ohms, mean ± standard error of the mean, p < 0.01, paired t test). In the normal subjects, when a nonsalt-containing coupling agent (ultrasound coupling gel) was compared with a salt-containing gel (Redux paste), TTI was 20% higher (65 ± 5 vs 54 ± 4 ohms, p < 0.01) with the nonsalt-containing gel. When no coupling agent was used, TTI was markedly higher, 160 ± 18 ohms (p < 0.01 vs control). After 1 hour, TTI decreased 6% in the normal subjects when salt-containing gel was used, but did not change when a nonsalt gel was used. Heavy pressure versus light pressure resulted in a 13% decline in TTI (54 ± 3 vs 62 ± 4 ohms, p < 0.01). It is concluded that the decline in TTI is in part due to the salt-containing coupling agent and in part to the direct effect of the shock. TTI in humans is affected by respiration, contact pressure, coupling agent and the total energy delivered.