Impaired heart rate regulation and depression of cardiac chronotropic and dromotropic function in polymicrobial sepsis

Abstract

The scope of cardiac pathophysiology in sepsis has not been fully defined. Accordingly, we evaluated the effects of sepsis on heart rate (HR), HR variability, and conduction parameters in a murine model of sepsis. Electrocardiograms were recorded noninvasively from conscious mice before and after cecal ligation and puncture (CLP) or sham surgery. Responses of isolated atria to tyramine and isoproterenol were quantified to assess the functional state of sympathetic nerves and postjunctional sensitivity to adrenergic stimulation. Cecal ligation and puncture mice had lower HR compared with sham at 16 to 18 h postsurgery (sham, 741 ± 7 beats/min; CLP, 557 ± 31 beats/min; n = 6/group; P < 0.001), and there was significant prolongation of the PR, QRS, and QTc intervals. Slowing of HR and conduction developed within 4 to 6 h after CLP and were preceded by a decrease in HR variability. Treatment of CLP mice with isoproterenol (5 mg/kg, intraperitoneally) at 25 h after surgery failed to increase HR or decrease conduction intervals. The lack of in vivo response to isoproterenol cannot be attributed to hypothermia because robust chronotropic and inotropic responses to isoproterenol were evoked from isolated atria at 25 °C and 30 °C. These findings demonstrate that impaired regulation of HR (i.e., reduced HR variability) develops before the onset of overt cardiac rate and conduction changes in septic mice. Subsequent time-dependent decreases in HR and cardiac conduction can be attributed to hypothermia and would contribute to decreased cardiac output and organ perfusion. Because isolated atria from septic mice showed normal responsiveness to adrenergic stimulation, we conclude that impaired effectiveness of isoproterenol in vivo can be attributed to reversible effects of systemic factors on adrenergic receptors and/or postreceptor signaling.

Fig. 1. Sepsis decreases heart rate and cardiac conduction in conscious mice

ECGs were recorded non-invasively and analyzed with eMouse software to determine heart rates (A) and PR (B), QRS (C), and QTc (D) intervals. Values are the mean ± SE (n=6 per group). #P<0.05 or ##P<0.005 compared to corresponding pre-surgery value (paired t-test).

Fig. 2. Sepsis does not affect the response of isolated atria to the indirectly-acting adrenergic agonist tyramine

Concentration-response curves are shown for the positive chronotropic (A) and inotropic (B) effects of tyramine in isolated atria from sham and CLP mice. Values are the mean ± SE (n=5 for sham and n=6 for CLP groups). Two-way ANOVA with repeated measures showed significant effects of concentration (P<0.001, A and B) but no effect of treatment or interaction between treatment and concentration (P>0.05 Sham versus CLP, A and B).

Fig. 3. Sepsis does not affect the response of isolated atria to the directly-acting adrenergic agonist isoproterenol

Concentration-response curves are shown for the positive chronotropic (A) and inotropic (B) effects of isoproterenol in isolated atria from sham and CLP mice. Values are the mean ± SE (n=5 for sham and n=6 for CLP groups). Two-way ANOVA with repeated measures showed significant effects of concentration (P<0.001, A and B) but no effect of treatment or interaction between treatment and concentration (P>0.05 Sham versus CLP, A and B).

Fig. 4. Heart rate, heart rate variability and body temperature decrease rapidly after the induction of sepsis

Graphs show the time course for early changes in heart rate (A), heart rate variability (B) and body temperature (C) after sham and CLP surgery. Values are the mean ± SE (n=7 per group for A and B, n=4 for C). For each parameter, no differences were deterred between groups prior to surgery (P>0.05). Post-surgical values for each parameter were evaluated by two-way ANOVA with repeated measures. A. Heart rate (HR): two-way ANOVA with repeated measures showed significant effects of treatment (F_1,12=28.13, P<0.0002), time (F_6,72=23.7, P<0.0001), and a treatment-time interaction (F_6,72=21.51, P<0.0001). B. Heart rate variability (HRV): two-way ANOVA with repeated measures showed a significant effect of treatment only (F_1,12=33.79, P<0.0001). C. Body temperature: two-way ANOVA with repeated measures showed significant effects of treatment (F_1,6=125.7, P<0.0001), time (F_6,36=13.14, P<0.0001), and a treatment-time interaction (F_6,36=15.97, P<0.0001). Sidak's multiple comparisons test was used to identify differences between sham and CLP groups at each post-surgical time.#P<0.05, ^**P<0.01, ##P<0.005, ###P<0.0005, ***P<0.0001.

Fig. 5. Cardiac conduction decreases rapidly after the induction of sepsis

Graphs show the time course for early changes in PR (A), QRS (B), and QTc (C) intervals after sham and CLP surgery. Values are the mean ± SE (n=7 per group). For each parameter, no differences were detected between groups prior to surgery (P>0.05). Post-surgical values for each parameter were evaluated by two-way ANOVA with repeated measures. A. PR interval: two-way ANOVA with repeated measures showedsignificant effects of treatment (F_1,12=9.880, P<0.01), time (F_6,72=5.731, P<0.0001), and a treatment-time interaction (F_6,72=8.062, P<0.0001). B. QRS interval: two-way ANOVA with repeated measures showed a significant effect of time (F_6,72=7.185, P<0.0001) and a treatment-time interaction (F_6,72=4.949, P<0.001), but the treatment effect alone was not significant (F_1,12=3.827, P=0.0741). C. QTc interval: two-way ANOVA with repeated measures showed significant effects of treatment (F_1,12=39.88, P<0.0001), time (F_6.72=10.97, P<0.0001), and a treatment-time interaction (F_6,72=9.206, P<0.0001). Sidak's multiple comparisons test was used to identify differences between sham and CLP groups at each post-surgical time. #P<0.05, ^**P<0.01,***P<0.0001.

Fig. 6. Isoproterenol has no effect on heart rate or conduction intervals of conscious mice at 25 hours after CLP

Graphs show heart rate (A) and PR (B), QRS (C), and QTc (D) intervals in septic mice before and after the intraperitoneal injection of isoproterenol. Values are the mean ± SE (n=4 per group). One-way ANOVA with repeated measures was performed on each data set and, in each case, showed a significant effect of time. Post hoc testing showed that all values after baseline were different from the baseline value but not different from each other. Differences compared to baseline are indicated in the figure (#P<0.05, *P<0.01, ###P<0.05, ^**P<0.001, and ^***P<0.0001).

Fig. 7. Isolated atria show robust rate and contractile response to isoproterenol at reduced incubation temperature

Graphs show concentration-response curves for the positive chronotropic (A) and inotropic (B) effects of isoproterenol in control atria maintained at 30° C. Values are the mean ± SE (n=3). The -log EC50 values for the chronotropic and inotropic responses were: 9.077 ± 0.087 and 8.107 ± 0.382 M, respectively.