Severe hypoglycemia-induced sudden death is mediated by both cardiac arrhythmias and seizures

Abstract

We previously demonstrated that insulin-induced severe hypoglycemia-associated sudden death is largely mediated by fatal cardiac arrhythmias. In the current study, a pharmacological approach was taken to explore the potential contribution of hypoglycemic seizures and the sympathoadrenergic system in mediating severe hypoglycemia-associated sudden death. Adult Sprague-Dawley rats were randomized into one of four treatment groups: 1) saline (SAL), 2) anti-arrhythmic (β₁ blocker atenolol), 3) antiseizure (levetiracetam), and 4) combination antiarrhythmic and antiseizure (β₁ Blocker+Levetiracetam). All rats underwent hyperinsulinemic severe hypoglycemic clamps for 3.5 h. When administered individually during severe hypoglycemia, β₁ blocker reduced 2nd and 3rd degree heart block by 7.7- and 1.6-fold, respectively, and levetiracetam reduced seizures 2.7-fold, but mortality in these groups did not decrease. However, it was combined treatment with both β₁ blocker and levetiracetam that remarkably reduced seizures and completely prevented respiratory arrest, while also eliminating 2nd and 3rd degree heart block, leading to 100% survival. These novel findings demonstrate that, in mediating sudden death, hypoglycemia elicits two distinct pathways (seizure-associated respiratory arrest and arrhythmia-associated cardiac arrest), and therefore, prevention of both seizures and cardiac arrhythmias is necessary to prevent severe hypoglycemia-induced mortality.

Keywords: cardiac arrhythmias; diabetes; hypoglycemia; seizures; sudden death.

Fig. 1.

Protocol timeline and treatment groups. A: on the day of the hyperinsulinemic hypoglycemic clamp, after the distal ends of vessel catheters were externalized, rats were allowed 1½ h to rest. The basal period samples were taken just before −150 min, before drug infusions. Rats were either infused with saline or β₁ blocker intravenously. After 30 min of infusion, a fixed insulin infusion (0.2 U·kg⁻¹·min⁻¹), and variable dextrose infusions were started. Saline or β₁ blocker was infused continuously throughout the clamp. About 30 min before the start of severe hypoglycemia, saline or levetiracetam infusion was started intravenously and continued throughout the duration of the clamp. Glucose levels were gradually reduced, and once glucose levels were 10–15 mg/dl, severe hypoglycemia was maintained for 210 min. Continuous ECGs were obtained throughout the clamp. Research design included four treatment groups receiving intravenous infusions of either saline, β₁ blocker, levetiracetam, or combined β₁ blocker+levetiracetam.

Fig. 2.

Hyperinsulinemic/severe hypoglycemic clamp. A: glucose levels were carefully lowered over a 120-min period. Once glucose levels were 10–15 mg/dl (time 0), this level of severe hypoglycemia was maintained for 210 min. By experimental design, blood glucose levels were equally matched among the saline (SAL; ○), β₁ blocker (■), levetiracetam (▲), and β₁ Blocker+Levetiracetam (▼). The basal period samples were taken just before −150 min, before drug infusions. B: glucose infusion rates during the hypoglycemic clamp were similar among the groups. C: epinephrine was similar among the groups during the basal period (open bar) and increased to a similar extent during severe hypoglycemia (solid bar) in all groups compared with their respective basal values. D: glucagon levels also increased from similar baseline to a similar extent in all groups during severe hypoglycemia. E: in the first 30 min of β₁ blocker infusion, heart rate decreased in both the β₁ blocker and β₁ Blocker+Levetiracetam Groups. Heart rate remained significantly lower in these two groups compared with SAL and levetiracetam. Compared with basal values, all groups showed a decline in heart rate during severe hypoglycemia (SAL: 272 ± 9; β₁ Blocker: 209 ± 5, Levetiracetam: 268 ± 6, β₁ Blocker+Levetiracetam: 204 ± 3 beats/min), with heart rate lower in the β₁ Blocker and β₁ Blocker+Levetiracetam Groups. *P < 0.05, RM-ANOVA. F: QTc prolongation occurred in all groups during severe hypoglycemia, but the extent of prolongation was significantly blunted with β₁ Blocker (172 ± 4 ms) and β₁ Blocker+Levetiracetam (172 ± 2 ms) infusions compared with SAL (203 ± 6 ms), and Levetiracetam (193 ± 3 ms). *P < 0.001, repeated-measures ANOVA. n = 13–15/group. Data are expressed as means ± SE.

Fig. 3.

Mortality and arrhythmias during severe hypoglycemia (time 0–210 min). A: percentage of mortality due to severe hypoglycemia was similar in the SAL (47%, open bar), β₁ Blocker (46%, solid bar), and Levetiracetam (46%, horizontal lines) Groups. β₁ Blocker+Levetiracetam (diagonal slash) completely prevented mortality. *P < 0.01, Fisher exact test, Freeman Halton extension. Seven of 15 SAL rats died, 6 of 13 β₁ blocker rats died, 6 of 13 Levetiracetam rats died, 0 of 13 β₁ Blocker+Levetiracetam rats died. B: premature ventricular contractions (PVCs) were reduced with β₁ Blocker and β₁ Blocker+Levetiracetam compared with SAL and Levetiracetam. *P < 0.01; ANOVA. C: 1st degree heart block (prolongation of the PR interval) tended to be reduced with β₁ blocker and β₁ Blocker+Levetiracetam and increased with Levetiracetam, but the differences did not reach statistical significance. D: frequency of 2nd degree heart block was decreased with β₁ blocker and β₁ Blocker+Levetiracetam compared with SAL and Levetiracetam. *P < 0.05, ANOVA. E: 3rd degree heart block was similar among the SAL (27%), β₁ blocker (17%), and Levetiracetam (50%) Groups, whereas β₁ Blocker+Levetiracetam prevented 3rd degree heart block. *P < 0.039; Fisher exact test, Freeman-Halton extension. F: overall chance of mortality was highly correlated with increased number of 2nd degree heart block per animal. A few rats died without experiencing 2nd degree heart block. However, if rats had more than 31 total 2nd degree heart blocks, mortality increased 100%. For rats that had 0 heart blocks, 3/27 total died. For rats with 1 heart block, 3/14 died. For rats with 5 heart blocks, 3/9 died. For rats with 11 heart blocks, 4/7 died. For rats that had more than 30 heart blocks, 3/3 died. *P < 0.002, Pearson correlation. n = 13–15/group. Data are expressed as means ± SE.

Fig. 4.

Representative ECG tracings during severe hypoglycemia. During prolongation of severe hypoglycemia, rats treated with SAL or levetiracetam exhibited increased frequency of 2nd degree heart block (dropped QRS complex indicated by the arrows). In the β₁ Blocker and β₁ Blocker+Levetiracetam Groups, the frequency of 2nd degree heart block was markedly decreased.

Fig. 5.

Seizure analysis during the final 30 min before death or the final 30 min of the clamp. A: duration of all seizures was significantly reduced in the Levetiracetam (16 ± 8 s) and β₁ Blocker+Levetiracetam (6 ± 6 s) Groups compared with the SAL (80 ± 33 s) and β₁ Blocker (43 ± 22 s) Groups. *P < 0.01; ANOVA. B: type 1 seizures occurred similarly in all groups. Rats that lived are indicated by open bars, whereas rats that died are indicated by hatched bars. C: Type 2 seizures were mostly present in the SAL group. Levetiracetam reduced the incidence of Type 2 seizures. D: duration of Type 3 seizures were markedly longer in rats that died (diagonal slash bar) in the SAL and β₁ blocker groups (note change in y-axis scale) compared with rats that lived (open bar). Levetiracetam significantly reduced the duration of Type 3 seizures. *P < 0.05 vs. rats that lived; #P < 0.05 vs. saline, ANOVA. E: Type 4 seizures were not prevented with Levetiracetam. F: in all of the experiments, seizure duration was associated with an increased mortality risk. Although some rats died without experiencing seizures, seizures lasting 10 s were associated with 81% mortality, and seizures lasting more than 40 s were associated with 100% chance of mortality. Out of the rats that had 0 seizures, 13/38 total rats died. For rats that had 1 s-long seizures, 13/18 total rats died. For rats with seizures more than 11 s long, 10/12 rats died. Seven out of 7 rats died with seizures longer than 40 s. P < 0.03, Pearson correlation. n = 13–15/group. Data are expressed as means ± SE.

Fig. 6.

Seizures, arrhythmias, heart rate, and respiration. To identify the timing and sequence of preterminal events, data are replotted as time before the end of the experiment in rats that lived (◇) or time before death (◆). Thus, time 0 represents the end of experiment (either sudden death or 210 min of severe hypoglycemia). A: in the saline-treated rats, duration of seizures increased toward the end of the experiment. 2nd degree heart block preceded 3rd degree heart block, the incidence of which peaked just before death. Heart rate sharply decreased following 3rd degree heart block. Respiration slowly decreased throughout the clamp, but preterminal respirations precipitously declined following severe seizures or fatal cardiac arrhythmias. B: in β₁ blocker-treated rats, seizure duration increased toward the end of the clamp and just before sudden death. 2nd degree heart block was nearly completely prevented, but 3rd degree heart block occurred in two of the rats that died. Heart rate did not decrease in the rats that died until respiratory depression occurred. Respirations slowly decreased during the hypoglycemic clamp, but consistent with impending respiratory arrest, preterminal respirations sharply declined in rats following prolonged seizures. C: in rats that received levetiracetam, seizure duration was markedly diminished; however, 2nd and 3rd degree heart block increased toward the end of the experiment. Rats that died had a precipitous drop in heart rate. Respirations slowly decreased during the hypoglycemic clamp. Immediately following fatal cardiac arrhythmias (3rd degree heart block), respiratory arrest occurred in the absence of prolonged seizures. D: rats that received β₁ Blocker+Levetiracetam had a reduction in seizures and heart block, and all treated rats survived. Heart rate and respirations did decrease slowly during severe hypoglycemia, but none of the rats experienced a sharp decline in heart rate or respiration characteristic of cardiorespiratory arrest. n = 13–15/group. Data are expressed as means ± SE.

Fig. 7.

Mechanisms of hypoglycemia-induced mortality. In response to insulin-induced hypoglycemia, it is hypothesized that two distinct mechanistic pathways led to sudden death. Hypoglycemia induced CNS effects led to coma and short-duration mild seizures, which were not fatal. However, longer-duration, severe seizures led to respiratory arrest and sudden death. From a cardiac perspective, hypoglycemia caused 2nd degree heart block, mediated through the sympathoadrenal activation of β₁ adrenergic receptors. This 2nd degree heart block often reverted back to normal sinus rhythm, and the rat survived. However, when 2nd degree heart block was followed by 3rd degree heart block, rats went into a fatal bradycardic rhythm, which led to sudden death. In this study, the antiseizure drug levetiracetam prevented seizures and respiratory arrest, but the rats died from cardiac arrhythmias. Conversely, treatment with β₁ blocker markedly reduced heart block, but the rats died from severe seizure-induced respiratory arrest. Combined treatment with antiseizure and antiarrhythmic drugs resulted in a 100% survival rate. CNS, central nervous system.