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Heart rate changes mediate the embryotoxic effect of antiarrhythmic drugs in the chick embryo A significant increase in cardiovascular medication use during pregnancy has occurred in recent years but only limited evidence on its safety profile is available. We hypothesized that drug-induced bradycardia is the leading mechanism of developmental toxicity. We tested metoprolol, carvedilol, or ivabradine for embryotoxicity and their acute effect on chick embryonic model. We used video microscopy and ultrasound biomicroscopy. Significant dose-dependent mortality was achieved in embryos injected with carvedilol and ivabradine. In ED4 embryos, metoprolol, carvedilol and ivabradine reduced the heart rate by 33%, 27%, and 55%, respectively, compared to controls (6%). In ED8 embryos this effect was more pronounced with a heart rate reduction by 71%, 54%, 53%, respectively (controls 36%). Cardiac output decreased in all tested groups but only proved significant in the metoprolol group in ED8 embryos. The number of -adrenergic receptors showed a downward tendency during embryonic development but a negative chronotropic effect of tested drugs was increasingly pronounced with embryonic maturity. This effect was associated with reduced cardiac output in chick embryos, probably leading to premature death....