Spontaneous beat-to-beat variation of PR interval in normal children.

MLA

Donnerstein, R. L., et al. “Spontaneous Beat-to-Beat Variation of PR Interval in Normal Children.” The American Journal of Cardiology, vol. 66, no. 7, Sept. 1990, pp. 753–54. EBSCOhost, https://doi.org/10.1016/0002-9149(90)91143-t.

APA

Donnerstein, R. L., Scott, W. A., & Lloyd, T. R. (1990). Spontaneous beat-to-beat variation of PR interval in normal children. The American Journal of Cardiology, 66(7), 753–754. https://doi.org/10.1016/0002-9149(90)91143-t

Chicago

Donnerstein, R L, W A Scott, and T R Lloyd. 1990. “Spontaneous Beat-to-Beat Variation of PR Interval in Normal Children.” The American Journal of Cardiology 66 (7): 753–54. doi:10.1016/0002-9149(90)91143-t.