This is a classic example of a case where to credit "an invention" to a single person is often to overlook the contributions made by many other people, which may have been equally crucial.
According to Wikipedia, John Alexander MacWilliam reported in the British Medical Journal in 1889 of his experiments in which application of an electrical impulse to the human heart in asystole caused a ventricular contraction. In 1926, Mark Lidwill of Sydney's Royal Prince Alfred Hospital, and Edgar Booth of Sydney University, devised a portable apparatus that has been described as "the first pacemaker". Six years later, the American physiologist Albert Hyman described an electro–mechanical instrument which he referred to as an "artificial pacemaker". It was only in 1952 that the Canadian electrical engineer John Hopps designed and built the first external pacemaker, based upon observations by cardio–thoracic surgeons Wilfred Gordon Bigelow and John Callaghan at Toronto General Hospital.
Note that even in the development in which Bigelow was involved, the device was actually "designed and built" by John Hopps.
Wikipedia doesn't even enlighten us as to what "observations" Bill Bigelow actually made. A quick Google search took me to the US National Center for Biotechnology Information, which reports that "In 1949, during experimental surgery, [Bigelow] found he was able to restart a dog's heart by stimulating it at regular intervals with a probe. The following year he co-developed the first electronic heart pacemaker with Dr Callaghan and electrical engineer John Hopps."
It strikes me that Bigelow's observations were not that different from the ones that John MacWilliam made some sixty years earlier.
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