The APS Archive of Teaching Resources is a repository of case histories, simulations, figures, lectures and course syllabi, animations, and links to physiology teaching resources for use by APS members and other educators. The Archive is a searchable database that can be used by teachers at all levels (K-12, undergraduate, graduate, and medical school) to enhance and supplement their current teaching resources and is part of the National Science Digital Library Project and the BiosciEd Net collaborative ( http://www.biosciednet.org ). The APS Archive of Teaching Resources was established as an initiative of the APS Education Committee and APS Council with additional support from the National Science Foundation (DUE 0226185). Authors submitting materials to the Archive for review and inclusion have the option of developing an abstract for publication in Advances in Physiology Education. The following abstracts are from items that have been accepted into the Archive following peer review.

Standard textbooks of physiology and medicine mention that heart rate (HR) is readily calculated from the ECG as follows: HR = 1,500/RR interval in millimeters, HR = 60/RR interval in seconds, or HR = 300/number of large squares between successive R waves. In each case, the authors are actually referring to instantaneous HR, which is the number of times the heart would beat if successive RR intervals were constant. However, because the above formula is almost always mentioned, students determine HR this way without looking at the ECG any further. We found out that this is indeed the case by administering an ECG showing marked sinus arrhythmia to 90 students, including postgraduates in medicine and other medical specialties and undergraduate medical students. We found that 63 of 90 students used the above formula immediately without examining RR regularity and either underestimated or overestimated average HR. The purpose of this learning object (an article in .pdf format with illustrative ECG and detailed explanatory notes for the teacher as well as the learner) and the accompanying PowerPoint presentation (for use in the classroom) is to make clear the difference between instantaneous HR and average HR. The material provided pertains to the educational requirements of undergraduate as well as graduate medical students. We have four suggestions. First, that the term “instantaneous HR” be used regularly in textbooks and a distinction be made between this and “average HR.” Second, to determine HR from an ECG, RR variability must be assessed. If successive RR intervals appear relatively constant, then average HR is ∼1,500/RR interval in millimeters. Third, if RR intervals vary, average HR (ventricular rate) should be estimated by determining the number of RR intervals in a 10- or 20-s ECG strip and multiplying this by 6 or 3. Finally, if cardiac cycle duration changes significantly and abruptly, then HR should be calculated over shorter periods of time (it may be as short as one cardiac cycle) to correctly interpret underlying physiology.