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We therefore tested the hypothesis that increasing maximum f<jats:sub>H<\/jats:sub>may increase maximum Q\u0307. To this end, we investigated the effects of elevating f<jats:sub>H<\/jats:sub>with right atrial pacing on Q\u0307 in the American alligator ( Alligator mississippiensis) at rest and while swimming. During normal swimming, Q\u0307 increased entirely by virtue of a tachycardia (29\u2009\u00b1\u20091 to 40\u2009\u00b1\u20093 beats\/min), whereas V<jats:sub>s<\/jats:sub>remained stable. In both resting and swimming alligators, increasing f<jats:sub>H<\/jats:sub>with right atrial pacing resulted in a parallel decline in V<jats:sub>s<\/jats:sub>that resulted in an unchanged cardiac output. In swimming animals, this reciprocal relationship extended to supraphysiological f<jats:sub>H<\/jats:sub>(up to ~72 beats\/min), which suggests that maximum f<jats:sub>H<\/jats:sub>does not limit maximum cardiac output and that f<jats:sub>H<\/jats:sub>changes are secondary to the peripheral factors (for example vascular capacitance) that determine venous return at rest and during exercise.<\/jats:p>","DOI":"10.1152\/ajpregu.00027.2018","type":"journal-article","created":{"date-parts":[[2018,4,25]],"date-time":"2018-04-25T15:17:07Z","timestamp":1524669427000},"page":"R296-R302","source":"Crossref","is-referenced-by-count":17,"title":["Maximum heart rate does not limit cardiac output at rest or during exercise in the American alligator (<i>Alligator mississippiensis<\/i>)"],"prefix":"10.1152","volume":"315","author":[{"ORCID":"https:\/\/orcid.org\/0000-0002-3782-1641","authenticated-orcid":false,"given":"William","family":"Joyce","sequence":"first","affiliation":[{"name":"Department of 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