The thick left ventricular wall of the giraffe heart normalises wall tension, but limits stroke volume and cardiac output

Morten Smerup, Mads Damkjær, Emil Brøndum, Ulrik T Baandrup, Steen Buus Kristiansen, Hans Nygaard, Christian Aalkjær, Cathrine Sauer, Rasmus Buchanan, Mads Frost Bertelsen, Kristine Østergaard, Carsten Grøndahl, Geoffrey Candy, J Michael Hasenkam, Niels H Secher, Peter Bie, Tobias Wang

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Abstrakt

Giraffes - the tallest extant animals on Earth - are renowned for their high central arterial blood pressure, which is necessary to secure brain perfusion. Arterial pressure may exceed 300 mmHg and has historically been attributed to an exceptionally large heart. Recently, this has been refuted by several studies demonstrating that the mass of giraffe heart is similar to that of other mammals when expressed relative to body mass. It thus remains unexplained how the normal-sized giraffe heart generates such massive arterial pressures. We hypothesized that giraffe hearts have a small intraventricular cavity and a relatively thick ventricular wall, allowing for generation of high arterial pressures at normal left ventricular wall tension. In nine anaesthetized giraffes (495±38 kg), we determined in vivo ventricular dimensions using echocardiography along with intraventricular and aortic pressures to calculate left ventricular wall stress. Cardiac output was also determined by inert gas rebreathing to provide an additional and independent estimate of stroke volume. Echocardiography and inert gas-rebreathing yielded similar cardiac outputs of 16.1±2.5 and 16.4±1.4 l min -1, respectively. End-diastolic and end-systolic volumes were 521±61 ml and 228±42 ml, respectively, yielding an ejection fraction of 56±4% and a stroke volume of 0.59 ml kg -1. Left ventricular circumferential wall stress was 7.83±1.76 kPa. We conclude that, relative to body mass, a small left ventricular cavity and a low stroke volume characterizes the giraffe heart. The adaptations result in typical mammalian left ventricular wall tensions, but produce a lowered cardiac output.

OriginalsprogEngelsk
TidsskriftThe Journal of Experimental Biology
Vol/bind219
Sider (fra-til)457-463
ISSN0022-0949
DOI
StatusUdgivet - 3. feb. 2016

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