How can a normal‐sized heart generate high blood pressure in the Giraffe?

Abstract

To secure cerebral perfusion, the giraffe heart generates a blood pressure that is twice as high as other mammals. Myocardial wall stress is alleviated by a pronounced thickening of the thick left ventricular wall, but because the relative heart mass of giraffes is similar to other mammals (0.6% of body mass), it was relevant to investigate whether a small end-diastolic volume of the left ventricle also contributes to the high pressure generated by the giraffe heart. In α-chloralose anaesthetized giraffes we determined a cardiac output and a stroke volume that are considerably lower than similar-sized mammals (500 kg). By means of intravascular echocardiography, we measured typical mammalian ejection fractions and normal myocardial function, but low end-diastolic and end-systolic volumes. The ability of the giraffe heart to generate high pressures, therefore, seems to reside in a decreased ventricular radius and a thickened left ventricular wall that normalizes wall stress. As such, the myocardial adaptation to the high afterload with increased blood pressure, entails a reduction in stroke volume that may render giraffes unable of sustaining a high cardiac output.