Factors influencing the ocular pulse — the heart rate

Abstract

Using pneumotonometry combined with a Langham ocular blood-flow system, measurements of pulsatile ocular blood flow (POBF) were performed in eight ocular normotensive patients with implanted cardiac pacemakers, with the subjects assuming both the erect and the supine postures. Sequential measurements of POBF were made at pre-set values of heart rate over the physiological range between 60 and 120 beats/min at intervals of 10 beats/min. With patients in the supine position, measurements of cardiac output and stroke volume indices were also recorded by impedance cardiography. The mean pulse amplitude of the intraocular pressure (the ocular pulse) decreased as heart rate increased, and this change was statistically significant in both postures according to repeated-measures analysis of variance (erect: f=18.7, P < 0.0001; supine: f=18.8, P<0.0001). As measured in supine patients following an increase in heart rate, the pulse amplitude decreased in parallel with a decline in stroke volume index (f=18.8, P<0.0001). Up to a level of 90 beats/min, the mean POBF increased with heart rate, but it declined above this rate in both erect and supine postures. At all heart rates, intraocular pressure was higher when subjects were supine than when they stood erect (f=4.3, P<0.001). At lower heart rates of 70 and 80 beats/min, ocular pulse volume and POBF were significantly lower in supine patients than in erect subjects (70 beats/min: t=3.89, P<0.01 vs; t=3.87, P<0.01; 80 beats/min: t=2.85, P< 0.05 vs ; t = 2.87, P< 0.05). We conclude that when the heart rate is under normal physiological drive, the decline in POBF that accompanies the act of lying down is determined both by the change in posture itself and by the fall in heart rate that is associated with it. These observations suggest a possible disadvantage inherent in the use of anti-glaucoma drugs that inter alia induce bradycardia.