The Equine Heart

functioning tissues.
Normal Rhythm How Normal is Normal?
What does this have to do with heart rhythm? First theres no such thing as a steady heart rate. An absolutely steady metronomelike rhythm is not normal. Normality involves constant change and readjustment the ability of the body to respond rapidly and appropriately to these changes is in fact an index of health. If you monitor the electrocardiogram ECG of a horse at complete rest and measure fluctuations in the interval between consecutive beats called instantaneous heart rate you get what is called a heart rate time series. In a normal animal this is anything but a straight line. The amount and pattern of variation from beat to beat called Heart Rate Variability or HRV changes tremendously from horse to horse circumstance to circumstance moment to moment.
Analysis of this sequence of instantaneous heart rates helps demonstrate that underlying control mechanisms such as blood pressure control for example dont simply turn on or off when necessary. Instead they tick along continuously in a cyclic or periodic manner at varying frequencies. When changes in blood pressure occur the amplitude and frequency of these control cycles change and this happens constantly. You can relate these cycles to the balance in underlying autonomic nervous system parasympathetic and sympathetic control.
Back to the mare being examined by the veterinary students. She is quiet and relaxed almost sleeping. She has very little sympathetic activity and lots of parasympathetic activity. This means she will have lots of high frequency changes in instantaneous heart rate making adjustments with every beat. Occasionally this will result in her dropping a beat completely at other times the interval between beats will be variably long. You would imagine that if everything was completely stable she ought to maintain the same slow basic rate but its not that simple because blood pressure is not the only thing stimulating her parasympathetic nervous system.
She is also breathing very slowly and deeply maybe six to eight breaths per minute and these breathing cycles are also adjusting both blood pressure and parasympathetic activity. As a result the actual effect on heart rate beattobeat sometimes adds to and sometimes subtracts from other influences. Since the input of the parasympathetic nervous system to the heart actually has a left and right branch and since those branches tend to innervate different parts of the heart there are also different options to slow down the heart