( Originally Published 1921 )
To understand the irregular heart, it is necessary for us to have a clear idea of what the regular heart is like. The heart is a two cylinder pump, or rather a set of two cylinder pumps, which are bound together and work in unison. In this two-cylinder pump, one chamber receives the blood and passes it on to the other chamber, which pumps it through the body.
The extreme simplicity of this arrangement has been copied in the kind of automobile tire pump that is now supplied with the Ford car. I took one of these pumps apart the other day to see what it was like, and I found its construction absurdly simple, but it seems to work quite effectively. It consists, essentially, of two tubes, in each of which there is a piston, both pistons being connected with the handle of the pump which is worked up and down. These pistons have leather washers of the usual type found in all pumps. The bottoms of the two cylinders are connected, and the top of the smaller cylinder is connected with the tire that is to be inflated. There are no valves or any other complicating structure in the whole arrangement, and like everything else about the Ford car, the surprise about it is that it works.
If a machinist had been building a human heart, he would have put in a number of things which nature has left out, because it would seem that a two-cylinder pump would need more than two valves. Until I ran across the Ford pump I had never seen a pump that worked without valves, and it reminded me of the auricle of the human heart. The auricle receives the blood and by a gentle contraction pushes it into the ventricle, but at the same time the ventricle is expanding and drawing it in, the mitral valve being open and the aortic valve closed.
There is no valve that keeps the blood from flowing back from the auricle to the veins, but the pressure in the veins seems to be enough to prevent any serious back flow, though the wave caused by the contraction of the auricle does pass up the veins into the neck. These waves, as I have explained elsewhere, give us the opportunity to study the motion of the auricle.
The contraction of the heart naturally begins at one point in the right auricle and spreads through the walls of both auricles, and then through a special band of tissues to the ventricles. This particular spot in the heart, where the heart beat starts, consists of a little mass of tissue known as the sino-auricular node, and the bundle of tissue that connects the auricle and the ventricles is called the bundle of His.
When the heart is beating naturally, the muscular tissue of the heart contracts in a wave starting in this point and spreading to the auricles and then to the ventricles, and these waves of contraction spread over the heart successively and regularly. When the heart is irregular, it may be that the contractions are not starting regularly from this place, and then we say that the heart is affected by sinus arrhythmia. In other words, the beats come from the proper part of the heart but do not come regularly. As a rule, this kind of irregularity is not of importance, unless, of course, the beats come altogether too quickly, causing severe palpitation, or when the disorder is the early stages of something more important.
Another common cause of irregularity of the heart is the origin of the heart beat in other parts of the heart than in the proper place. The ventricle of the heart ought to beat only when told to do so by the auricle ; but sometimes when the ventricle is irritable it will originate its own beats. It will also originate its own beats when it does not receive any word at all from the auricle that it is time to beat. When the ventricle works independently of the auricle, it always works slowly about twenty-eight to the minute.
So we have irregularities due to the irregular action of the pace-maker of the heart, which is another name given to the sino-auricular node, or we have irregularities because some other part of the heart impertinently assumes the office ofthe pace-maker and starts to beat on its own account. These latter are called extrasystoles.
Another source of irregularity is when there is some trouble with the transmission of the beat from the auricle to the ventricle. In all there are seven kinds of irregular hearts, but these three will be a good basis to begin with.