The deflection P is due to the auricular contraction and QRS marks the beginning of the ven tricular contraction. Thus the P-R interval represents the period between the auricular and ven tricular contractions. The total duration of the excitatory state of the ventricle is measured by the distance between Q and T. A case of A-V block, as for instance in Adams-Stokes' disease, is shown at once on the electrocardiogram by the dissociation of the normal relations between the auricular and ventricular deflections. A delay in conduction of the excitatory wave is accompanied by a prolongation of the P-R interval, while a beat originating from the A-V node instead of the S-A node is immediately shown by a or even disappearance of the P-R interval. The exact origin of the T wave is not known. The electrocardiogram has become an important aid in the study and diagnosis of abnormal heart activities.
The Isolated Mammalian nutrition of the mammalian heart is carried out by means of the coronary arteries, which leave the aorta at the place of its origin. A mammalian heart, which has been removed after the death of the animal, can easily be revived if the coronary arteries are perfused under pressure with blood or a salt solution which resembles the saline medium of the blood in composition. This procedure was first introduced by Ludwig for the frog's heart, and by Langendorff for the mammalian heart. Sydney Ringer first determined the exact amount and type of salts necessary for the most successful survival of the frog's heart, and Locke modified Ringer's solution for the mammalian heart. With the use of such a solution, a mam malian heart can be restored to activity as long as 7 days after death. The beat of the isolated heart of a child can be restored 20 hours after death from pneumonia. The excised heart of a cat can be kept beating for 4 days. The heart of a monkey was restored after freezing the dead body of the animal.
Conditions Essential for the Heart perfusion experiments, a cannula is tied in the aorta pointing to the heart. The pressure of the column of fluid closes the aortic valves and the only way of escape is through the coronary arteries ; after having passed through the heart muscle, the fluid flows out of the coronary veins. In this manner it is possible to study the influences directly affecting the heart beat and the coronary blood vessels.
The first and most essential condition for reviving a heart is an abundant supply of oxygen ; the second is the maintenance of the perfusion fluid at a reaction similar to that of blood, i.e., slightly
on the alkaline side of neutrality (sodium bicarbonate is usually added for this purpose).
A third important factor is the maintenance of the temperature within physiological limits. An increase in temperature causes an increase in heart rate but at about 44° C to C the beat ceases entirely. At temperatures ranging from I3°–I9° C the beat ceases, but on rewarming the co-ordinated contractions are re-established. In all these cases the effect of temperature is primarily on the S-A node, and warming or cooling the node duplicates the effect of warming or cooling the perfusion fluid, except that at some stage the lower rhythm centre of the heart will begin to be dominant, whereas in the cooling of the whole heart the rhythm of all the centres becomes depressed.
The main purpose of the sodium chloride in the fluid is to keep the osmotic pressure of the fluid the same as that of blood. Cal cium and potassium have however a direct influence on the con tractility of the heart. After a short perfusion with a fluid that lacks calcium and potassium, the heart soon stops beating alto gether. Addition of calcium salts will immediately evoke con tractions, which will grow in force, but the heart will soon fail to relax completely and will gradually stop in systole. On addition of potassium salts, the heart resumes a normal beat. Excess of cal shortening cium salts leads to a systolic standstill, and excess of potassium salts leads to a diastolic standstill. The normal activity can proceed only if salts of both calcium and potassium are present in the proper proportions.
The Heart-lung Preparation.—A great advance in the study of the heart was made when Starling succeeded in investigating the heart under conditions like those of the perfused heart, but with the great advantage that the heart performed work and pumped blood in exactly the same way as in the whole animal. The arrangement of the method is shown in fig. 8.
Artificial respiration being maintained, the chest is opened under an anaesthetic. Cannulae are placed in the brachiocephalic artery and the superior vena cava. All other blood vessels going to and from the heart are tied off. The blood emerging from the heart is made to flow against an artificial variable resistance (R), through a glass spiral immersed in warm water, into a reservoir. From the reservoir the blood flows through the superior vena cava into the heart. The aortic blood pressure can be varied in this preparation by changing the artificial resistance against which the heart is made to work.