The cardiac muscle fibers of birds differ from that of mammals in a number of ways.
The M band of mammals consists of a line of proteins that connect adjacent myosin filaments and forms an observable band when viewed with the electron microscope. This band is not present in birds and its significance, from the ability to provide contractile properties, is unknown. The T-tubule, or transverse tubule system, of mammals represents the invaginations of the outer plasmalemma at regular intervals perpendicular to the long axis of the muscle fibers. The T-tubule system increases the surface area in relation to the volume of the muscle cells. However, in birds, this T-tubule system does not exist. This means that there are more muscle cells in the same volume in birds than in mammals. The surface-to-volume ratio of the cardiac cells with these factors taken into account in the finch heart is the same as a mouse heart with similar cardiac frequencies.
Excitation-contraction coupling produces an electrical signal to the heart muscle, and it produces an action potential. This wave of electrical activity travels along the sarcoplasmic reticulum, and this causes the cardiac muscles to contract. It is the movement of calcium across the membrane of the sarcoplasmic reticulum that causes a change in the actin-myosin fibrils effecting muscle contraction. In cardiac muscle, there is all-or-none muscle contraction. Control of the work performed is by regulating the amount of calcium entering through the sarcoplasmic reticulum and the amount of calcium released from it.
The cardiac conduction system of the avian heart consists of a sinoatrial (SA) node, atrioventricular (AV) node, AV Purkinje ring, bundle of His, and three bundle branches. There are three types of cells that provide the histologic components of this conduction system. The pacemaker cells or P-cells are small, spherical cells in the nodes that have repetitive spontaneous depolarizations. Transitional cells or T-cells are intermediate in morphology as they have smaller numbers of myofibrils than cardiac cells and are much smaller. Purkinje cells are large, elongated cells that are more rectangular in shape. They transmit the electrical impulse through the substance of the myocardium or heart muscle.
The SA node consists of a loose collection of P-cells and T-cells within a connective tissue sheath. The node has a variable location depending on the species of bird. It may be found near the area where the venae cavae open into the right atrium. It has been noted that the pacemaker region appears to change location spontaneously within the node.12 The T-cells within the SA node transmit electrical impulses to the atrial muscles for contraction to occur. It is unknown if there is a true conduction system that transmits the electrical impulse through the atria but one does exist in the ventricles.
All of the muscle fibers of each chamber must contract at roughly the same time to efficiently move blood from one chamber to the next. The conduction system also needs a time delay so that blood can fill the ventricles (or the atria) before this chamber ejects blood to perfuse the lungs or the rest of the body. After the electrical impulse is initiated in the SA node, it spreads through the atrial muscles after which there is a slowing of the conduction velocity at the AV node. From the AV node, the electrical impulse is conducted though the Purkinje cells in the ventricles. From the AV node there is a bundle of His with three main branches of these cells that conduct the impulse. The right and left bundle branches from the wall between the ventricles form a network that follows the course of the coronary arteries. This results in a relatively short distance of travel for these fibers as they penetrate the thick endocardium of bird hearts. The third branch encircles the aorta and connects with an AV ring of fibers that forms around the right AV valve. This peculiar arrangement of the Purkinje fibers forming rings around valves of the heart suggests a reptilian ancestry.
In mammals, the Purkinje cells conduct the electrical impulse at a faster rate than the myocytes (muscle cells). The size of birds’ Purkinje cells is greater than their myocytes, so the conduction velocity of birds is greater than that of mammals.2 You would expect that because bird hearts beat much faster than mammals!
The Purkinje cells take a short course through the thick left ventricular myocardium so that the surface of the heart is depolarized relatively rapidly. This causes the burst effect of electrical impulse, which, on an ECG produces the deep S wave in birds and is not found in mammals.14 Studies indicate that the wave of depolarization is from the right ventricular apex, or the tip of the heart, to the right ventricular base, then the left ventricular base, and finally the left ventricular apex.
The electrocardiogram can be useful clinically for evaluating and diagnosing diseases that cause vague signs of weakness, fatigue, lethargy, collapse, and/or seizures in our avian patients.14 However, these signs are not specific to the cardiovascular system. The electrocardiogram is useful in monitoring anesthesia in avian patients as well, as it can alert the clinician of possible hypoxia (deficiency in the amount of oxygen delivered to the body tissues).
In birds, the electrical activity of the heart is recorded using electrodes placed on the body using standard leads. The body is a surface conductor of electrical activity with waves of depolarization and repolarization such that it can be recorded as a single dipole. The dipole has magnitude as recorded in volts of direction and sense (positive and negative). The polarity of the recording in mammals is such that the majority of the contraction (i.e., ventricular contraction) is positive, while in birds, ventricular contraction is negative. For this reason, the mean electrical axis or its vector in birds is negative.
The avian electrocardiogram has a positive P wave, and its duration represents the period of depolarization of the atria. The Ta wave is observed commonly in normal racing pigeons and represents repolarization of the atria.14 The QRS wave of birds is actually a RS wave that is deeply negative. This negative wave represents the period of ventricular activation. The PR interval includes the wave of activation through the atria and the conduction delay at the AV node. The RT interval represents the complete cycle of activation and relaxation of the ventricle. The T wave represents repolarization of the ventricles. The shape and duration of the T wave is commonly observed during anesthesia as is often increases in size with hypoxia and/or electrolyte changes.
The heart rate of many avian patients requires that the paper speed on the electrocardiogram be at least 100 Millimeters per second, which is much faster than in mammals and requires a special machine designed to record it. Electrocardiograms can be recorded in anesthetized or unanesthetized patients. ECG recordings are useful in evaluating for primary heart disease, treatment(s), and during anesthesia.