Each heart beat consists of two parts , known as systole and diastole . When measuring blood pressure , these give the systolic and diastolic readings .
There are two audible sounds that can be heard on auscultation of the heart , which could be described as a ‘ lub ’ and a ‘ dub ’ sound . The first half of the heartbeat is the sound of the tricuspid and bicuspid valves closing . The second half of the heartbeat is the sound of the aortic and pulmonary valves closing [ 3 ] .
There are other structures within the heart that play an important role in its function . These structures aid in the heart ' s electrical activity and include :
The sinoatrial ( SA ) node
This is often referred to as the pacemaker of the heart . It is located in the wall of the right atrium . Nerves that are connected to the SA node cause an impulse that initiates the contraction of the atrium . The left and right atria contract at the same time [ 1 ] .
The atrioventricular ( AV ) node
The AV node sits at the top of the interventricular septum of the heart . The ventricular walls do not allow electrical messages to pass through them , so this specialised group of cardiac muscle cells sends impulses to a structure called the bundle of His [ 1 ] .
Bundle of His
The bundle of His runs down the interventricular septum of the heart and branches into both the left and right sides at the bottom of the heart . These branches connect with a network of specialised neurons called the Purkinje fibres [ 1 ] .
Purkinje fibres
The Purkinje fibres spread throughout the ventricular muscles . The contraction of the ventricles starts at the base of the ventricles , then spreads up through the ventricles and pushes the blood towards the pulmonary artery and the aorta [ 1 ] .
Common cardiovascular diseases
When an animal suffers from a disease , it compromises different organs in the body .
If a disease affects the cardiovascular system , it can have drastic effects on the function of almost every organ in the body , unless treatment is initiated . There are a variety of conditions that can affect the function of the heart .
Heart murmurs
Heart murmurs can be both congenital and chronic , and occur when the cardiac tissue structure functions incorrectly . A heart murmur can be auscultated with the use of a stethoscope , with the sound being generated from a turbulent flow of blood within the heart [ 5 ] . The intensity of the audible murmur can increase depending on the severity of the damage to the heart .
A heart murmur can usually be associated with either the tricuspid or the bicuspid valve , which separate the heart ' s atria from the ventricles . If the valves are not working correctly , it can become difficult for blood to pass from the atrium to the ventricle , causing an audible murmur . In some instances , a murmur can develop due to factors other than cardiac disease , such as anaemia [ 6 ] .
Heart murmurs can be graded depending on severity [ 7 ] :
• Grade I : Quiet murmur , often difficult to auscultate .
• Grade II : Soft murmur but easy to distinguish .
• Grade III : Audible murmur with moderate intensity .
• Grade IV : Audible , loud murmur , but cannot be palpated via the thorax .
• Grade V : Audible , loud murmur , which can be palpated via the thorax .
• Grade VI : Loud , intense murmur , which can still be auscultated when the stethoscope has been moved away from the chest . The murmur can be palpated via the thorax .
Mitral valve disease ( MVD )
The mitral valve can become thickened , making it unable to close properly . As a result , blood can start to leak between the left atrium and ventricle , producing the sound of a murmur [ 8 ] . The heart then has to work harder to pump blood around the body effectively . If left untreated , MVD can become progressively worse and lead to heart failure .
Dilated cardiomyopathy ( DCM )
When an animal suffers from DCM , its heart muscles become weaker and thinner . The heart becomes bigger due to dilation and struggles to pump blood effectively . An arrhythmia , which is where the heart has an irregular beat or rhythm , is usually associated with DCM .
DCM can affect a number of breeds , but large-breed dogs such as the dobermann and great dane are more likely to suffer from the disease [ 8 ] .
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