The Circulatory System

Transcription

1 The Circulatory System The heart The heart is a four chambered muscular organ that pumps the blood around the body The heart lies in the mediastinum within the thoracic cavity It is conical in shape and lies at a slight angle in the thorax The heart lies to the left on the midline with its apex near its sternum The heart is enclosed by a double layered membrane called the Pericardium The inner layer of this is called the Epicardium, which is a serous membrane and directly covers the heart wall Within the pericardial cavity is a little serous fluid which lubricates the heart as it beats The wall of the heart consists of a layer of cardiac muscle Myocardium and a thin inner epithelial layer that lines the heart called the Endocardium

2 The heart is divided up into right and left chambers by a partition called the Septum Each side is then divided up into two chambers They are: The thin walled collecting chambers = Atrium The thicker walled pumping chambers = Ventricles The right side of the heart pumps blood from the heart to the lungs and back The left side of the heart pumps blood all around the body Heart valves Within the heart there are two sets of valves, the function of these valves is to prevent backflow of blood They are: 1. Atrioventricular valves 2. Semi lunar valves Atrioventricular valves These lie between the atria and ventricles They prevent the flow of blood back into the atria when the ventricles contract They are attached to the papillary muscles in the heart wall by fibres called Chordea tendinae these prevent the valves being turned inside out by the pressure of the blood The right atrioventricular valve lies between the right atrium and right ventricle

3 It comprises of three fibrous cusps and is also known as tricuspid valve The left atrioventricular valve lies between the left atrium and left ventricle It comprises of two cusps and is also known as the bicuspid valve Semilunar valve These are situated at the base of the two major vessels leaving the heart The Pulmonary Valve lies at the base of the pulmonary artery and prevents backflow of blood from the pulmonary artery to the right ventricle The Aortic Valve lies at the base of the aorta and prevents backflow of blood from the aorta to the left ventricle It is the semi lunar valve closing that can be heard in the second heart sound (dubb) if the valve is faulty it doesn t close properly causing turbulence. This is now heard as a whoosh instead of a lubb or dub this is known as a Heart murmur

4 Circulation of blood through the heart Deoxygenated blood is carried back to the heart in Veins Two major veins collect all the blood and enter the right side of the heart these are called: Cranial vena cava Caudal vena cava The two atria (left and right) and the two ventricles (left and right) contract in unison Both vena cava empty into the Right Atrium which contracts when it is full This sends deoxygenated blood into the right ventricle via the right atrioventricular valve The right ventricle contracts, pumping blood into the pulmonary artery via the pulmonary valve The deoxygenated blood is carried in the pulmonary circulation to the Lungs The blood is oxygenated in the lungs and is then carried back to the left side of the heart in the pulmonary veins which enter the Left Atrium When the left atrium is full of blood it contracts forcing oxygenated blood into the left ventricle via the left atrioventricular valve The left ventricle contracts and pumps blood into the Aorta (by the aortic valve) which then carries the oxygenated blood around the body. Oxygenated blood is delivered to the tissues; the deoxygenated blood is collected up by the veins and transported back to the heart

5 Control of the heart beat The heart is made of cardiac muscle which is a specialised type of muscle that has the ability to initiate contraction from within the muscle itself (without nervous control The mechanism that is responsible for controlling the rate of contraction of heart muscle is called the conduction mechanism Within the wall of the right atrium is an area of modified cardiac muscle called the Sino atrial node This nodes job is to determine the rate of the heart beat and is referred to as the Pacemaker If the muscles have been working hard and require more oxygen the sinoatrial node will increase the heart rate so that the oxygen can get to those muscles During sleep the sinoatrial node slows the heart rate down as the muscles are relaxed The sinoatrial node starts a wave of contraction, which passes over the walls of the atria The impulse passes to another specialised area called the Bundle of his which is situated within the septum The impulse is then conducted to the apex of the heart where it spreads out into the ventricles in specialised nerve cells called Purkinje Fibres The wave of contraction in the heart muscle (myocardium) of the ventricles starts at the apex and spreads upwards forcing blood into the arteries that are situated at the top of the ventricles. This is called the cardiac cycle The period of contraction within the heart is called Systole and is when the blood is being pumped into the ventricles or circulation

6 The period of relaxation of the heart is called Diastole and is when the atria are filling with blood The circulatory system is made up of a network of channels that transport the blood from the heart to the tissues, where oxygen and nutrients are delivered, and then transport it back again to the heart. This network consists of arteries, capillaries and veins Arteries A large vessel which carries blood under pressure away from the heart Most arteries carry oxygenated blood except for the pulmonary artery which carries deoxygenated blood Arteries have thick muscular walls which enables the vessel to dilate or constrict Blood travels along the arteries reflecting the heartbeat which can be felt as the pulse As the arteries enter the tissues they branch getting smaller and smaller these are then called Arterioles which then flow into capillary networks Capillaries The capillaries form a branching network in all the tissues and link the arteries and the veins They are narrow with thin walled consisting of a single layer of endothelial cells with no muscle or elastic tissue They are permeable to gases, nutrients and waste products which diffuse between the blood and tissues and from the tissues back into the blood

7 Veins A vein is a large vessel which carries blood towards the heart The walls are thinner than arteries and they contain less muscle and elastic tissue Blood flows slowly under low pressure and valves may be present to prevent pooling of the blood Most veins carry deoxygenated blood from the tissues except for the pulmonary veins which carry oxygenated blood from the lungs back to the heart Systemic and pulmonary circulation The circulation in the mammal is referred to as a Double circulation as blood passes through the heart twice during a complete cycle There are two parts to the circulatory system: 1. Systemic circulation carries oxygenated blood around the body and returns deoxygenated blood to the heart 2. Pulmonary circulation - Carries deoxygenated blood from the heart to the lungs where it is oxygenated and returned to the heart Systemic circulation Arterial supply Oxygenated blood leaves the left ventricle of the heart in the major artery known as the aorta The aorta gives off a number of arteries that then supply various parts of the body

8 Venous return Deoxygenated blood returns to the heart from the tissues in veins which follow the pattern of the arteries and often have the same name (renal artery and renal vein) The caudal vena cave empties into the right atrium of the heart, venous blood returns: From the head in the Jugular vein From the neck and forelimbs in the cephalic veins, brachial veins and subclavian veins which drains into the cranial vena cava which empties into the right atrium of the heart Hepatic portal system The liver has its own modified circulatory system within the systemic circulation Its function is to carry blood straight from the digestive system to the liver so that the products of digestion can be used immediately rather than transporting them around the body Pulmonary system Deoxygenated blood is pumped from the right ventricle of the heart and is carried to the lungs in the pulmonary artery Within the lung tissue the artery then divides into lots of capillaries that wrap around the thin walled alveoli of the lungs Oxygen in the inspired air diffuses into the blood and carbon dioxide in the blood diffuses into the air in the alveoli

9 Exotic animals Small mammals The cardiovascular system of small mammals is similar to cats and dogs except for the ferret which only has one vessel arising from the aorta Birds The avian heart is four chambered as with mammals but is proportionally larger with respect to its body size The major artery (aorta) curves to the right side as it leaves the heart rather than to the left in mammals There are several other differences in the blood vessels in birds Reptiles The heart is generally considered to be three chambers in lizards, snakes and chelonians as there is no physical division between the right and left ventricle They function as a four chambered heart with deoxygenated blood being directed towards the lungs and oxygenated blood to the rest of the body Lizards, snakes and chelonians have paired aorta leaving the heart one going to the right one to the left. It then fuses dorsally to become one aorta Fish The heart has only one atrium and one ventricle. Blood returns to the heart via two blood vessels the common cardinal vein which receives blood from the head and body and the hepatic veins

10 These enter into the a small chamber which then empties into the ventricle The ventricles then enter into a vessel like the aorta which divides into brachial arteries and supplies the gills with blood These then move on to the dorsal aorta which moves caudally to supply the rest of the body with oxygenated blood

11 Circulatory Disorders The primary function of the cardiovascular system is to supply adequate blood flow to meet the body s metabolic demands. Cardiac output is tightly controlled which maintains blood pressure in normal circumstances. Cardiac output depends on the heart rate and the amount of blood ejected from the heart with each contraction Common Circulatory disorders Congestive heart failure Acute heart failure Congenital heart diseases to include, Patent ductus arteriosis, pulmonic stenosis, Aortic stenosis, ventricular septal defects Acquired heart disease to include, Dilated cardiomyopathy, hypertrophic cardiomyopathy, endocardiosis, pericardial effusion Vascular disease hypertension Dysrhythmias to include; atrial fibrillation, ventricular premature contractions, heart block Questions to ask an a owner Has the animal previously had signs of heart disease? Heart disease often goes undetected for many years before clinical signs develop Is the animal able to exercise normally? Heart disease will often cause exercise intolerance or collapse at exercise

12 Is the animal eating and drinking normally? Patients in heart failure often go off their food and are cachexic (loss of weight, fatigue, general poor condition) In early stages they may become polydipsic Have there been episodes of weakness or collapse? This could be a sign of poor circulation (reduced cardiac output) Does the animal seem unsettled at night? Nocturnal restlessness is common in congestive heart failure, they find it difficult to breathe when lying down Are there any breathing issues or is the animal coughing? Respiratory issues may be associated with pulmonary oedema Coughing is more commonly a sign of respiratory disease but could be caused due to increased pressure of a large heart on the bronchus

13 The most common type of heart failure is congestive heart failure. This can affect the left or right sides of the heart but usually progresses to involve both sides Right sided heart failure results in congestion of the venous circulation in the liver and spleen which causes ascites Left sided CHF causes congestion of vessels in the lungs and fluid leaking from here causes pulmonary oedema. Oedema and congestion reduce oxygen transfer in the lungs Causes of CHF include: Pooling in the venous system due to: Systolic failure (reduced efficiency if the myocardium to contract Diastolic failure (reduced ability of the myocardium to relax and fill with blood) Volume overload, excess volumes of blood entering the chamber causing myocardial stretching Pressure overload where resistance to the outflow of blood which increases the force of contraction needed and muscle hyperatrophy increases pressure in the ventricle Dysrhythmias which reduce efficiency of the pumping action Signs of CHF include: Exercise intolerance Cough Dyspnoea Syncope/collapse Weight loss Pale mucous membrane (due to poor supply to the peripheral circulation) Ascites Restlessness

14 Diagnostic testing: Auscultation of the heart for the presence of murmurs Increased heart rate and loss of sinus dysrhythmia Compare heart and pulse rate Pulse quality and strength MM colour Cyanotic with pulmonary oedema or pale if output is reduced Thoracic radiographs to assess size and shape of the heart and presence of pulmonary oedema or pleural fluid Ultrasonography for internal cardiac anatomy valves, septa, vessels and myocardium ECG provides information about cardiac chamber size but also for detection, recognition, classification and monitoring dysrhythmias Treatment options: Early heart disease (no signs but a murmur is detected) require the underlying cause to be diagnosed and corrective treatment started If corrective treatment is not possible it is likely that CHF will progress so the patient should be checked every 3-6 months A reduced calorie diet should be started to control weight Exercise restrictions should be imposed, if pulmonary oedema is present then ACE inhibitors and diuretics should be started Supportive care such as draining pleural effusions and oxygen therapy should be started

15 Congenital abnormalities Patent ductus arteriosus (PDA) The ductus arteriosus is a blood vessel that connects the two main arteries of the body - the aorta and the pulmonary artery. This blood vessel is normal in the foetus, but shortly after birth, it should close. When the ductus arteriosus remains open or patent after birth, this abnormal communication between the aorta and pulmonary artery passes extra volumes of blood into the lungs. Patent ductus arteriosis (PDA) is a birth defect representing the second most common congenital heart defect of dogs. Approximately seven out of 1000 live birth puppies are affected. Generally, there are no serious symptoms of PDA unless congestive heart failure has caused fluid build-up in the lungs. The condition is typically identified in puppies during a routine veterinary visit for vaccinations. Continual blood flow through the PDA into the lungs produces a continuous heart murmur. Even when a PDA has been identified, most people believe their dog is normal. In some cases, the dog can be smaller than littermates or play less vigorously. However, the situation can be very misleading as symptoms usually occur within a year of diagnosis. If untreated, about 60 percent of affected dogs die within a year of diagnosis. When caught early, and following treatment with successful closure of the PDA, most dogs live a normal life. Unless there are complications from other heart defects or heart failure has already developed, there is rarely any future need for medication. While special circumstances can influence the prognosis, most cases are straightforward.

16 What to look for: Breathing difficulties Coughing Exercise intolerance Lethargy Diagnosis Various diagnostic tests are needed to recognize PDA, and exclude other diseases. Some of the necessary tests may include: Complete medical history and physical examination including auscultation (stethoscope examination) of the heart and lungs. The heart murmur of PDA is characteristic and most experienced veterinarians learn to make the diagnosis simply by listening. Since other birth defects can also cause heart murmurs, a veterinary cardiologist may be consulted if the diagnosis is in doubt.. A chest X-ray (radiograph) can help determine the severity of the problem An electrocardiogram (ECG) can assist with the diagnosis. An echocardiogram with Doppler (cardiac ultrasound) is the definitive diagnostic test. This may require referral. Routine blood tests may be performed prior to any anaesthesia Treatment The conventional treatment is an operation done shortly after diagnosis. The PDA is closed with surgical suture. Surgery should not be delayed by waiting for symptoms to develop. Medical treatment may be necessary before surgery if symptoms (coughing, difficult breathing) are present.

17 In some referral centres, the PDA may be closed using special catheterization techniques. PDA is common in: Miniature poodle, collie, Maltese Shetland sheepdog, German shepherd dog, cocker spaniel, Pomeranian, and Labrador retriever. Female dogs are predisposed. Pulmonic stenosis Cause: A congenital narrowing of pulmonary valve or the artery leaving the heart Signs: A murmur is often detected at routine examination, this can then progress to right sided heart failure Diagnosis Thoracic radiographs show enlargement of the right side of the heart Ultrasound examination shows thickening of the right heart wall Doppler ultrasound shows narrowing of the vessel which can identify the severity of stenosis Treatment Severe cases require dilation of the artery. Symptomatic medical treatment in non surgical cases

18 Aortic Stenosis Cause: A narrowing of the outflow of the left side of the heart (or abnormal development of the aortic valve). This causes resistance to outflow resulting in hypertrophy of the left side of the heart Congenital aortic stenosis is probably the most common heart defect seen in large breed dogs. Newfoundland dogs have the highest risk for this disorder. It is also seen in the golden retriever, Rottweiler, and boxer. There is a mildly increased risk in: the German shepherd, German short-haired pointer, Great Dane, Samoyed bulldog. Signs Depending on the severity signs may be present at a young age or develop into adulthood Fainting Collapse Sudden death Diagnosis Thoracic radiographs may show dilation of aorta after the point of stenosis Ultrasound can be used to visualise the stenotic area Doppler to measure blood flow through the area ECG examination shows cardiac arrythmias Treatment Symptomatic treatment with anti-dysrhythmic drugs and beta blockers Mild cases don t require treatment

19 Venticular septal defect (hole in the heart) A failure of the heart to develop properly which results in an opening in the division between the left and right sides of the heart. Blood flows from left to right meaning that too much blood is returned and the left side is overloaded. VSD is a congenital disease in dogs and affected dogs should not be used for breeding. When a dog is diagnosed with ventricular septal defect, its parents should also not be used for breeding any longer. As a dog embryo develops, the heart begins as a single tube. This tube will then gradually separate into four different chambers. Abnormalities can appear at several different steps in this intricate process and this can lead to ventricular septal defect in a dog Symptoms The nature and extent of the symptoms will depend on both the size of the abnormality and exactly where it is located. In mild cases of ventricular septal defect, most dogs will display no symptoms except heart murmur. It is even possible for a small defect to close on its own as the dog matures. In a dog with a larger defect, the pressure in the left side of the heart will be higher than the pressure in the right side, and there will be a blood flow from left to right via the defect. The left side of the dog s heart will be forced to work harder than normally and more blood will circulate to the lungs. This will in turn cause a higher than normal work load for the lungs. The symptoms can develop over the course of several months or even years. Common Symptoms exercise intolerance Increased respiratory effort In severe cases, abnormal heart rhythm can lead to premature death.

20 Dilated cardiomyopathy Dilated cardiomyopathy (DCM) is a disease affecting the heart muscle. It is the second most common heart disease in dogs (after mitral valve disease). In DCM the myocardium is thinned resulting in loss of contractibility It affects: Mainly middle-aged large and giant breed dogs some spaniels Dobermans are the main breed affected. Cats and small breeds Male dogs are more likely to be affected than females. What causes DCM? Cardiomyopathy literally means disease of the heart muscle (cardio = heart and myopathy = muscle disease). In DCM the heart muscle becomes thin and weakened. The heart muscle can be damaged in a number of ways including: viral infections Dietary deficiencies of taurine (an essential amino acid only found in meat protein) Dietary deficiencies in carnitine have been reported as causes of DCM in some groups of dogs. In most cases of DCM there is no apparent cause of the damage to the heart and this is termed idiopathic cardiomyopathy. The heart muscle is damaged it becomes weak and so does not contract well. Because heart contractions are weak the heart does not empty with each contraction and the blood supply to the body is reduced. Pulses are weak and the paws may feel cold. With time the heart muscle stretches and heart becomes a flabby sac.

21 DCM eventually results in heart failure with fluid build-up in the lungs (pulmonary oedema), the chest (pleural effusion) and abdomen (ascites). What are the signs of DCM: an irregular heart beat on a routine examination exercise intolerance Increased respiratory effort Anorexia Depression Polydipsia Coughing Collapse. Cardiac cachexia. ascites Sudden death How do you diagnose DCM? Thoracic radiographs show increased cardiac size Pulmonary oedema Ultrasound shows thin myocardium and poor contractibility ECG can show dysrhythmias particularly atrial fibrillation Treatment In almost all cases there is no treatment for the underlying muscle disease. Signs of heart failure can be managed according to its severity. Taurine supplementation in some cases

22 Endocardiosis Endocardiosis is the most common acquired disease in dogs particularly in Cavalier King Charles Spaniels Causes: Degenerative condition of the atrioventricular valves (usually the mitral valve) which causes faulty valve function and the leakage of blood through valves when they are closed Signs Cardiac murmur progressing over years to CHF (usually left sided) Diagnosis Thoracic radiographs shows left sided heart enlargement (atrium) Ultrasound shows distortion of valves and turbulent blood flow with leakage Treatment Symptomatic management of CHF

23 Pericardial effusion Often seen in middle sized and large breed dogs. Fluid accumulates inside the pericardial sac which prevents the heart from filling with blood. Signs Lethargy Dyspnoea Muffled heart sounds Weak pulses Pale mucous membranes Muffled heart sounds Ascites Treatment Drainage of the fluid (pericardiocentesis) Pericardectomy

24 Drugs commonly used in the treatment of cardiac conditions Positive inotropes e.g. digoxin Increase the force of contraction of the heart muscle Slow the heart rate Can cause nausea at high doses Local anaesthetics Given by I/V injection to treat some dysrhythmias They reduce the sensitivity of the heart muscle so that abnormal contractions are reduced Only used for short term control of severe dysrhythmias Diuretics Promote renal excretion of fluids and reduce oedema Inhibits the resorption of salt and water filtered by the kidneys so that more fluid is lost in urine Vasodilators Reduce the workload of the heart by: Dilating systemic veins which reduces pressure making oedema less likely to form Dilating arterioles making it easier for the heart to pump blood Betablockers Slow heart rate Reduce the force of contraction so not used in DCM

25 Nursing the patient with congestive heart failure: Choose a suitable kennel which can be lined with absorbent material. This should be in a calm quiet area Monitor and record TPR, urinary and faecal output, abnormalities etc A low salt/fat diet, which has a protein of a high biological value and is highly digestible. This will reduce oedema and ascites Water should be available at all times Medication should be correctly administered Exercise should be kept to a minimum