Investigation Of The Cardiovascular System

By | 2013-08-07

History

As always, the animal’s vaccination status, past medical history, diet and details of any previous or present treatment (including the response to any treatment given) should be recorded.

Dogs, and to a lesser extent, cats in heart failure often present with a history of coughing. In most cases this is due to pulmonary venous congestion and oedema, or is caused by an enlarged left atrium impinging on the bronchi at or slightly below the level of the carina- The frequency and time of day when the cough appears most pronounced should be noted. A cough of cardiac origin may become worse with exercise or excitement, or at night when the animal has been lying on one side (a nocturnal cough is highly suggestive of cardiac disease). Frequently the cough is paroxysmal terminating with an episode of pharyngeal retching and the production of clear phlegm (a productive cough).

Depending on the severity, pulmonary oedema or pleural effusion may result in orthopnoea, tachypnoea or dyspnoea (unlike pulmonary oedema a pleural effusion generally does not cause coughing). Young animals with congenital heart defects may fail to thrive and will appear stunted; this failure to gain weight should be differentiated from weight loss which may occur during the later stages of heart failure. Cardiac decompensation often results in a mild degree of polydipsia. Cardiac disorders resulting in a significant decrease in cardiac output (low output heart failure) may result in exercise intolerance, syncope and / or cyanosis of the mucous membranes. The in appetence and weight loss which is often associated with the more advanced srages of cardiac decompensation is usually attributed to cardiac cachexia.

Physical examination

Examination of the cardiovascular system can be broken down into five phases: (1) inspection; (2) temperature, pulse and respiration; (3) auscultation; (4) palpation; (5) percussion.

Inspection

Does the animal appear thin or cachexic? Is there evidence of tachypnoea or dyspnoea? Animals (particularly cats) with pleural effusion become dyspnoeic and develop a characteristic posture with elbow abduction. Jugular distension or the presence of a jugular pulse indicates an increase in central venous pressure and possible right-sided congestive heart failure. Other signs of right-sided heart failure include aseites and oedema of the ventral abdomen or limbs.

Temperature, pulse and respiration

The mucous membranes should be examined for signs of pallor or cyanosis. Pallor of the mucous membranes may be due to anaemia or poor peripheral perfusion. Cyanosis implies inadequate oxygenation of blood and may occur with congenital right to left cardiovascular shunts, severe pulmonary oedema (due to impaired diffusion of oxygen across the alveolar membrane) and acute airway obstruction. The normal capillary refill time is less than 2 s.

Palpation

The position of the apex beat over the 5th intercostal space on the left side at the level of the costochondral junction should be ascertained. Displacement of the apex beat may occur with large intrathoracic masses. The presence of a pre-cordial thrill generally indicates a severe murmur. Both femoral pulses should be assessed noting rate, rhythm, quality and presence of a pulse deficit. Finally, the abdomen should be palpated for evidence of hepatomegaly and ascites (presence of a fluid thrill) indicative of right-sided congestive heart failure.

Percussion

Percussion of the thoracic wall is a more useful technique in cattle and horses when it can be used to outline the cardiac silhouette and detect the presence of a fluid line.

Auscultation

Auscultation of the thorax is discussed in site. Auscultation should be performed in a quiet room, preferably when the animal is not panting, using both the bell and diaphragm of the stethoscope to detect low and high frequency tones, respectively. The heart rate, rhythm and intensity of the heart sounds and the presence of a heart murmur or abnormal respiratory sounds should be recorded. The regions of the thorax over which the mitral, tricuspid, aortic and pulmonic valves can be auscultated are described in detail in site.

Obesity, perieardial or pleural effusion, diaphragmatic hernia or a large intrathoracic (for example mediastinal) mass may result in muffled heart sounds; conversely heart sounds may be accentuated with tachycardia, anaemia, excitement and in deep-chested animals.

Normal heart sounds and their association with the cardiac cycle

Gallop rhythms

A gallop rhythm consists of three beats. The most common type of gallop rhythm in dogs is a protodiastolic gallop which represents an accentuation of the third (S3) heart sound. This may occur in cases of mitral or tricuspid insufficiency when rapid ventricular filling results in diastolic overloading of the ventricles. A summation gallop is caused by fusion of the third (S3) and fourth (S4) heart sounds. The third heart sound produced is audible during diastole and its presence always indicates cardiac pathology (for example ventricular dilation). A presystolic gallop represents accentuation of the fourth (S4) heart sound and is occasionally heard in dogs with mitral insufficiency.

Cardiac murmurs

Laboratory investigation of cardiac disease

Routine haematology and a full biochemistry screen may help to exclude the presence of an underlying disease process as a cause for a cardiac abnormality, for example anaemia may be ruled out as a cause of a systolic murmur. Arrhythmias may be the result of electrolyte or acid-base disturbances, or endotoxaemia. Feline hyperthyroidism is frequently associated with myocardial disease therefore, where appropriate, thyroid hormones should be assayed.

There are presendy no specific laboratory tests for assessing myocardial damage. Lactate dehydrogenase may be released from damaged myocardium but other tissues, most notably kidney, skeletal muscle and liver also contain high concentrations of the enzyme. Low output cardiac failure may result in a significant increase in the plasma concentrations of urea and creatinine due to prerenal azotaemia; passive venous congestion of the liver associated with right-sided congestive heart failure usually results in a moderately increased level of plasma alkaline phosphatase (ALP) and alanine aminotransferase (ALT).

Haematological examination of animals with congestive or low output cardiac failure is often unremarkable. Hypoxia may result in increased numbers of circulating normublasts and animals with severe ascites and / or pleural effusion may have low or low normal concentrations of total plasma proteins due to hypoalbuminaemia.

Electrocardiography

Radiological examination

Echocardiography

Diagnostic ultrasound provides a safe, non-invasive technique of obtaining qualitative and quantitative data about cardiac anatomy and function. The information gained is complementary to that obtained from thoracic radiography. Real time, two-dimensional (2D) imaging provides the most easily recognizable and understandable images of the heart. Complete examination of the heart requires a systematic approach using both the right and left parasternal acoustic windows.

M-mode echocardiography allows measurement of chamber size and wall thickness and provides characterization and timing of motion relative to the ECG during the cardiac cycle. M-mode has a high sampling rate which facilitates accurate resolution and definition of rapidly moving Structures. M-mode can be used to measure left ventricular indices, mitral valve measurements and systolic time intervals.

Doppler echocardiography provides information about the flow of blood within the heart chambers, across the valves and in the great vessels. It is complementary to 2D and M-mode echocardiography and provides a more complete, non-invasive evaluation of cardiac function.

There are two basic Doppler techniques: continuous wave and pulsed wave Doppler. A continuous wave Doppler transducer has two piezoelectric crystals, one co transmit a continuous wave of ultrasound and the other to receive back-scattered echoes. This system is very sensitive and allows the measurement of high maximum velocities. The pressure gradient across a valve can be calculated using the modified Bernoulli equation:

Pressure gradient = 4 x (maximum velocity)2

An important drawback of continuous wave Doppler, however, is that all velocities along the length and width of the ultrasound beam are recorded and as no structural information is gained, the region of abnormal flow cannot be localized accurately.

This problem is circumvented by using pulsed wave Doppler, which uses a single crystal to transmit ultrasound in pulses and is then set up to receive echoes after a time delay. In this wav reflected signals are recorded only from a particular depth and localization within an echocardiographic image is then possible. However, the pulsed wave system is unable to detect high frequency Doppler shifts (that is high velocities) due to a phenomenon known as aliasing. This disadvantage is greatly reduced by colour flow mapping, a two-dimensional version of pulsed wave Doppler.

Cardiac catheterization and angiocardiography

The use of cardiac catheterization and angiocardiography has declined with the advent of echocardiography. However, the technique is still indicated in some cases when the diagnosis proves difficult to make or during interventional procedures for the management of heart disease, for example balloon valvuloplasty.

It is essential to obtain a good concentration of contrast medium within the heart. This is best achieved by injecting the contrast medium through a catheter positioned in the jugular vein close to the heart (non-selective angiocardiography) or directly into a specific cardiac chamber or great vessel (selective angiocardiography). Low osmolar water-soluble contrast media should be used in patients with compromised cardiac function. A pressure injector is necessary in large dogs to deliver the required amount of contrast rapidly. The flow of contrast through the heart can be recorded using a rapid film changer or on video from image intensified fluoroscopy.

Selective angiocardiography is the most versatile and reliable method of angiocardiography and provides optimal information when supplemented with pressure measurements and blood gas analyses from the cardiac chambers and great vessels. Various types of catheter are available and are designed to assist the positioning of the catheter tip in selected chambers or vessels. The placing of the catheter must be monitored by image intensification or spot films. Careful planning before the investigation and continuous monitoring of the patient during the examination are essential to increase the quality and safety of the procedure.