6.8.1 Pericardial effusions
Pericarditis can be bacterial in origin, but most commonly it is idiopathic. It is sometimes found in association with an infectious disease such as viral respirainfections. Traumatic pericarditis resulting from penetrating wounds has also been recorded. Neoplastic causes are very rare. Pericarditis appears to be much less common in the UK than in the USA. This may be because pericarditis is sometimes associated with pleuro-pneumonia, which is more common in animals transported long distances overland between two climatic extremes.
Effusive pericarditis results in right-sided CHF when the pressure within the pericardium rises so that it exceeds that within the heart, restricting diastolic filling of the cardiac chambers. The low-pressure RA is the first chamber to collapse under the pressure of the intra-pericardial fluid. This limits venous return. The increase in filling pressure leads to venous congestion and the limited preload means that cardiac output cannot be raised to meet any increased demands. The greater the pressure within the pericardium, the higher the pressure required to fill the RA and ventricle. The rate at which the effusion develops also affects the severity of the condition, because the pericardium can stretch to accommodate larger volumes of fluid if it accumulates slowly. If an acute effusive episode occurs, the fibrous sac will have little time to dilate and intra-pericardial pressure will be high.
A spectrum of clinical signs may be attributable to a pericardial effusion, depending on the quantity of fluid, the rate of accumulation and the aetiology of the condition. Pericarditis may go unrecognised or may result in relatively minor clinical signs if the volume of the effusion is small. In the athletic horse, a reduction in performance may be noted. In more severe cases, clinical signs of right-sided CHF are evident, indicating cardiac tamponade. In other animals, signs of systemic disease such as fever, anorexia, weight loss, dyspnoea, tachypnoea and tachycardia are seen. Horses with a pericardial effusion may appear to be in pain and discomfort.
Clinical examination If the pericardium is inflamed and only a small quantity of pericardial fluid is present, a pericardial friction rub may be heard on auscultation. This is a harsh sound which may be confused for a murmur. It is usually triphasic, in time with atrial systole, ventricular systole and ventricular diastole. Occasionally one or more of these sounds will be absent; however, the sound can be distinguished from a pleural rub because it is in time with the cardiac cycle. The sound of a pericardial friction rub is similar to that of a creaking branch or door. Heart sounds are muffled if a significant effusion is present and the friction rub may be lost. If anaerobic bacteria are present, gas formed by these organisms may result in splashing sounds being heard, although this is unusual.
Further diagnostic aids
Electrocardiography Small complexes and electrical alternans (variable size complexes which are alternately large and small) may be seen on an ECG, although these findings are not specific for a pericardial effusion.
Echocardiography The technique of choice for diagnosis of pericardial disis echocardiography. The effusion may be sufficiently clear to be seen with linear-array transducers or sector scanners of up to 5 MHz. The main echofeature is the presence of an anechoic space (fluid) between the echogenic pericardium and the myocardium (Figure 4.29). In some animals with septic pericarditis, fibrin tags forming echogenic fronds within the fluid, or thickening of the pericardial or epicardial surface will be seen. Pleural fluid may also be present, either as a result of right-sided CHF or due to the effects of systemic disease which also involves the pleura. It is important that pleural fluid is not confused with fluid within the pericardium.
The volume of pericardial fluid and its effect on cardiac function should be assessed; this will dictate whether immediate drainage is required. If cardiac tamponade is present, the RA will be seen to collapse during diastole. If the condition is more severe, restriction of RV filling may be observed. An M-mode image is useful to identify paradoxical septal motion, which occurs when RV pressure is high. The heart may be seen to swing from side to side in the fluid and this may make septal motion appear unusual.
Radiography Radiography may be a helpful diagnostic technique in animals with large pericardial effusions if equipment of a suitable output is available. A globular or pumpkin-shaped cardiac outline may be seen. A gas cap may be present if anaerobic organisms are involved.
Clinical pathology Clinical pathological tests are useful in animals which show signs of systemic disease. A plasma fibrinogen assay and haematology should be performed to investigate the likelihood of active inflammation of the pericardium. Cytological, biochemical and bacteriological examinations of pericardial fluid are indicated if pericardiocentesis is performed (see below).
Effusive pericarditis which results in cardiac tamponade is a life-threatening condition which requires immediate, aggressive treatment. Pericarditis may also occur without compromising venous return, in which case the need for treatment depends on whether there are systemic signs of infectious disease. It is essential that the horse receives box rest during the course of treatment and conAlthough pericardial effusions carry a relatively poor prognosis, recent reports show that treatment can be rewarding, with a successful long-term outcome.
Where pericarditis is suspected, echocardiographic examination is invaluable in order to identify the severity of the condition. It is important to identify fibrin tags or echogenic particles within the pericardial fluid which may suggest that an inflammatory or infectious process is occurring. In anaerobic infections, very echogenic particles may be seen indicating the presence of gas-forming organand suitable antibiotics (e.g. metronidazole) must be used (ideally based on the results of bacterial culture and antibiotic sensitivity tests). The cause of the effusion may also be determined from cytological examination of pericardial fluid. Thus for both accurate diagnosis and treatment, placement of a catheter into the pericardium is desirable. However, many horses with CHF, or effusions in other body cavities, have a small amount of pericardial fluid which may be seen echocardiographically but which does not result in tamponade. In these instandrainage is unwarranted and should be avoided.
Drainage/sampling technique The optimal site for pericardiocentesis can be selected from echocardiographic examination. Alternatively, an area over the left or right fifth intercostal space between the level of the costochondral junctions and the shoulder can be selected. The region should be aseptically prepared. A bleb of local anaesthetic is placed under the skin and is infiltrated into the underlying muscle. The skin can be moved slightly so that the puncture site does not directly overlie the muscular puncture site. A stab incision can be made with a No.11 scalpel blade. A 3~21 in or longer 10-12 gauge catheter or a chest drain is advanced into the chest until a slight 'pop' is felt and fluid is aspirated. If a large-bore catheter is used for the puncture, a longer, finer catheter such as a dog urinary catheter can be advanced through it and left in place. If a pleural effusion is also present it may be difficult to distinguish whether the fluid has been aspirated from the pleural or pericardial space. Echocardiographic guidance is particularly helpful in this situation.
An indwelling drain can be left in place, secured with a purse string suture, if drainage or lavage and infusion of drugs is required. Isotonic fluids can be used for lavage. Flushing of the pericardial space with one litre of isotonic fluid, followed by instillation of drugs in a similar volume, has been reported to be successful.
Drainage of pericardial fluid is not without risk. The most worrying compliare laceration of major thoracic or coronary vessels and resultant haeor induction of ventricular arrhythmias. The equine epicardium is reported to be particularly sensitive to stimulation. VPCs are not uncommon and could precipitate ventricular fibrillation. An ECG should therefore be recorded during the procedure so that arrhythmias can be recognised early and treatment given if necessary. It is advisable to have lignocaine or quinidine gluconate on hand for treatment of arrhythmias; however, the arrhythmia will usually resolve when the catheter is withdrawn. An indwelling intravenous catheter should be placed prior to the procedure to allow rapid administration of drugs if necessary.
Long-term complications of the condition and of pericardiocentesis include constrictive pericarditis.
Drug treatment Bacterial pericarditis should be treated with systemic antibut, in addition, broad spectrum antibiotics such as a combination of penicillin and gentamycin, can be infused into the pericardial sac two to four times daily. Idiopathic pericarditis carries a better prognosis than bacterial pericarditis. Animals with idiopathic effusions have been treated with corticossystemically and by local infusion, with success (see Appendix). Cortiare only advisable if an infectious cause has been ruled out and the fluid collects repeatedly following drainage.
Non-steroidal anti-inflammatory drugs such as flunixine meglumine may also be useful in pericardial disease as analgesic and anti-inflammatory agents. If CHF is present, drainage of the effusion is the only suitable form of treatment. Diuresis is unlikely to result in any significant improvement of clinical signs. Vasodilators are contraindicated. Digoxin will have no beneficial effect and should not be used unless atrial tachycardia is present.