Heart disease is not uncommon in guinea pigs, with the most common abnormality seen on echocardiography (ultrasound scan of the heart) being a pericardial effusion (reported to be found in 63% of guinea pigs presenting with heart disease). Pericardial effusions in guinea pigs are not well documented and are still not completely understood in terms of their cause.
Guinea pigs with a pericardial effusion appear to present most frequently with sudden onset dyspnoea (difficulty breathing). Other general signs of poor heart function can also occur, including lethargy, anorexia, respiratory noise, nasal discharge, cold extremities.
What is a pericardial effusion?
The heart is surrounded by a thick fibrous bag known as the pericardium. The pericardium has two layers, one of which is intimately associated with the heart. In a normal individual the pericardial space (the space between the two layers of the pericardium) will contain a small amount of fluid. This fluid acts as a lubricant, allowing for the smooth movement of the heart as it beats. In some circumstances the pericardium fills up with too much fluid. Since the outer layer of the pericardium is firm and non-elastic the effect of this fluid is to put pressure on the heart. The pericardial fluid, or effusion, effectively squashes the heart. This is known as cardiac tamponade. The result is that the heart cannot refill with sufficient blood during the relax phase of heart contractions, so less blood is pumped forwards when the heart contracts. This means that blood backs up in the vessels leading to the heart (congestion) and can lead to fluid leaking out of the vessels into the tissues of the body or the body spaces (ie the thoracic or abdominal cavities).
The diagrams below illustrate a normal heart and one where a pericardial effusion has developed, showing the expansion of space between the visceral and parietal pericardium (the pericardial effusion) and the pressure this puts on the heart (cardiac tamponade, illustrated by the green arrows).
PE – Pericardial effusion RA – Right atria RV – Right ventricle LA – Left atria LV – Left ventricle
How can a pericardial effusion be diagnosed?
An indication of a pericardial effusion can be obtained from the clinical exam. The patient will present with signs or heart disease as listed above. When listening to the heart with a stethoscope the heart sounds may be muffled, due to the fluid in the pericardial space.
A chest x-ray will often show an enlarged cardiac silhouette (outline of the heart), which will have a more well-defined edge, since it will not show the movement blur seen in normal circumstances. In a patient with pericardial effusion the movement of the heart is within the pericardial fluid and does not cause movement of the outer layer of the pericardium.
The most sensitive method of diagnosis is by performing an echocardiogram. This is an ultrasound scan of the heart. This will clearly show the enlarged pericardial space, filled with fluid. It also allows you to visualise the heart chambers, in order to attempt to ascertain the cause of the effusion.
PE – Pericardial effusion RA – Right atria RV – Right ventricle LA – Left atria LV – Left ventricle
What is the cause of a pericardial effusion?
The cause of pericardial effusions in guinea pigs is still poorly understood. Possible causes can include:
· Septic, infection causing a build-up of purulent fluid.
· Cardiogenic, as a result of heart failure.
· Tumours, bleeding into the pericardial space.
· Idiopathic, unknown cause.
How can the pericardial effusion be treated?
If a pericardial effusion is seen on an echocardiogram the best route of action is to drain the fluid. This is achieved by clipping the hair, cleaning the skin and inserting a needle into the pericardial space, through the skin, in order to drain the fluid. This will rapidly improve the cardiac filling (ability of the heart to fill with blood), resulting in the heart working more efficiently and an improvement in clinical signs. The fluid can also be saved and sent to a laboratory to analyse which will aide in determining the cause of the effusion.
Cases are often also treated with diuretics (to remove water from the system) and heart medication (to help with the efficiency of the heart). If the fluid is septic then antibiotics would also be indicated.
Reggie’s case
Reggie presented with acute onset dyspnoea (increased rate and effort of breathing), having been fine the previous day. Pericardial effusion was suspected from clinical exam. An echocardiogram was performed which revealed a pericardial effusion. The pericardial effusion was drained under heavy sedation. Reggie’s heart looked grossly normal on exam and no tumours were visible. He was started on diuretics and heart medication as a precaution. The fluid was sent for analysis (see pic below).
The fluid showed no evidence of infection or neoplastic cells, but was thought to be an idiopathic, haemorrhagic effusion. The round cells evident on the slide are mesothelial cells found normally in the pericardium.
Within 2-3 days Reggie was virtually back to normal. His heart medications were discontinued 2 weeks later. No reformation of the pericardial effusion has been found and Reggie continues to do very well.