October 2024

In this VETgirl online veterinary continuing education blog, Dr. Natasha Yeh and Dr. Christopher Kennedy, DACVECC, DECVECC discuss the use of focused cardiac ultrasound (FCU) to identify pericardial effusion and tamponade in dogs and cats. The most common causes of pericardial effusion in dogs are cardiac neoplasia and idiopathic pericardial effusion (idiopathic pericarditis). In cats, congestive heart failure/volume overload are common but rarely require pericardiocentesis. While commonly encountered and treated in the emergency room, pericardial diseases benefit from investigation by a cardiologist whenever possible.

The goals of FCU in pericardial effusion:

  • Identify pericardial effusion
  • Suspect cardiac tamponade physiology
  • Recognize inversion/collapse of the right atrium (+/- right ventricle)
  • Integrate this information with your clinical examination

The targeted views:

  • PLAX4
  • PSAX-apex
  • PSAX-base
  • SX-heart
  • SX-CVC

Definitions:

Pericarditis is inflammation of the pericardial sac.

Pericardial effusion (PE) is accumulation of any fluid.

Cardiac tamponade (CT) occurs when the pressure in the pericardial space is greater than the intra-cardiac pressures, leading to compression of the heart chambers.

Identifying pericardial effusion

Remember our blog Focused Cardiac Ultrasound (FCU) Examination with Dr. Christopher Kennedy to establish a systematic approach? The first goal in each view is to evaluate for the presence of PE. Identifying greater than mild PE is straightforward. Smaller volumes can be missed. Using multiple views improves sensitivity. As fluids are good conductors (poor reflectors) of soundwaves, they appear hypoechoic to anechoic (black) on the ultrasound display. Fluid tends to accumulate around the apex before the base.

Video 1 – PLAX4 with PE

Video 2 – PSAX-apex with PE

Video 3 – PSAX-base with PE

The SX-heart view can also be used to identify PE.

Video 4 – SX-heart with PE. The ECG is not attached to the patient.

Figure 1 – Still image of video 4. The heart is outlined in pink. PE is outlined in blue.

PE can be quantified semi-objectively using ultrasound; however, quantity does not perfectly correlate with clinical severity. A simple linear measurement can be used to compare PE volume before and after pericardiocentesis. Alternatively, subjective assessment of quantity can be sufficient.

Cardiac tamponade

As fluid accumulates within the pericardium, intrapericardial pressure increases. The stiffness of the pericardium determines the volume-pressure relationship (figure 2). Slowly accumulating effusions may reach larger volumes before CT occurs, as the pericardium has time to stretch.

 

Figure 2: Pericardial Volume-Pressure Graph. Pressure increases as volume increases. Past the limit of pericardial stretch, pericardial pressure increases rapidly and exceeds intracardiac pressures. When outside pressure (pericardial pressure) is greater than the inside pressure (e.g., right atrial pressure), collapse (tamponade) of that chamber occurs.

CT is a physiology, not a disease itself. It is a clinical diagnosis made by:

  • Presence of pericardial effusion
  • + Signs of low cardiac output (e.g., syncope, shock, hypotension, pulsus paradoxus)
  • + Compensation (tachycardia and/or ascites due to fluid retention)

With FCU, we can identify inversion (collapse) of the right atrium (RA) in the PLAX4 view – this occurs when pericardial pressure > RA pressure, i.e. CT.

Video 5 – PLAX4 showing PE with right atrial collapse.

Figure 3 – Still image of video 5. The RA walls are highlighted and the RA cavity is shaded in green. The right ventricular wall is highlighted in pink. The interventricular septum and the left ventricular free wall are highlighted in red. The pericardial effusion is shaded blue. Note the inversion, or buckling, of the right atrial wall in toward the right atrial cavity, reducing the potential space for systemic venous blood to return to the right atrium.

RA collapse is sensitive, but not 100% specific for CT. Right ventricular collapse is less sensitive, though more specific. CT is a clinical diagnosis: RA collapse alone is not a perfect diagnostic tool.

Video 6 – This is the patient associated with video 5. This patient was diagnosed with non-hemorrhagic idiopathic pericardial effusion that resolved after two pericardiocentesises one week apart. He never showed signs of hemodynamic compromise.

With CT, we expect congestion of the systemic venous circulation. The SX-CVC view should show a distended caudal vena cava (CVC) with minimal respiratory-induced size variation. A non-distended, collapsible CVC suggests CT is not present – look for a different problem somewhere else.

Video 7 – SX-CVC showing a distended/fat caudal vena cava with minimal respiratory-associated variation in size.

Two exceptions exist. First, volume underloaded patients may experience tamponade physiology with a collapsible CVC – “low-pressure tamponade” may also lack other classical findings of CT. This is uncommon in dogs and cats. Second, brachycephalic patients with marked inspiratory efforts can reduce their intrathoracic pressures enough to collapse their CVC.

Cardiac masses

Canine cardiac masses may be identified with FCU. However, I have missed these; additionally, pericardial fat can be mistaken for masses. Consultation with a cardiologist is strongly recommended in all PE cases even if the effusion is hemorrhagic – idiopathic effusion is often hemorrhagic and may not be terminal.

Video 8 – PLAX4 showing a cardiac mass in the right atrioventricular groove.

Video 9 – PSAX-base showing a cardiac mass in the right atrioventricular groove.

Left atrial tear and pericardial blood clots

Left atrial tearing occurs in dogs with myxomatous mitral valve disease (MMVD). Sometimes a blood clot can be seen in the pericardial space and should not be mistaken for a neoplasm (video 10). Pericardiocentesis is generally avoided unless CT is present, in which case aspirating a small volume may be beneficial; however, lowering the pericardial pressure potentially risks re-bleeding via the atrial tear, so a risk-benefit assessment is necessary. Fortunately, we can use clinical judgement and prioritize differentials based on signalment: small dogs get MMVD, so we avoid pericardiocentesis unless the patient is clinical; big dogs experience neoplastic or idiopathic PE, where pericardiocentesis may be diagnostic and/or therapeutic.

Video 10 – PLAX4 showing a pericardial thrombus following left atrial tear in a dog with MMVD. Note that the left atrium is enlarged and the mitral valve is irregular.

Figure 4 – Still image of video 10. The thrombus is highlighted in red. Note how it is outside of the heart, within the pericardium. It is also not in the typical right atrial/auricular or heart base locations that we expect to find tumors.

Pericardiocentesis

Considering the volume-pressure curve, pericardiocentesis to decompress the pericardial space is the treatment of choice for CT. It is important to confirm that this procedure is necessary first and then to perform it safely, in a controlled manner. Fluids may be helpful in some cases, though will only be temporarily effective. Patients with PE without CT may be referred emergently to a cardiologist for investigation prior to pericardiocentesis.

References and further reading:

  1. Adler Y, Ristić AD, Imazio M, Brucato A, Pankuweit S, Burazor I, Seferović PM, Oh JK. Cardiac tamponade. Nature Reviews Disease Primers. 2023 Jul 20;9(1):36.
  2. Fernández de Palacio, M.J. Pericardial diseases. In. Bussadori C, editors. Textbook of cardiovascular medicine in dogs and cats.  Florida: Edra Publishing. 2023. p. 403-439.

Please note that the opinions in this blog are expressed by the authors, and not directly endorsed by VETgirl.

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