The Heart, Exposure of the Heart, Coronary Vessels, Removal of the Heart, Bisecting the Heart, In Situ Dissection of the Heart

Trauma: Cardiac box: The “cardiac box” is the region demarcated by the sternal notch (superiorly), xiphoid process (inferior), and midclavicular lines (laterally). Penetrating trauma or blunt force to the cardiac box can injure the heart. The right ventricle, being the most anterior structure of the heart, is most prone to penetrating injury. A life-threatening consequence of cardiac trauma is cardiac tamponade where blood accumulates within the pericardial sac and compresses the heart. For more information of thoracic trauma, review the “deadly dozen”. 

Procedure: Atrial fibrillation, the left atrial appendage, and TEE: Atrial fibrillation (Afib) impairs the contraction and emptying of the atria which allows blood to pool and clot. These clots generally form in the left atrial appendage (LAA), a pouch-like portion of the left atrium. Return to sinus rhythm can dislodge clots into the aorta (causing stroke) therefore high-risk patients should undergo work-up for LAA thrombus prior to cardioversion. Because its far posterior location and interference from the bones of the spine/rib cage, the left atrium cannot be reliably viewed through transthoracic echo (external ultrasound probe placed on external chest). Importantly, the esophagus is located immediately posterior to the left atrium, Therefore, the gold-standard for diagnosing LAA thrombus is transesophageal echo (TEE) which entails passage of an ultrasound probe through the mouth and into the esophagus of an anesthetized patient.  

Embryology: Foramen ovale and ductus arteriosus: In utero, lungs are fluid-filled (i.e. high-pressure system) therefore most blood pumped by the right heart bypasses the lungs through two structures:  

• the ductus arteriosus: allows blood to move from pulmonary artery to aorta
• the foramen ovale: allows blood to move from the right atrium to the left atrium

At birth, the lungs become a low-pressure system prompting signaling events that precipitate closure of these structures to leave remnants:  

• ductus arteriosus becomes the ligamentum arteriosum  
• fossa ovalis becomes the foramen ovale 

If these structures remain inappropriately open (i.e. patent), inappropriate shunting of blood may occur. A patent ductus arteriosus (PDA) allows blood to move from the higher-pressure aorta to the lower pressure pulmonary trunk, pathologically increasing blood flow across the heart and lungs. A patent foramen ovale (PFO), while usually asymptomatic, is associated with intermittent right to left shunting when RA pressure rises (see paradoxical emboli for more information). 

Surgery/Autopsy: Cardiac surgery and phrenic nerve: Within the thorax, the right and left phrenic nerves pass between the fibrous pericardium and the mediastinal parietal pleura. As a result, cardiac surgeries that involve manipulation of the pericardium carry a risk of phrenic nerve injury. Clinical presentations include hiccups and difficulty breathing. Imaging may show diaphragmatic dysfunction, paralysis, or paradoxical movements. 

Anatomical Variations: Coronary “dominance”: The posterior descending artery (PDA) feeds the posterior third of the interventricular septum. The PDA arise from the right coronary artery (70%; “right-dominance”), left coronary artery (10%; “left-dominance”), or both (20%; “co-dominance”). Knowing a patient’s cardiac dominance is critical for appropriate endovascular navigation to alleviate PDA stenosis or appropriate installation of vascular grafts during bypass surgery. 

Imaging: Chest Xray is the first modality often used to image the chest, as it is fast, safe and inexpensive. However, although good for looking at large lung findings and assessing the size of the heart or placement of support devices, it does not allow for visualization of discrete cardiac anatomy. Instead, CT with contrast can be use. Echocardiogram (ultrasound) is one of the best and most common ways to evaluate the cardiac chambers and function. Cardiac MRI can be used for specialized applications to evaluate the myometrium and cardiac function, but requires much more time and is only performed at subspecialty centers. 

Imaging correlate: Head to Table 13 to see a pacemaker or Table 26 to see a pacemaker, cardiac valve replacement (can you figure out which one?) and other support devices. What anatomic structures are these support devices located in? Challenge: How are the cardiac pacemaker leads placed via an endovascular approach…how did they get there??