The Cardiac Muscle and Pericarditis

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As the circulatory pump of the human body, it is vital that the cardiac muscle has structures compatible with its necessary function. The heart is vulnerable to disease, but this risk is decreased by its inherited features crucial for survival.

At a cellular level:

The photo below presents a microscopic image of a stained cardiac muscle sample, presenting the following structures[4]:

  • Striations, or striped cardiac muscle – this is the lighter actin and darker myosin muscle fibres arranged in sarcomeres to enhance contractive force for pumping
  • Nuclei – contain DNA for cell regeneration
  • Intercalated discs – crucial for cell-to-cell communication. Also contain gap junctions and desmosomes that maintain the heart’s structure
  • Gap junctions – a channel between adjacent myocytes for impulse transmission, allowing for intrinsic rhythmicity
  • Desmosome – structures which hold muscle fibres together to prevent tearing during contractions
  • Mitochondria – the site of aerobic respiration to produce ATP needed for muscular contraction

If the flow of oxygenated blood to the heart is restricted then ATP will not be produced, meaning the heart cannot contract, and a myocardial infarction occurs. This can lead to Dressler’s syndrome due to cellular death triggering inflammation, a type of pericarditis[3].

Chapter 7, Page 8 - HistologyOLM
A microscopic image of the cardiac muscle [1]

The general structure

There are four chambers of the heart; the left and right atria and the left and right ventricles. These are separated by the vertical septum down the middle, and the atrioventricular valves, allowing each chamber to function at different times. The valves, rooted by tendinous cords and papillary muscles, prevent backflow of blood to ensure efficient delivery of oxygenated blood to working muscles.

The structure of the cardiac tissue from inner to outer is the endocardium, myocardium (responsible for contractions), epicardium (lining of the myocardium), pericardial cavity and adipose tissue.

The pericardium is a sac-like structure surrounding the cardiac muscle, enclosing the pericardial cavity. There are two layers of the pericardium; the serous pericardium (including the pericardial cavity) and fibrous pericardium (keeps the heart in place). Within the cavity there is pericardial fluid; the lubricant between the cardiac walls and the layers of the pericardium. This substance is crucial to prevent friction from occurring but can lead to a serious diagnosis when at an abnormal level[3].

Pericarditis is a self-explanatory translation of ‘around heart inflamed’, involving swelling/inflammation of the pericardium. Although most diagnoses are idiopathic, one of the common causes is a viral infection. More often than not, the swelling is an indirect cause of this virus or autoimmune disease, stimulated by the hyperactive immune system attacking healthy bodily tissues. This can lead to a crushing, central chest pain commonly associated with myocardial infarction.

Pericarditis - Symptoms and causes - Mayo Clinic
A diagram showing the swelling of the pericardium in pericarditis[2]

Other than the aforementioned result of Dressler’s syndrome, pericarditis can also be caused by kidney problems, leading to a high concentration of urea and nitrogen in the blood (uremic pericarditis). This irritates the serous pericardial layers, stimulating a release of fluid comprised of fibres and lymphocytes which causes the pericardial cavity to swell.

Regardless of the cause, the inflammation in the pericardial layers causes fluid and immune cells to move into the pericardial tissues, causing its swelling appearance. If more fluid moves in than it does out, pericardial effusion can occur whereby there is an abnormal accumulation of fluid around the heart. If not treated quickly, this accumulation can cause so much pressure on the cardiac muscle that the ventricles no longer expand enough to fill with sufficient oxygenated blood (cardiac tamponade) which can be fatal.[5]

Another possible result of pericarditis is the immune system’s response of simulating fibrosis of the cardiac tissue, preventing expansion due to the inelastic nature of the scar tissue. This leads to more difficulty contracting and relaxing, leading to a decreased stroke volume, causing a need for an increased heart rate – constrictive pericarditis.[4]

Some symptoms of pericarditis[6]

  • Fever
  • Chest pain that improves when sitting up
  • Heavy, short breathing
  • Light-headedness and fatigue
  • Crushing chest pain

Diagnosis of pericarditis[6]

  • Friction between pericardial layers can be heard as a leathery sound
  • On an echocardiogram, the heart can be seen ‘dancing’ from’ side to side as it bathes in the surplus of fluid in a tamponade
  • If the patient has constrictive pericarditis, the heart may be presented as stiff and immobile.
  • On an ECG (electrocardiogram) the QRS wave may be smaller due to the weakened contraction the ST wave may be flat and the T wave may be irregular or inverted – caused by heart moving back and forth
  • On an X-ray, the cardiac muscle appears bottle-shaped and larger

Some treatments for pericarditis[7]

  • The swollen pericardial sac may be pierced with a scalpel to release fluid and relieve the cardiac muscle of the pressure – pericardiocentesis
  • Viral medication
  • For constrictive pericarditis, a pericardiectomy must be done to remove the pericardium

Overall, the heart is a vital yet complex organ that is essential to keeping us alive. Although some diagnoses are idiopathic, preventive measures must always be taken to reduce the risk of undiscovered or prevalent disorders.


References

  1. ‘Microscopic Study of Cardiac Muscle in Long Section’, HistologyOLM
  2. ‘Pericarditis’, Mayo Clinic, 06 March, 2018
  3. ‘Pericarditis and pericardial effusions’, Osmosis on YouTube, 09 Jul 2019
  4. ’69 10.7 CARDIAC MUSCLE TISSUE’, OpenTextBC
  5. ‘Cardiac Tamponade’, Dr Laurence Knott, 16 Sep 2020
  6. ‘Pericarditis’, Harvard Health Publishing, Dec, 2018
  7. ‘Prevention and Treatment of Pericarditis’, Heart.org, 31 Mar, 2016

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