Comprehensive Cardiovascular Science and Pharmacology Review

Cardiac Valve Anatomy and Functional Mechanisms

The heart incorporates four primary valves that maintain unidirectional blood flow: the tricuspid valve, the mitral valve, the pulmonary valve, and the aortic valve. The tricuspid and mitral valves are classified as atrioventricular (AV) valves, while the pulmonary and aortic valves are termed semilunar valves. Structural differences exist between these groups; specifically, AV valves are equipped with chordae tendineae and papillary muscles, whereas semilunar valves lack chordae tendineae. The opening and closing of these valves are governed by intrachamber pressure changes. The chordae tendineae serve a critical function by connecting the AV valve leaflets to the papillary muscles, thereby preventing AV valve prolapse into the atria during ventricular systole.

Atrial and Ventricular Chambers and Septal Structures

The atria function as reservoirs and assist in ventricular filling. The right atrium receives deoxygenated blood from three main sources: the superior vena cava (SVC), the inferior vena cava (IVC), and the coronary sinus. In contrast, the left atrium receives oxygenated blood from the four pulmonary veins. Anatomically, the heart is divided by septa. The interatrial septum separates the two atria and contains the fossa ovalis, a remnant of fetal circulation. The interventricular septum separates the ventricles and consists of both muscular and membranous portions.

Cardiovascular Embryology and Fetal Shunts

Cardiac development begins around week $3$ with the formation of the primitive heart tube. This tube undergoes dextral looping, a process where it folds to the right to position the future cardiac chambers correctly. The primitive heart tube is composed of several segments: the truncus arteriosus, which develops into the ascending aorta and the pulmonary trunk; the bulbus cordis, which develops into the right ventricle and the outflow tracts for both ventricles; the primitive ventricle; the primitive atrium; and the sinus venosus. Endocardial cushions are vital during development as they evolve into the AV valves, the membranous interventricular septum, and portions of the atrial septum.

Fetal circulation relies on three specialized shunts: the foramen ovale, which directs blood from the right atrium to the left atrium; the ductus arteriosus, which shunts blood from the pulmonary artery to the aorta; and the ductus venosus, which connects the umbilical vein to the IVC. At birth, pulmonary vascular resistance drops dramatically following the first breath, leading to the closure of these shunts. Post-natal oxygenation is managed via one umbilical vein carrying oxygenated blood and two umbilical arteries carrying deoxygenated blood.

Vascular Histology and Systemic Anatomy

Blood vessels are composed of three distinct layers: the tunica intima, the tunica media, and the tunica adventitia. Arteries are characterized by a thick tunica media, high internal pressure, and significant elasticity. Veins possess thinner walls, a larger lumen, and valves to prevent backflow. The abdominal aorta begins at the level of $T12$ after passing through the diaphragm and terminates at $L4$, where it bifurcates into the common iliac arteries. The coronary circulation includes the right coronary artery (RCA), the left coronary artery (LCA), the left anterior descending (LAD) artery, and the circumflex artery. Venous blood from the myocardium drains into the coronary sinus, a large vein on the posterior aspect of the heart, which then empties into the right atrium.

Cardiac Physiology and Hemodynamics

Cardiac muscle contains abundant mitochondria for the production of adenosine triphosphate (ATP), and its functional contractile unit is the sarcomere. Movement of blood is categorized by oxygenation status: oxygenated blood is found in the pulmonary veins, the aorta, and systemic arteries, while deoxygenated blood is found in the pulmonary arteries, the vena cava, and systemic veins. Standard oxygen saturation levels are approximately $75\%$ in the right atrium, right ventricle, and pulmonary artery, while the left atrium, left ventricle, and aorta maintain levels between $95\%\text{--}100\%$.

Normal physiological pressures vary by chamber: the aorta is typically $120/80\,\text{mmHg}$, the left ventricle ranges from $120/5\text{--}12\,\text{mmHg}$, the right ventricle is $25/5\,\text{mmHg}$, and the pulmonary artery is $25/10\,\text{mmHg}$. The normal systolic pulmonary artery pressure is generally between $15\text{--}30\,\text{mmHg}$, averaging approximately $25\,\text{mmHg}$. Key physiological concepts include preload, which is the ventricular filling or stretch prior to contraction, and afterload, which is the resistance the ventricles must pump against.

The Cardiac Cycle and Heart Sounds

The cardiac cycle consists of five stages: 1. Ventricular filling, 2. Atrial systole, 3. Isovolumic contraction, 4. Ventricular ejection, and 5. Isovolumic relaxation. During isovolumic contraction, the ventricles contract, pressure rises, but volume remains unchanged because all valves are closed. This phase corresponds with the QRS complex and the onset of systole. During ventricular ejection, the semilunar valves open while the AV valves remain closed. During isovolumic relaxation, all valves are again closed.

Heart sounds provide clinical markers for the cycle. $S1$ is caused by the closure of the mitral and tricuspid valves at the beginning of systole. $S2$ is caused by the closure of the aortic and pulmonary valves at the beginning of diastole. Physiologic splitting of $S2$ occurs during inspiration. Cardiac output ($CO$) is calculated as $CO = HR \times SV$ (Heart Rate times Stroke Volume), with a normal range of $4\text{--}8\,\text{L/min}$. During inspiration, right heart filling increases while left heart filling briefly decreases.

Electrocardiography (ECG)

An electrocardiogram (ECG) is the electrical recording of heart activity. Einthoven's triangle involves electrodes on the right arm (RA), left arm (LA), and left leg. Lead $II$ is the most commonly used lead for rhythm monitoring because it best aligns with the heart's normal electrical axis, which ranges from $-30^{\circ}$ to $+90^{\circ}$. Lead systems include bipolar leads ($I$, $II$, $III$) and unipolar leads ($aVR$, $aVL$, $aVF$, and the precordial leads $V1\text{--}V6$). An upward deflection occurs when an electrical impulse moves toward a positive electrode.

Key ECG components include the P wave (atrial depolarization), the QRS complex (ventricular depolarization), and the T wave (ventricular repolarization). The ST segment represents the period when ventricles are fully depolarized and early ventricular contraction occurs. A normal PR interval lasts $0.12\text{--}0.20\,\text{sec}$. In terms of grid measurement, a large ECG box represents $0.20\,\text{seconds}$. Normal sinus rhythm is defined as $60\text{--}100\,\text{bpm}$, with bradycardia falling below $60\,\text{bpm}$ and tachycardia exceeding $100\,\text{bpm}$. Essential ions for myocardial function include Calcium ($Ca^{2+}$) for contraction, Sodium ($Na^+$) for depolarization, and Potassium ($K^+$) for repolarization. The refractory period is the interval during which cardiac cells cannot be re-excited.

Diagnostic Testing and Clinical Murmurs

Stress testing is used to evaluate ischemia, exercise tolerance, and arrhythmias. Tests should be terminated if the patient experiences chest pain, serious arrhythmias, severe blood pressure changes, significant ST-segment changes, or severe dyspnea. Stress echocardiography specifically detects wall motion abnormalities, while nuclear imaging assesses myocardial perfusion and viability. Pharmacological stress tests may utilize Dobutamine, a $\beta1$ agonist that increases heart rate and contractility for patients unable to exercise.

Clinical signs such as an ejection click indicate the opening of an abnormal semilunar valve. A systolic ejection murmur characterized by a crescendo-decrescendo pattern is associated with aortic stenosis, typically auscultated at the $2$nd right intercostal space. For hemodynamics, the pulmonary artery wedge pressure (PCWP) is best measured using a Swan-Ganz (pulmonary artery) catheter.

Cardiovascular Pharmacology and Nursing Considerations

Pharmacological interventions for hypertension and heart failure include various classes of drugs. Diuretics, such as loop and thiazide diuretics, can cause the dangerous complication of hypokalemia. Potassium-sparing diuretics include Spironolactone, Eplerenone, Amiloride, and Triamterene. Effectiveness is indicated by weight loss, reduced edema, improved breathing, and increased urine output. Diabetics on thiazides must monitor blood glucose levels closely as these drugs can increase glucose. The FDA is responsible for drug approval, safety, and monitoring adverse effects.

Angiotensin-Converting Enzyme (ACE) inhibitors block the conversion of Angiotensin $I$ to Angiotensin $II$, reducing vasoconstriction and aldosterone; a life-threatening side effect is angioedema. Angiotensin Receptor Blockers (ARBs) can cause hyperkalemia, hypotension, and renal impairment. Alpha blockers induce vasodilation by blocking $\alpha1$ receptors, while Beta blockers lower heart rate, contractility, and renin release. Direct vasodilators include Hydralazine and Minoxidil. Natriuretic peptides promote the excretion of sodium and water and lower both preload and afterload.

Digoxin requires careful monitoring; the apical pulse must be checked for $1$ full minute before administration and held if the heart rate is less than $60\,\text{bpm}$. Toxicity signs include nausea, vomiting, yellow/green vision, and arrhythmias. Intravenous (IV) potassium is a high-alert medication; it must never be given via IV push due to the risk of fatal cardiac arrest. Older patients on opioids require monitoring for sedation, respiratory depression, and constipation. For lipid management, bile acid sequestrants like Cholestyramine should be mixed with water and taken with increased fluids and fiber; other meds should be taken $1$ hour before or $4\text{--}6$ hours after. Nicotinic acid is contraindicated in those with active liver disease, peptic ulcer disease, or severe gout.

Hematology and Stroke Management

Hematological monitoring involves the International Normalized Ratio (INR) to manage warfarin therapy. Erythropoiesis-stimulating agents (ESAs) are used to treat anemia associated with chronic kidney disease or chemotherapy. In stroke management, ischemic stroke is treated with thrombolytics like alteplase if within the window, followed by antiplatelet therapy. Hemorrhagic stroke treatment avoids thrombolytics and focuses on blood pressure control and neurosurgical intervention.