Pediatric Cardiac Circulation and Hemodynamics

Hemodynamics: Cardiac Output and its Determinants

Cardiac output (CO) is the volume of blood ejected by the left ventricle in one minute. It is determined by two primary factors: Heart Rate (HR) and Stroke Volume (SV).

$\text{Cardiac Output} = \text{Heart Rate} \times \text{Stroke Volume}$

The components of this equation are influenced by several hemodynamic variables:

• Heart Rate: The number of beats per minute, which depends on the heart's rate and rhythm.
• Stroke Volume: The amount of blood ejected per beat. This is influenced by:
  • Preload: The "stretch" or volume of blood in the ventricles at the end of diastole.
  • Afterload: The "squeeze" or the resistance the heart must pump against to eject blood.
  • Contractility: The innate strength of the cardiac muscle contraction.

Clinical Pearl: Pediatric Differences Unlike adults, who can adjust both heart rate and stroke volume to maintain cardiac output, children (especially neonates) have a limited stroke volume. Therefore, they are highly dependent on Heart Rate to maintain adequate cardiac output.

Cardiac Conduction and Assessment

The heart's rhythm is maintained by a specialized electrical conduction pathway that ensures coordinated contraction of the atria and ventricles. When assessing a child, clinicians often listen for murmurs—sounds created by turbulent blood flow across pressure differences.

Murmurs are graded on a scale of I to VI:

• Grade I: Very faint; may only be heard by an expert.
• Grade III: Moderately loud; no palpable thrill.
• Grade IV: Loud and associated with a palpable thrill (a vibratory sensation).
• Grade VI: Loudest; can be heard even with the stethoscope not touching the skin.

While "innocent" murmurs occur in up to 80% of children with normal anatomy, any murmur associated with a thrill is considered abnormal.

Congenital Heart Defects (CHDs)

Congenital defects are abnormalities present at birth, often resulting from genetic or environmental factors during the first eight weeks of gestation. These are classified by how they affect blood flow:

1. Increased Pulmonary Blood Flow

These defects cause blood to shunt from the left side of the heart to the right, overloading the lungs.

• Atrial Septal Defect (ASD): An opening between the atria resulting in left-to-right shunting.
• Ventricular Septal Defect (VSD): An opening between the ventricles. Large VSDs can lead to heart failure and failure to thrive.

2. Decreased Pulmonary Blood Flow

These defects limit the amount of blood reaching the lungs for oxygenation.

• Tetralogy of Fallot (ToF): Characterized by four defects: VSD, pulmonic stenosis, overriding aorta, and right ventricular hypertrophy. Children may experience "tet spells"—sudden episodes of cyanosis and hypoxia often triggered by agitation.

3. Obstructive Defects

These block the flow of blood through the systemic circulation.

• Coarctation of the Aorta (CoArc): A localized narrowing of the aorta. This results in high blood pressure in the upper extremities and weak or absent pulses in the lower extremities (e.g., femoral pulses).

4. Mixed Blood Flow

• Transposition of the Great Arteries (TGA): The aorta and pulmonary artery are switched, creating parallel circulations that do not mix unless a VSD or PDA is present.
• Hypoplastic Left Heart Syndrome (HLHS): The left side of the heart is severely underdeveloped. This is a critical condition often requiring a three-stage surgical palliation (Norwood, Glenn, and Fontan procedures) to allow the right ventricle to support systemic circulation.

Cardiomyopathy and Heart Failure

Cardiomyopathy refers to abnormalities of the myocardium that impair the heart's ability to contract or relax.

• Dilated Cardiomyopathy: Enlarged chambers and reduced contractility.
• Hypertrophic Cardiomyopathy: Thickened walls that may obstruct outflow.
• Restrictive Cardiomyopathy: Stiff walls that impair ventricular filling.

When these conditions lead to Heart Failure (HF), symptoms vary based on which side of the heart is failing:

• Left-Sided Failure: Results in pulmonary congestion (crackles, wheezes, dyspnea, tachypnea).
• Right-Sided Failure: Results in systemic congestion (ascites, hepatosplenomegaly, peripheral edema).

Nursing Management Goals For children with cardiac dysfunction, nursing interventions focus on reducing the workload of the heart (rest), improving respiration, maintaining nutrition/fluid balance, and supporting the family.