Cardiac and Pulmonary Physiology: ECG, Heart Valves, Lung Volumes, and Acid-Base Balance

What is the electrical conduction pathway of the heart?

SA node → AV node → Bundle of His → Right and Left Bundle Branches → Purkinje fibers

What are the phases of the ventricular action potential?

Phase 0: Depolarization (Na⁺ influx), Phase 1: Initial repolarization (K⁺ out), Phase 2: Plateau (Ca²⁺ influx balances K⁺ out), Phase 3: Repolarization (K⁺ out), Phase 4: Resting potential (Na⁺/K⁺ pump activity)

What are the phases of the cardiac cycle?

Atrial systole → Isovolumetric contraction → Ventricular ejection → Isovolumetric relaxation → Ventricular filling

Which heart valves are open and closed during systole?

Systole: AV valves (mitral, tricuspid) closed; Semilunar valves (aortic, pulmonary) open

Which heart valves are open and closed during diastole?

Diastole: AV valves open; Semilunar valves closed

What causes the first heart sound (S1)?

Closure of AV valves (mitral and tricuspid) at the beginning of systole

What causes the second heart sound (S2)?

Closure of semilunar valves (aortic and pulmonary) at the beginning of diastole

What causes heart murmurs?

Turbulent blood flow, often due to valve stenosis or regurgitation

What do the P, QRS, and T waves represent on ECG?

P wave: atrial depolarization; QRS complex: ventricular depolarization; T wave: ventricular repolarization

What does the PR interval represent?

Time from onset of atrial depolarization to onset of ventricular depolarization (AV nodal delay)

What does the QT interval represent?

Duration of ventricular depolarization and repolarization

What is the standard ECG paper speed and calibration?

25 mm/sec; 1 mV = 10 mm

What are the limb leads in ECG?

I, II, III, aVR, aVL, aVF

Where are the precordial (chest) ECG leads placed?

V1-V6 are placed across the anterior chest in specific intercostal spaces (V1/V2 = 4th ICS, V4 = 5th ICS midclavicular)

What are Type I pneumocytes and their function?

Type I cells cover 95% of alveolar surface and are involved in gas exchange

What are Type II pneumocytes and their function?

Type II cells produce surfactant and can proliferate to replace damaged Type I cells

What is the function of pulmonary surfactant?

Reduces surface tension, preventing alveolar collapse (especially during expiration)

Which lung volumes make up vital capacity (VC)?

VC = Tidal Volume (TV) + Inspiratory Reserve Volume (IRV) + Expiratory Reserve Volume (ERV)

Which lung volumes make up total lung capacity (TLC)?

TLC = VC + Residual Volume (RV)

What is tidal volume (TV)?

The amount of air inhaled or exhaled in a normal breath (~500 mL)

What is residual volume (RV)?

Volume of air remaining in the lungs after maximal exhalation

What distinguishes the conducting zone from the respiratory zone?

Conducting zone (trachea to terminal bronchioles): no gas exchange; Respiratory zone (respiratory bronchioles to alveoli): gas exchange occurs

What is anatomical dead space?

Volume of air in the conducting zone that does not participate in gas exchange (~150 mL)

What is alveolar dead space?

Alveoli that are ventilated but not perfused; contributes to physiologic dead space

What are the three lung zones and their characteristics?

Zone 1: PA > Pa > Pv (least perfused), Zone 2: Pa > PA > Pv, Zone 3: Pa > Pv > PA (most perfused)

What is the normal V/Q ratio and its significance?

Normal V/Q ≈ 0.8; balance of ventilation and perfusion necessary for optimal gas exchange

How does V/Q ratio vary from apex to base of lung?

Apex: high V/Q (more ventilation than perfusion); Base: low V/Q (more perfusion than ventilation)

What is a physiologic shunt?

Perfusion without ventilation; blood bypasses the alveoli (e.g., airway obstruction)

What is alveolar dead space?

Ventilation without perfusion; air reaches alveoli but no gas exchange occurs (e.g., pulmonary embolism)

What is a normal arterial blood pH range?

7.35-7.45

What do PaCO₂ and HCO₃⁻ represent in ABG interpretation?

PaCO₂ = respiratory component; HCO₃⁻ = metabolic component

What defines respiratory acidosis?

Low pH, high PaCO₂ (e.g., hypoventilation, COPD)

What defines respiratory alkalosis?

High pH, low PaCO₂ (e.g., hyperventilation, anxiety)

What defines metabolic acidosis?

Low pH, low HCO₃⁻ (e.g., DKA, diarrhea)

What defines metabolic alkalosis?

High pH, high HCO₃⁻ (e.g., vomiting, diuretic use)

Which acid-base disorder is seen in COPD?

Chronic respiratory acidosis (high PaCO₂, compensated with high HCO₃⁻)

Which acid-base disorder is seen in diabetic ketoacidosis (DKA)?

Metabolic acidosis (low HCO₃⁻ and low pH)

Which acid-base disturbance is caused by vomiting?

Metabolic alkalosis (loss of H⁺ and Cl⁻ from stomach)

Which acid-base disturbance is caused by aspirin overdose?

Early: respiratory alkalosis; Late: mixed metabolic acidosis and respiratory alkalosis

What is the pacemaker of the heart and why?

The SA node, because it has the highest rate of spontaneous depolarization

What is the function of the AV node?

Delays conduction to allow time for ventricular filling after atrial contraction

What happens during the isovolumetric contraction phase?

Ventricles contract with all valves closed, causing a rapid rise in pressure

What happens during the isovolumetric relaxation phase?

Ventricles relax with all valves closed, pressure falls without volume change

What causes the third heart sound (S3)?

Rapid ventricular filling; may be normal in children/athletes or pathologic (heart failure)

What causes the fourth heart sound (S4)?

Atrial contraction against a stiff ventricle; associated with LV hypertrophy

Which murmur is heard in aortic stenosis?

Systolic crescendo-decrescendo murmur best heard at right upper sternal border

Which murmur is heard in mitral regurgitation?

Holosystolic murmur best heard at the apex, radiates to axilla

What is the significance of the ST segment on ECG?

It represents the isoelectric period between ventricular depolarization and repolarization; elevation/depression indicates ischemia or infarct

Which ECG leads view the anterior heart?

V1-V4 (especially V2-V4)

Which ECG leads view the lateral heart?

Leads I, aVL, V5, V6

What is the effect of hyperkalemia on the ECG?

Peaked T waves, widened QRS, flattened P waves

What is the effect of hypokalemia on the ECG?

Flattened T waves, U waves, ST depression

What are the three layers of the heart wall?

Endocardium, myocardium, epicardium (outermost)

What stimulates surfactant production in the fetus?

Cortisol (glucocorticoids) stimulate Type II pneumocytes to produce surfactant

At what gestational age does surfactant production become sufficient?

About 34-36 weeks of gestation

What is compliance in pulmonary physiology?

The ease with which the lungs expand; defined as ΔV/ΔP

How does emphysema affect lung compliance?

Increases compliance (loss of elastic recoil)

How does pulmonary fibrosis affect compliance?

Decreases compliance (stiff lungs)

What is functional residual capacity (FRC)?

The volume of air in lungs after normal exhalation; balance point of inward lung recoil and outward chest wall force

What is the most accurate method to measure residual volume?

Helium dilution or body plethysmography

What does Dalton's Law state?

Total pressure of a gas mixture is the sum of the partial pressures of each gas

What does Fick's Law describe?

Rate of gas transfer across a membrane is proportional to surface area and partial pressure gradient and inversely proportional to thickness

What is ventilation-perfusion mismatch?

A mismatch between air reaching alveoli (ventilation) and blood flow (perfusion); leads to impaired gas exchange

What is the A-a gradient and what does it signify?

Alveolar-arterial gradient; increased in V/Q mismatch, diffusion problems, or shunt

What are common causes of hypoxemia with normal A-a gradient?

Hypoventilation and high altitude

What are common causes of hypoxemia with increased A-a gradient?

V/Q mismatch, diffusion impairment, right-to-left shunt

What happens to V/Q ratio during a pulmonary embolism?

Increases; ventilation is preserved but perfusion is blocked (dead space)

What is the Haldane effect?

Oxygenation of blood in the lungs displaces CO₂ from hemoglobin, increasing CO₂ removal

What is the Bohr effect?

Increased CO₂ and H⁺ in tissues decreases hemoglobin's affinity for O₂, enhancing O₂ delivery

What is the bicarbonate buffer system equation?

CO₂ + H₂O ⇌ H₂CO₃ ⇌ H⁺ + HCO₃⁻

What compensatory response occurs in metabolic acidosis?

Respiratory compensation via hyperventilation (↓PaCO₂)

What compensatory response occurs in respiratory acidosis?

Renal compensation by increasing HCO₃⁻ reabsorption

What acid-base abnormality is seen in salicylate poisoning?

Mixed: Early respiratory alkalosis, later metabolic acidosis

How can you quickly distinguish respiratory vs metabolic acid-base disorders?

Check pH first (acidic or basic), then see if PaCO₂ or HCO₃⁻ is the primary change

What is Winter's formula and its use?

Predicts expected PaCO₂ in metabolic acidosis: PaCO₂ = (1.5 × HCO₃⁻) + 8 ± 2

What defines anion gap metabolic acidosis?

Anion gap > 12; indicates addition of unmeasured acids (MUDPILES)

What are causes of normal anion gap metabolic acidosis?

HARDASS: Hyperalimentation, Addison's, RTA, Diarrhea, Acetazolamide, Spironolactone, Saline