Fick's Principle

The Core Statement

"The Fick principle states that there is conservation of mass... In the steady state, the rate of O₂ consumption by the body must equal the amount of O₂ leaving the lungs in the pulmonary vein minus the amount of O₂ returning to the lungs in the pulmonary artery."

• Costanzo Physiology, 7th Ed.

1. Application to Cardiac Output (Most Important Use)

Formula

Numerical Example (from Guyton & Hall / Costanzo)

• O₂ absorbed by lungs: 200 mL/min
• Arterial O₂ content (left heart): 200 mL/L blood
• Venous O₂ content (right heart): 160 mL/L blood
• A-V difference: 200 - 160 = 40 mL/L
• CO = 200 ÷ 40 = 5 L/min

2. Practical Measurement - Cardiac Catheterization

• Mixed venous blood is sampled from the pulmonary artery (or right ventricle) via a catheter inserted through the brachial or subclavian vein
• Arterial blood is sampled from any systemic artery
• VO₂ can be measured directly using a metabolic cart (measuring O₂ disappearance from expired air) = Direct Fick method, or estimated using a nomogram (125 mL/min × BSA) = Indirect Fick method

• Estimated VO₂ = 125 mL O₂/min × body surface area
• A-V O₂ difference = Hb (g/100 mL) × 1.36 (mL O₂/g Hb) × 10 × fractional O₂ saturation difference

3. Application to Renal Plasma Flow (RPF)

4. General Organ Blood Flow (Conceptual Extension)

• Renal blood flow = O₂ consumed by kidney ÷ (renal arterial O₂ - renal venous O₂)
• Brain perfusion, hepatic blood flow, etc. can all be estimated the same way

5. Indicator Dilution - the Fick-Derived Methods

Summary Table

• Costanzo Physiology, 7th Ed. - core derivation and RPF application
• Guyton and Hall Textbook of Medical Physiology - illustrated numerical example
• Harrison's Principles of Internal Medicine, 22E - clinical context in cardiac catheterization
• Fuster and Hurst's The Heart, 15th Ed. - direct vs. indirect Fick, gold standard status
• Morgan & Mikhail's Clinical Anesthesiology, 7th Ed. - anesthetic/perioperative application

Physiological Basis of Fick's Principle

1. The Foundational Law: Conservation of Mass

"The Fick principle states that there is conservation of mass, a concept that can be applied to the utilization of O₂ by the body."

• Costanzo Physiology, 7th Ed.

2. The Oxygen Transport Chain - the Physiological Framework

Atmosphere → Lungs (alveoli) → Pulmonary capillaries
  → Left heart → Systemic arteries → Tissues (consumption)
  → Systemic veins → Right heart → Pulmonary arteries → Lungs

3. The Steady-State Assumption - Why It Matters

• Cardiac output of the left and right hearts must be equal (no net blood pooling anywhere)
• O₂ consumption by tissues is constant (no sudden changes in metabolic rate)
• O₂ content of arterial and venous blood is not changing over time

"The fundamental assumption is that, in the steady state, the cardiac output of the left and right ventricles is equal."

• Costanzo Physiology, 7th Ed.

4. Physiological Components in the Equation

a. Oxygen Consumption (VO₂)

• Measured directly at the mouth using a metabolic cart (expired gas analysis)
• Estimated as 125 mL O₂/min × body surface area (indirect Fick)

b. Oxygen Content (CaO₂ and CvO₂)

• Arterial O₂ content (CaO₂): ~20 mL/100 mL blood (SaO₂ ~95%, Hb ~15 g/100 mL)
• Mixed venous O₂ content (CvO₂): ~15 mL/100 mL blood (SvO₂ ~75%)
• A-V O₂ difference: ~5 mL/100 mL blood (= 50 mL/L)

c. Mixed Venous Blood - Why the Pulmonary Artery?

5. The Fick Equation and Oxygen Delivery/Consumption Balance

1. SaO₂ falls (hypoxemia)
2. VO₂ rises (fever, sepsis, exercise)
3. Q falls (low cardiac output)
4. Hgb falls (anemia)

6. Pulmonary Application - the "Mass Balance" at the Lung

O₂ added to blood by alveoli (q₂) = O₂ leaving lungs in pulmonary veins (q₃) - O₂ entering lungs in pulmonary artery (q₁)

7. Assumptions and Conditions Required for Validity

Summary Diagram (Conceptual)

                    LUNGS
          O₂ absorbed = VO₂ = 250 mL/min
                    ↓
    ┌───────────────────────────────────┐
    │  Pulmonary artery blood           │
    │  (mixed venous, CvO₂ = 15 mL/dL) │
    └───────────────────────────────────┘
                    ↓  Cardiac output (Q)
    ┌───────────────────────────────────┐
    │  Systemic arterial blood          │
    │  (CaO₂ = 20 mL/dL)               │
    └───────────────────────────────────┘
                    ↓
              TISSUES consume O₂
         A-V difference = 5 mL/dL
         
   Q = VO₂ / (CaO₂ - CvO₂)
     = 250 / (20-15) mL/dL × 10
     = 5000 mL/min

• Costanzo Physiology, 7th Ed. - conservation of mass derivation, steady-state assumption
• Medical Physiology (Boron & Boulpaep) - mass balance at the lung
• Miller's Anesthesia, 10th Ed. - conservation of mass framing, SvO₂ determinants
• Goldman-Cecil Medicine - O₂ content formula and normal A-V difference
• Mulholland & Greenfield's Surgery, 7th Ed. - DO₂/VO₂/OER relationship
• Creasy & Resnik's Maternal-Fetal Medicine - O₂ consumption = CO × C(a-v)O₂

Windkessel Vessels

Origin of the Term

1. Which Vessels Are Windkessel Vessels?

2. The Core Mechanism: Systole and Diastole

During Systole

• The elastic walls of the aorta stretch and distend, storing roughly 50% of the stroke volume as potential energy (elastic deformation) in the vessel wall
• Aortic pressure rises to ~120 mmHg (systolic)

"The entry of blood into the arteries during systole causes the walls of these arteries to stretch and the pressure to increase to about 120 mmHg."

• Guyton & Hall, Medical Physiology

During Diastole

• The stretched elastic walls recoil, converting stored potential energy back into kinetic energy of blood flow
• This propels blood continuously through the peripheral vessels even though the heart is in diastole
• Aortic pressure falls gradually to ~80 mmHg (diastolic) - maintained by this elastic recoil, not by ventricular contraction

"After the aortic valve closes, pressure in the aorta decreases slowly throughout diastole because the blood stored in the distended elastic arteries flows continually through the peripheral vessels back to the veins."

• Guyton & Hall

3. Quantitative Framework

Compliance (C)

Pulse Pressure

• High compliance → aorta accommodates stroke volume with little pressure rise → narrow pulse pressure
• Low compliance (stiff aorta) → same stroke volume causes a greater pressure rise → wide pulse pressure

Arterial Compliance and Pulse Pressure

RC Time Constant

4. Pulse Wave Propagation - What Happens Downstream

• The wave speeds up (stiffer wall = faster propagation)
• The waveform becomes taller and narrower - peripheral systolic pressure can exceed central aortic systolic pressure

5. Structural Basis: The Elastic Artery Wall

• Tunica media: concentric layers of elastin fibers + smooth muscle cells + collagen. "The high elastin content of the media allows these vessels to expand during systole and recoil during diastole." (Robbins & Cotran)
• Tunica intima: thick subendothelial layer; endothelium regulates tone via NO and other mediators
• Vasa vasorum: small vessels within the adventitia perfuse the outer wall of these large vessels

6. Clinical and Pathophysiological Significance

Aging and Arteriosclerosis

• Arterial compliance falls
• Systolic pressure rises, diastolic pressure falls slightly
• Pulse pressure widens (isolated systolic hypertension in the elderly)
• Left ventricular afterload increases → LV hypertrophy
• Pulse wave velocity increases (aortic stiffness index)

"Aging decreases the distensibility of arteries. The decreased compliance elevates systolic pressure, slightly decreases diastolic pressure, and thus widens pulse pressure."

• Medical Physiology, Boron & Boulpaep

"In larger elastic arteries, arterial stiffness, associated with aging or disease, increases left ventricular afterload and portends a poor outcome."

• Harrison's Principles, 22nd Ed.

Aortic Aneurysm

Pulmonary Windkessel

7. Summary Table

• Medical Physiology, Boron & Boulpaep - Windkessel model diagram, pressure wave propagation
• Guyton & Hall Textbook of Medical Physiology - aortic pressure curve mechanism
• Robbins & Cotran Pathologic Basis of Disease - elastic artery wall structure
• Harrison's Principles of Internal Medicine, 22nd Ed. - arterial stiffness and afterload
• Fishman's Pulmonary Diseases and Disorders - pulmonary Windkessel model and PAC
• Costanzo Physiology, 7th Ed. - compliance and pulse pressure relationship

Carotid Sinus vs. Carotid Body

At a Glance

1. Carotid Sinus - Baroreceptor

Anatomy

"The carotid sinus is a small dilation of the internal carotid artery just above the bifurcation of the common carotid into external and internal carotid branches. Baroreceptors are in the adventitia of the vessel within this dilation."

• Ganong's Review of Medical Physiology, 26th Ed.

Stimulus

• Activated by increased intraluminal pressure → vessel wall stretches → receptor fires more
• They are more sensitive to pulsatile pressure than constant pressure
• Most sensitive to rapid changes in pressure; rate of change is actually a stronger stimulus than absolute pressure
• Operational range: ~60-180 mmHg (fully activated at ~170 mmHg; silent below ~60 mmHg)

Afferent Pathway

• Carotid sinus nerve (nerve of Hering) → branch of CN IX (glossopharyngeal)
• Synapses on nucleus tractus solitarius (NTS) in medulla
• NTS then activates:
  • Caudal ventrolateral medulla (CVLM) → GABAergic inhibition of rostral VLM (RVLM)
  • Dorsal motor nucleus / nucleus ambiguus → vagal output

The Baroreceptor Reflex (Carotid Sinus Reflex)

• Sympathetic outflow decreases → heart rate ↓, contractility ↓, vasodilation (↓TPR)
• Parasympathetic outflow increases → further heart rate ↓
• Net result: blood pressure falls back toward normal

• Sympathetic outflow increases → heart rate ↑, contractility ↑, vasoconstriction (↑TPR)
• Parasympathetic withdrawal
• Net result: blood pressure rises

Key Clinical Points

• In chronic hypertension, baroreceptors "reset" to the new higher pressure - they no longer signal it as abnormal, so hypertension is maintained rather than corrected
• Carotid sinus hypersensitivity - exaggerated baroreceptor response to neck pressure → severe bradycardia/syncope (e.g., when wearing a tight collar)
• Carotid sinus massage - used clinically to terminate supraventricular tachycardias by triggering vagal slowing of the AV node
• Volatile anaesthetics (especially halothane) inhibit the heart rate component of this reflex

2. Carotid Body - Peripheral Chemoreceptor

Anatomy

"The carotid body is located in the adventitia of the posteromedial aspect of the bifurcation of the common carotid artery. The normal carotid body measures 3 to 5 mm in diameter... average weight 12 mg."

• Cummings Otolaryngology

Cell Types

Stimuli

1. Hypoxia (PaO₂ < 60 mmHg) - the primary, most potent stimulus
2. Hypercapnia (raised PaCO₂)
3. Acidosis (low pH)

Mechanism of O₂ Sensing (Type I Glomus Cells)

• Hypoxia inhibits O₂-sensitive K⁺ channels (outward rectifier, TASK channels) on Type I cells
• Reduced K⁺ efflux → membrane depolarisation → Ca²⁺ influx via L-type voltage-gated Ca²⁺ channels
• Ca²⁺ influx triggers action potentials and neurotransmitter release (dopamine, ACh, substance P, ATP)
• These neurotransmitters activate the cuplike endings of glossopharyngeal afferent fibers

• Mitochondrial cytochromes and NADPH oxidases act as O₂ sensors
• Hypoxia alters reactive oxygen species (ROS) generation → downstream signalling

Afferent Pathway

• Nerve of Hering (a branch of CN IX, glossopharyngeal) → petrosal ganglion → medulla
• Drives the respiratory centre → increased rate and depth of breathing
• Also activates the parasympathetic system → bradycardia, ↓contractility (at normal O₂; masked by hypoxia's sympathetic drive)

Responses to Stimulation

"Acidemia, hypoxia, and hypercapnia stimulate the carotid body to initiate an autonomic reflex, which leads to increased respiratory rate and depth along with increased heart rate, blood pressure, and cerebral cortical activity."

• Cummings Otolaryngology

Key Clinical Points

• High altitude: People living at altitude have enlarged carotid bodies (up to 2× normal size) - chronic hypoxia causes hypertrophy
• Cyanotic congenital heart disease: Chronically blunted chemoreceptor sensitivity
• Carotid endarterectomy (CEA): Surgical manipulation can damage the nerve supply → postoperative respiratory insufficiency, reduced hypoxic ventilatory response
• Obstructive sleep apnea: Intermittent hypoxia sensed by carotid body mediates the hypertensive response
• Sudden infant death syndrome (SIDS): Some cases linked to small carotid bodies or decreased ratio of Type I to Type II cells - attenuated response to hypoxic crises
• Carotid body tumour (paraganglioma / glomus caroticum): Benign neuroendocrine tumour; associated with SDH gene mutations (SDHB, SDHC, SDHD)

3. Side-by-Side Comparison

• Ganong's Review of Medical Physiology, 26th Ed. - baroreceptor anatomy, carotid body cell types, afferent pathways
• Costanzo Physiology, 7th Ed. - baroreceptor reflex arc and medullary centres
• Miller's Anesthesia, 10th Ed. - baroreceptor reflex mechanism, chemoreceptor reflex
• Cummings Otolaryngology - carotid body anatomy, physiology, clinical associations
• Fishman's Pulmonary Diseases - O₂ sensing mechanisms (membrane vs. metabolic hypothesis)

Coarctation of the Aorta

Definition

• Incidence: ~1 in 3,000-4,000 births; accounts for ~10% of all congenital heart defects
• Male:female ratio = 2:1
• Associated bicuspid aortic valve in 50-85% of cases

1. Anatomy and Classification

A. Juxtaductal (most common - ~90%)

B. Postductal (Adult type)

• Constriction distal to the ductus arteriosus
• Time for collateral circulation to develop - so often well-compensated, may present in adolescence/adulthood with incidental hypertension
• Rib notching and collaterals are well-developed

C. Preductal (Infantile type)

• Constriction proximal to the ductus arteriosus; often tubular/extensive
• Before birth: the ductus provides blood flow to the lower body
• After ductal closure at birth: sudden cardiovascular collapse - ductal-dependent circulation
• Differential cyanosis: upper body pink (left ventricle → ascending aorta), lower body cyanotic (right-to-left shunt through PDA into descending aorta)

2. Embryological Basis

1. Ductal tissue theory: Smooth muscle from the ductus arteriosus is incorporated into the adjacent aortic wall. When the ductus constricts at birth, this ectopic ductal muscle in the aorta also constricts, forming a shelf.
2. Involution theory: Abnormal involution of a segment of the left dorsal aorta leads to a stenotic region that moves cranially with the left subclavian artery.
3. Isthmus theory: The aortic isthmus (between the left subclavian and ductus) carries very little blood in fetal life and is normally narrow; if it fails to enlarge after birth, a coarctation results.

3. Pathophysiology and Haemodynamics

Above the coarctation (proximal):

• Increased afterload on the left ventricle
• Hypertension in the upper body, arms, and head
• LV pressure overload → left ventricular hypertrophy

Below the coarctation (distal):

• Reduced blood pressure in the lower body, kidneys, and legs
• Weak or absent femoral pulses; delayed brachial-femoral pulse timing

Mechanism of hypertension

"When a constrictor is placed on the aorta above the renal arteries, the blood pressure in both kidneys falls at first, renin is secreted, angiotensin and aldosterone are formed, and hypertension occurs in the upper body."

• Guyton & Hall

1. Coarctation reduces renal perfusion pressure initially
2. Kidneys secrete renin → angiotensin II → aldosterone
3. Volume retention and vasoconstriction → upper body hypertension
4. As upper body pressure rises, renal perfusion normalises → renin normalises
5. Hypertension persists due to autoregulatory remodelling of upper body vessels

Collateral Circulation

• Internal mammary artery → anterior intercostals → posterior intercostals → descending aorta
• Scapular and subscapular arteries → posterior intercostals → descending aorta
• Superior intercostals → posterior intercostals → descending aorta

4. Clinical Features

Symptoms

• Neonates/infants (severe): circulatory failure, shock, poor feeding, tachypnoea, differential cyanosis (preductal)
• Children/adults (mild-moderate): often asymptomatic until detected on screening
• Upper body: headaches, epistaxis, pulsatile tinnitus
• Lower body: leg fatigue, claudication, cold feet
• Older adults: angina, heart failure symptoms

Signs

5. Investigations

ECG

• Left ventricular hypertrophy (adults)
• Left axis deviation
• In neonates: right ventricular hypertrophy initially (immature LV)

Chest X-ray

• Upper convexity: dilated left subclavian artery (pre-stenotic dilatation)
• Central indentation: the coarctation site itself
• Lower convexity: post-stenotic dilatation of the descending aorta Together these form the silhouette of the number "3"

• Bilateral erosion of the inferior margins of the posterior 3rd-8th ribs
• Caused by pulsatile dilated posterior intercostal collateral arteries pressing on rib undersurfaces
• Typically not visible before age 5 (collaterals haven't fully developed)
• Unilateral left rib notching: if coarctation is proximal to origin of the right subclavian artery
• Unilateral right rib notching: if left subclavian is sacrificed in repair

Echocardiography

• Documents gradient across stenosis in descending aorta
• Assesses LV hypertrophy and function
• Detects associated lesions (bicuspid valve, VSD)
• Continuous-wave Doppler: "diastolic tail" (persistent antegrade flow throughout diastole across the stenosis) is a key feature of significant coarctation

MRI/CT (Gold standard imaging)

• MRI (CMR) is the best modality - shows anatomy, length of stenosis, collateral vessels, arch morphology, LV mass
• CT angiography: excellent if MRI contraindicated
• Both used for post-operative surveillance

Cardiac Catheterization

• Measures peak-to-peak pressure gradient across coarctation
• Gradient ≥20 mmHg = haemodynamically significant

6. Complications

7. Treatment

Neonates with ductal-dependent CoA

• Immediate prostaglandin E₂ (PGE₂) infusion to keep the ductus open and maintain lower body perfusion - buys time until surgical repair

Surgical Options (preferred in neonates and infants)

1. Resection with end-to-end anastomosis - gold standard for localised coarctation
2. Extended end-to-end anastomosis - for longer or arch involvement
3. Subclavian flap aortoplasty - uses left subclavian as a patch (sacrifices left arm pulse)
4. Patch aortoplasty - higher risk of late aneurysm formation

Catheter-Based Intervention (preferred in older children and adults)

• Balloon angioplasty - effective for recurrent coarctation after surgery (74% mean gradient reduction)
• Stent implantation - treatment of choice for adults; avoids surgery; allows future re-expansion as child grows

Intervention Thresholds (Braunwald's)

• Hypertensive patient with peak-to-peak gradient ≥20 mmHg → intervene
• Coarctation diameter ≤50% of aortic diameter at diaphragm → intervene regardless of gradient if hypertensive

Post-repair Surveillance

• Hypertension persists or recurs in >50% of adults even after technically successful repair - especially if intervention was delayed
• Re-coarctation in up to 35% (particularly after neonatal surgical repair)
• Lifelong imaging surveillance with CMR every 3-5 years
• Aggressive blood pressure control

8. Associated Syndromes

Summary

            COARCTATION OF THE AORTA
                     │
        ┌────────────┴─────────────┐
   PREDUCTAL                  POSTDUCTAL/JUXTADUCTAL
  (Infantile type)             (Adult type)
   Ductal-dependent            Collaterals develop
   Neonatal shock              Adolescent hypertension
   Differential cyanosis       Radio-femoral delay
        │                            │
  PGE₂ + surgery           Stent / Surgery
  
  KEY SIGNS: Figure-3 sign + Rib notching on CXR
  KEY MECHANISM: Upper body HTN via renin-angiotensin
  KEY ASSOCIATION: Bicuspid aortic valve (50-85%)
  KEY COMPLICATION: Berry aneurysm, aortic dissection

• The Developing Human (Moore), Clinically Oriented Embryology - embryology and classification
• Guyton & Hall Textbook of Medical Physiology - haemodynamics and renin mechanism
• Braunwald's Heart Disease, 2-Vol Set - clinical features, imaging, intervention thresholds
• Goldman-Cecil Medicine - diagnosis, collateral circulation, complications
• Rosen's Emergency Medicine - differential cyanosis, neonatal presentation
• Grainger & Allison's Diagnostic Radiology - CXR signs, MRI/CMR approach