Hypoxia mbbs 1st year physiology

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Hypoxia - MBBS 1st Year Physiology

Definition

Hypoxia is O2 deficiency at the tissue level - it is a decrease in O2 delivery to, or utilization by, the tissues. The term "anoxia" (complete absence of O2) is rarely accurate; "hypoxia" is more correct. - Ganong's Review of Medical Physiology, 26th Ed.

Distinguishing Hypoxemia vs Hypoxia:

Hypoxemia = decreased arterial PaO2 (a blood gas problem)

Hypoxia = decreased O2 at tissue level (can occur even with normal PaO2) Hypoxemia is one cause of hypoxia, but not the only one.

Costanzo Physiology, 7th Ed.

Classification / Types of Hypoxia

The classic 4-type system (Ganong) is the most widely used in MBBS:

Type	Also Called	Arterial PaO2	Mechanism	Example
Hypoxic hypoxia	Hypoxemic hypoxia	Decreased	Low O2 in blood	High altitude, pneumonia, hypoventilation
Anemic hypoxia	-	Normal	Reduced O2-carrying capacity	Anemia, CO poisoning
Ischemic hypoxia	Stagnant hypoxia	Normal	Reduced blood flow to tissue	Heart failure, peripheral vascular disease
Histotoxic hypoxia	-	Normal	Tissues cannot utilize O2	Cyanide poisoning

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

Detailed Description of Each Type

1. Hypoxic (Hypoxemic) Hypoxia

The arterial PaO2 is reduced. Causes include:

High altitude - low barometric pressure → low inspired PO2 (A-a gradient: normal)
Hypoventilation - e.g., narcotic overdose, neuromuscular disease (A-a gradient: normal)
Diffusion defect - e.g., pulmonary fibrosis (A-a gradient: increased)
V/Q mismatch - e.g., pneumonia, COPD (A-a gradient: increased)
Right-to-left shunt - e.g., cyanotic heart disease (A-a gradient: increased; O2 therapy: limited benefit)
Costanzo Physiology, Table 5.5

2. Anemic Hypoxia

Arterial PaO2 is normal, but O2-carrying capacity is reduced
Anemia - decreased hemoglobin → less O2-hemoglobin
CO poisoning - CO binds hemoglobin with 200x affinity compared to O2, occupies O2-binding sites AND causes a left-shift of the O2-dissociation curve
Costanzo Physiology, Table 5.6

3. Ischemic (Stagnant) Hypoxia

Blood flow to tissue is so low that adequate O2 is not delivered, despite normal PO2 and hemoglobin
Examples: heart failure (generalized), peripheral vascular disease, cerebral ischemia, coronary artery disease (localized)
Tissue edema also contributes by increasing the diffusion distance for O2

4. Histotoxic Hypoxia

O2 delivery is normal, but tissues cannot utilize O2
Classic cause: Cyanide poisoning - blocks cytochrome oxidase (the terminal enzyme of the electron transport chain), so cells cannot use O2 even when it is plentiful
Other causes: vitamin B deficiency (e.g., beriberi - disrupts oxidative phosphorylation steps), certain metabolic poisons
Guyton & Hall, 14th Ed.

Guyton's Classification (Causes of Hypoxia)

Guyton classifies hypoxia by the underlying cause:

Inadequate oxygenation in lungs (extrinsic) - O2 deficiency in atmosphere, hypoventilation due to neuromuscular disorders
Pulmonary disease - airway obstruction, decreased compliance, V/Q mismatch, impaired diffusion
Venous-to-arterial shunts (right-to-left cardiac shunts)
Inadequate O2 transport - anemia, abnormal Hb, circulatory failure, tissue edema
Inadequate tissue capability to use O2 - cyanide poisoning, enzyme toxicity, vitamin deficiency

Guyton & Hall Textbook of Medical Physiology

Effects of Hypoxia on the Body

On the Brain (First Affected Organ)

The brain is the most sensitive organ to hypoxia:

Mild hypoxia: Impaired judgment, drowsiness, euphoria, disorientation, loss of time sense, headache - resembles alcohol intoxication
Moderate: Anorexia, nausea, vomiting, tachycardia, hypertension
Severe: Mental and muscle fatigue, twitchings, seizures
Very severe: Coma → death

Acute effects at altitude (Guyton):

12,000 ft: Drowsiness, lassitude, fatigue, headache
18,000 ft: Twitchings or seizures
Above 23,000 ft: Coma → death

At 15,000 ft for 1 hour: mental proficiency falls to ~50% of normal.

On Respiration

Hypoxia stimulates peripheral carotid and aortic chemoreceptors → increased ventilation (hyperpnea)
Severe hypoxia: dyspnea (conscious sensation of breathlessness)
A PO2 drop to < 20 mmHg → loss of consciousness in 10-20 seconds, death in 4-5 minutes

On Cells (HIF Pathway)

Hypoxia triggers production of Hypoxia-Inducible Factors (HIFs):

In well-oxygenated tissue: HIF-α subunits are ubiquitinated and rapidly destroyed
In hypoxic cells: HIF-α dimerizes with HIF-β → activates genes producing:
- Erythropoietin (EPO) - stimulates red blood cell production
- VEGF (angiogenic factors) - promotes new blood vessel formation
- Enzymes for anaerobic glycolysis
Ganong's Review of Medical Physiology, 26th Ed.

Acclimatization to Hypoxia (High Altitude)

With prolonged exposure, the body compensates through 5 mechanisms (Guyton):

Mechanism	Details
1. Increased pulmonary ventilation	Chemoreceptors stimulated → up to 5x normal ventilation after days
2. Increased RBCs (polycythemia)	EPO released → bone marrow stimulation
3. Increased diffusing capacity	More alveolar recruitment, better membrane function
4. Increased peripheral vascularity	New capillaries form (angiogenesis via HIF/VEGF)
5. Increased cellular O2 utilization	Tissues adapt to use O2 at lower PO2

Guyton & Hall Textbook of Medical Physiology

Note: Hyperventilation initially causes respiratory alkalosis (low PCO2, high pH), which blunts further ventilatory drive. Over 2-5 days, the kidneys compensate by excreting bicarbonate, restoring pH and allowing full ventilatory response.

Oxygen Therapy in Hypoxia

Type of Hypoxia	O2 Therapy Useful?	Reason
Hypoxic (hypoxemia)	Yes	Raises alveolar and arterial PO2
Anemic	Partial	PaO2 already normal; O2 slightly increases dissolved O2
Ischemic/Stagnant	Partial	Cannot increase blood flow
Histotoxic	No	Tissues cannot use O2 regardless
Right-to-left shunt	Limited	Shunted blood bypasses lungs entirely

Guyton & Hall; Costanzo Physiology

Key Points to Remember for Exams

Hypoxia ≠ Hypoxemia - know the distinction
The 4 classic types: Hypoxic, Anemic, Ischemic, Histotoxic (remember H-A-I-H)
Cyanide = histotoxic hypoxia (blocks cytochrome oxidase)
CO poisoning = anemic hypoxia (displaces O2 from Hb + left-shifts ODC)
Brain is first to show effects of hypoxia
HIF is the master molecular switch for hypoxic response
O2 therapy is most effective in hypoxic hypoxia; useless in histotoxic hypoxia
Acclimatization involves 5 compensatory mechanisms
Right-to-left shunt: A-a gradient is increased but O2 therapy gives limited benefit

Sources:

Ganong's Review of Medical Physiology, 26th Ed. - Chapter 35, pp. 648-649
Guyton & Hall Textbook of Medical Physiology, Chapter 44 - Hypoxia and Oxygen Therapy
Costanzo Physiology, 7th Ed., Chapter 5 - Tables 5.5 & 5.6

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