Mismatch in V/Q. Respiratory failure

what is ventilation- perfusion mismatch

V/Q mismatch

• represents the balance between air reaching the alveoli (V)

• and blood flow to alveoli (Q)

• normal V/Q ratio is approximately 0.8- 1.0

• ventilation and perfusion are closely matched

what is respiratory failure

Failure to maintain adequate gas exchange

Lungs fail to oxygenate blood adequately or remove CO2

• hypoxemia, PaO₂ < 60 mmHg

• hypercapnia, PaCO₂ > 50 mmHg

types of respiratory failure

• Acute Respiratory Failure (ARF)

  • Develops rapidly

  • requires urgent medical intervention.

• Chronic Respiratory Failure (CRF)

  • Develops slowly

  • compensatory mechanisms

(PaO₂) formula

Lower limit of arterial oxygen (PaO₂)

• This formula estimates normal PaO₂ levels with aging.

• If PaO₂ falls below this, ARF is suspected.

Mechanisms Leading to Respiratory Failure

1. Ventilation-Perfusion (V/Q) Mismatch

• most common cause of hypoxemia

• Airflow and blood flow are not matched

• Low V/Q

  • ⇓ ventilation,

  • normal perfusion

  • pneumonia

  • pulmonary edema.

• High V/Q

  • normal ventilation

  • ⇓ perfusion

  • pulmonary embolism

1. Altered Diffusion

   • Impaired O2 transfer

   • across the alveolar-capillary membrane

   Causes:

   • increase diffusion distance

     • Pulmonary edema

     • Pulmonary fibrosis

   • reduced alveolar surface area

     • emphysema

   • Disruption in gas exchange

     • Abnormal Hb binding affinity

     • CO poison

2. Alveolar Hypoventilation

   • Reduced air movement into the lungs

   • CO₂ retention

     • hypercapnia

     Causes:

   • CNS depression

     • opioid

     • brainstem injury

   • Neuromuscular disorders

     • Guillain-Barré syndrome

   • Obesity hypoventilation syndrome

Pathogenesis of Acute Respiratory Failure

Type 1: Hypoxemic Respiratory Failure

• PaO₂ < 60 mmHg (low oxygen levels)

• Develops over hours to days

• Caused by: V/Q mismatch

• Shunt

  • (blood bypasses ventilated alveoli)

  • (e.g., ARDS)

• Impaired diffusion

Compensation: hypoxic pulmonary vasoconstriction (HPV)- alv/cap reflex

Type 2: Hypercapnic Respiratory Failure

• PaCO₂ > 50 mmHg (high CO₂ levels)

• Develops abruptly

• drunk like, vessels in eyes

• Life-threatening if untreated

  • respiratory acidosis

• Common mechanisms:

  • Alveolar hypoventilation

    • ⇓ respiratory drive

    • muscle weakness.

    • Airway obstruction (e.g., COPD, asthma)

  • V/Q mismatch

    • ⇓air reaching alveoli

    • CO₂ ⇑

Etiology of Acute Respiratory Failure

Airway Abnormalities

• Obstruction → foreign bodies, severe asthma, anaphylaxis.

• Air trapping → COPD, cystic fibrosis

CNS Abnormalities

• Brainstem damage (stroke, trauma, tumors).

• Drug overdose (opioids)

Chest Wall Abnormalities

• Obesity hypoventilation syndrome (restricted lung expansion)

• kyphoscoliosis

• fractured ribs

Neuromuscular Conditions

• Guillain-Barré Syndrome

• Amyotrophic lateral sclerosis (ALS)

  • progressive nerve degeneration

Chronic Respiratory Failure (CRF)

• Develops over months to years

• compensatory mechanisms to take effect.

• Three Stages of CRF Progression

  1. Bronchopulmonary syndrome

  • persistent respiratory symptoms.

  • Dyspnea, wheezing

  2. b/p syndrome

  • compensation

  • HR increases

  • angiogenesis

  • ⇑ erythropoietin

  3. Neuropsychiatric syndrome

  • Chronic hypoxia + hypercapnia

  • affected the brain and heart

  • confusion, memory loss

  • chronic pul. hypertension

    results in cor pulmonale

  • Hypercapnic encephalopathy

Blood Gas Changes in Respiratory Failure

• Type 1 (Hypoxemic R/F):

  • ↓PaO₂

  • normal or ↓PaCO₂.

• Type 2 (Hypercapnic R/F):

  • ↓PaO₂,

  • ↑PaCO₂

  • respiratory acidosis.

explain the stages of CRF and what leads to what?

Chronic Respiratory Failure (R/F)
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    V
Stage 1: Initial Disease 
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    V
Initial Symptoms 
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    V
Stage 2: Compensation
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    V
Body Compensates for R/F
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    V
Stage 3: Complications
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Complications Develop
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Symptoms of Complications