Case 6: Devi Gopal - V/Q Mismatch

Bronchial Arteries

Supply lung parenchyma

Main bronchi has:

• 1 right artery

• 2 left arteries (superior and inferior)

Pulmonary Arteries

Thin and distensible walls

• Large compliance

• Accommodate large stroke volume

Deliver deoxygenated blood from systemic circulation to alveolar capillaries (gas exchange)

Pulmonary Veins

Deliver oxygenated blood from alveolar capillaries to heart (gas exchange)

• Pump through systemic circulation

Blood leaves lungs (to heart) through pulmonary veins (most) and bronchial veins (some)

Pulmonary vs Systemic Circulation

Systemic: High-pressure, low-flow

• Include bronchial arteries

Pulmonary: Low-pressure, high-flow

• Pulmonary arteries and veins

Pulmonary Capillary Pressure

Low for gas exchange

Pulmonary Pressure Control

Mean Pulmonary Arterial Pressure (mPAP) = Cardiac Output (CO) x Pulmonary Vascular Resistance (PVR) + Pulmonary Venous Pressure (PVP)

Hypoxic Pulmonary Vasoconstriction

Low PO2 (ventilation) = Alveolar vasoconstriction

• Divert blood flow (Q) to well-ventilated (V) alveoli

Ventilation (V)

Air moving in and out of lungs

Dead Space Ventilation: Air entering conducting airways not in gas exchange

Alveolar Ventilation: Air entering alveolus

Perfusion (Q)

Blood flow through alveolus (gas exchange)

V/Q Ratio

Normal = 0.80

PO2: 104 mmHg

PCO2: 40 mmHg

Measuring V/Q

Nuclear medicine imaging techniques (V/Q scan, PET, CT)

A-a gradient

Measuring V/Q: A-a Gradient

Difference between PO2 in alveoli (PAO2) and PO2 in arteries (PaO2)

Normal Range: 5-10 mmHg (increase with exercise)

V/Q Mismatch: Increase A-a gradient

V/Q Mismatch

V/Q > or < 0.80

Decreased/no gas exchange

Low V/Q (< 0.80)

Low ventilation

Normal perfusion

Air in alveolus = Blood O2 and CO2 (no air supply)

High V/Q (> 0.80)

Normal ventilation

Low perfusion

Air in alveolus = Inspired air O2 and CO2 (no blood flow)

Normal Lungs

Apex → Base = Ventilation decrease and perfusion increase (relative)

• V and Q low in apex

• V and Q high in base

Apex:

• Low Q (High V/Q)

Base:

• Low V (Low V/Q)

V/Q Mismatch: Clinical Features

Hypoxemia

Increased A-a gradient

Supine position improve V/Q match

Severe:

• Dyspnea

• Resp distress

V/Q Mismatch: Effects on Blood Gas

Hypoxia:

Causes

• Shunt

• Hypoventilation (decrease alveolar ventilation = decrease PO2 and increase PCO2)

Hypercapnia:

Causes

• Increase dead space ventilation

• Hypoventilation

Dead Space Ventilation

Obstruction prevent perfusion to ventilated alveoli

• Ventilation with no pay off

• Ex: Pulmonary embolism

High V/Q

Redistribute to regions with blood flow (decrease V/Q)

Give O2 = Increase saturation

Dead Space Ventilation: Pulmonary Embolism

Blood flow obstruction in pulmonary circulation

No perfusion to ventilated area = Physiological dead space

• Normal V, low Q

• V/Q > 0.80

Increase CO2 retention

Shunting

Blood flow diversion prevent ventilation to perfused alveoli

• Perfusion with no benefit

Low V/Q

Give O2 = Small saturation increase (shunted blood not exposed to increased O2)

Shunting: Normal Lung Unit

Normal V and Q to both lungs (gas exchange units)

Shunting: Anatomical Shunt

From structural abnormalities

Venous blood bypass gas-exchange unit to mix with arterial blood

Low V, normal Q

• V/Q < 0.80

Shunting: Physiological Shunt

From ventilation blockage (lung collapse in atelectasis)

Venous blood bypass one gas-exchange unit to mix with arterial

Low V, normal Q

• V/Q < 0.80

O2 Saturation

% of O2-saturated hemoglobin in blood

Arterial: 95-100%

Venous: Variable based on tissue metabolic activity

O2 Sat Measurement

Pulse oximetry (SpO2)

SpO2

Red/infrared light absorption determine oxygenated (O2Hb) vs deoxygenated (HHb) hemoglobin

• O2Hb: Absorb infrared, red pass through

• HHb: Absorb red, infrared pass through

SpO2: Inaccurate Measurements

Nail polish

Poor perfusion

CO poisoning

• CO-hemoglobin indistinguishable from O2-hemoglobin

Darker skin

• Overestimate at <85% (false negative for hypoxemia)