VALUES to remember

Question 1

Hyperventialtion increases dramatically when pO2 is less than

Answer 1

5.3 kPa

Question 2

Loss of consciousness when pO2 is

Answer 2

4.3 kPa

Question 3

Death when pO2 is

Answer 3

2.7 kPa

Question 4

types of hypoxia

Answer 4

1. circulatory hypoxia
2. anaemic hypoxia
3. toxic hypoxia
4. alveolar hypoxia - hypoventilation

Question 5

NORMAL V/Q ratio

Answer 5

0.8

Lung ventilation and perfusion vary by gravity due to differences in blood flow and air distribution.
🔹 Apex (Top of Lung) → Highest V/Q Ratio (Dead Space-like)
* Ventilation is okay, but perfusion is very low (blood flow is reduced due to gravity).
* High V/Q (~3.0) → Like physiologic dead space, meaning O₂ is wasted.
🔹 Base (Bottom of Lung) → Lowest V/Q Ratio (Shunt-like)
* Perfusion is high, but ventilation is slightly lower due to mild alveolar compression.
* Low V/Q (~0.6) → More like a mild shunt, meaning less effective oxygenation.
🔹 Middle of the Lung → Near-Normal V/Q (~0.8)
* This is where ventilation and perfusion are best matched for efficient gas exchange.
✅ Greatest Overall Ventilation & Perfusion = Lung Base
✅ Greatest V/Q Ratio = Lung Apex

Question 6

Hypoxemia, low O2

Answer 6

PaO2 < 60 mmHg

Question 7

Hypercapnia

P

Answer 7

PaCO2 > 50 mmHg

Question 8

High CO2, failure of ventilation

Answer 8

PaCO2 > 45 mmHg

Question 9

Key indicator of decompensated respiratory acidosis

Answer 9

Severely low pH (< 7.25) + High CO₂ (↑ PaCO₂) + Inadequate HCO₃⁻ compensation

Question 10

A pO₂ of ~60 mmHg corresponds to an SaO₂ of

Answer 10

~90% (on the oxygen-hemoglobin dissociation curve).

Question 11

A pO₂ of ~100 mmHg usually gives

Answer 11

~98–100% SaO₂.

Question 12

FiO2 in different conditions

FiO₂ (Fraction of Inspired Oxygen) is the percentage of oxygen in the air that a person breathes. It is expressed as a decimal (e.g., 0.21 for room air) or as a percentage (e.g., 21%).

Answer 12

Room Air → FiO₂ = 0.21 (21%)
Nasal Cannula (1-6 L/min) → FiO₂ ~ 0.24 to 0.44 (24-44%)
Simple Face Mask (6-10 L/min) → FiO₂ ~ 0.40 to 0.60 (40-60%)
Non-Rebreather Mask (10-15 L/min) → FiO₂ ~ 0.60 to 0.90 (60-90%)
Mechanical Ventilation → Can deliver FiO₂ = 0.21 to 1.0 (21% to 100%)

Question 13

FiO₂ and PaO₂ Relationship (Alveolar Gas Equation Concept)

Answer 13

Normally, PaO₂ ≈ FiO₂ × 5
Example:
FiO₂ = 0.21 (21%) → Normal PaO₂ ~ 100 mmHg
FiO₂ = 0.50 (50%) → Expected PaO₂ ~ 250 mmHg

Question 15

PEFR

Peak Expiratory Flow Rate (PEFR) is the maximum speed at which a person can exhale air forcefully after a full inspiration. It is an important measurement in respiratory function tests to assess airway obstruction, especially in asthma and COPD.

Answer 15

Group Normal PEFR (L/min)
Adult Males 450–650 L/min
Adult Females 300–500 L/min
Children 150–400 L/min

• Measured using a peak flow meter, a handheld device.
  
  • The patient inhales deeply, then exhales as forcefully and quickly as possible into the device.
  • The highest reading from multiple attempts is recorded in liters per minute (L/min).

Question 16

FiO2

Answer 16

FiO₂ (Fraction of Inspired Oxygen)
🔹 Definition: The percentage (or fraction) of oxygen in the air a person is inhaling.

Key FiO₂ Levels:
Room air: 21% (0.21 FiO₂)
Nasal cannula (1-6 L/min): ~24-44%
Simple face mask (5-10 L/min): ~40-60%
Mechanical ventilation: Can be set between 21-100% based on oxygenation needs.

Question 17

Tidal Volume Vt

Answer 17

Normal Values:
Healthy adults: ~500 mL (6-8 mL/kg of ideal body weight)
Mechanically ventilated patients: Typically set at 6-8 mL/kg
In ARDS (Acute Respiratory Distress Syndrome) → Lower tidal volumes (4-6 mL/kg) are used to prevent ventilator-induced lung injury (VILI).