Components of Gas Exchange physiology

Oxygenation and Ventilation

Oxygenation and Ventilation are two separate concepts....

The term Oxygenation refers to mechanics involved in delivering oxygen to the arteriolar network of the lungs. Optimal oxygenation occurs when an inspired oxygen rich mix is delivered to an adequate number of "normally" aerated alveoli so that passive diffusion of O2 can occur across the thin alveolar / arterial membrane.

Ventilation refers to mechanics of clearing CO2 from the body via the lungs. This process is dependent on adequate perfusion within the lung capillary networks, adequate passive gas movement into the alveoli and rapid elimination by rate of breathing.

Ventilator settings that affect Oxygenation:

FiO2
PEEP -> recruits alveolar beds

Ventilator settings that affect Ventilation

Respiratory Rate
Tidal volume*

A straightforward, short video regarding respiratory mechanics involved in oxygenation and ventilation can be found in from the respiratory care exam prep course @ Respiratory HQ. https://www.youtube.com/watch?v=8wEhoQH73xY&list=PPSV

Minute Ventilation

Minute ventilation (V̇) is the total volume of air moved in and out of the lungs per minute, calculated by multiplying Tidal Volume (VT) (air per breath) by Respiratory Rate (RR) (breaths per minute).

V̇ = TV × RR

It's a crucial measure in respiratory medicine, reflecting the lungs' overall gas exchange, though it differs from alveolar ventilation because it includes air in dead space (airways) that doesn't participate in gas exchange. Normal resting values are around 5-8 L/min, increasing significantly with exercise.

A-a gradient

The A-a gradient (Alveolar-arterial gradient) represents the difference between oxygen in the alveoli (A) and oxygen in the arterial blood (a), and reflects how effectively oxygen moves from the lungs into the bloodstream.

What it measures

Oxygen Transfer Efficiency: It quantifies the oxygen that doesn't cross the alveolar-capillary membrane into the blood, indicating the extent of gas exchange in the lungs at a given point in time.

A-a Gradient = PAO2 (Alveolar PO2) – PaO2 (Arterial PO2)

PAO2 (Alveolar PO2): Estimated using the Alveolar Gas Equation, which considers atmospheric pressure, inspired oxygen (FiO2), and arterial CO2 (PaCO2).

PaO2 (Arterial PO2): Directly measured from an arterial blood gas (ABG).

Normal ranges

General: Around 5-10 mmHg for young adults breathing air, but increases with age (roughly 1 mmHg per decade).

What an elevated gradient means (Increased A-a)

Problems: Suggests issues within the lungs affecting oxygenation, such as:
- Ventilation/Perfusion (V/Q) Mismatch.
- Impaired Diffusion (like lung fibrosis).
- Right-to-Left Shunt (blood bypassing oxygenation)

What a normal gradient with low oxygen (Hypoxemia) means (Normal A-a)

Problems: Indicates lack of oxygen getting to the alveoli, not a lung issue:
- Hypoventilation (breathing too slowly/shallowly).
- Low Inspired Oxygen (e.g., high altitude, low FiO2

PaO2 / FiO2 ratio

The P/F ratio (PaO2/FiO2 ratio) is calculated by dividing the arterial oxygen partial pressure (PaO2) by the fraction of inspired oxygen (FiO2). It assesses the degree of hypoxemia during acute lung injury and is also used to classify the severity of acute respiratory distress syndrome (ARDS).

How it's Calculated

Formula: P/F Ratio = PaO2 (mmHg) / FiO2 (as a decimal).
Example: A patient with a PaO2 of 90 mmHg on 40% oxygen (FiO2 = 0.40) has a P/F ratio of 90 / 0.40 = 225.
Room Air: For room air (21% oxygen), the expected PaO2 is around 105 (0.21 * 500), giving a normal ratio of about 400-500.

What it Means (ARDS Classification)

Normal: ≥ 400.
Mild ARDS (Acute Lung Injury): ≤ 300.
Moderate ARDS: 100-200.
Severe ARDS: < 100