ultra-minimal clinical notes style
Core mental model
The lung is a matching organ.
Gas exchange works when air reaches alveoli, blood reaches those same alveoli, the membrane is thin, and partial pressure gradients favor movement.
Bedside split
PaO2
oxygenation
PaCO2
alveolar ventilation
Gas exchange pathway
Atmosphere -> conducting airways -> alveoli -> alveolar-capillary membrane -> pulmonary capillary blood -> systemic arterial blood -> tissues -> mitochondria
760
mmHg Patm
47
mmHg PH2O
100
mmHg PAO2
40
mmHg PACO2
Ventilation
Fresh air reaches alveoli
Failure: hypoventilation, airway obstruction, rapid shallow breathing, or increased dead space.
Diffusion
Gas crosses a thin barrier
Failure: fibrosis, edema, reduced surface area, or short capillary transit time during exercise.
Perfusion
Blood reaches matched alveoli
Failure: embolism, low cardiac output, capillary destruction, shunt, or V/Q mismatch.
Equation board
Each equation explains a failure mode
connect symbols to physiology
Minute ventilation
VE = VT x RR
Total air moved per minute; includes wasted dead space air.
Alveolar ventilation
VA = (VT - VD) x RR
Only fresh air reaching gas-exchanging alveoli controls PaCO2.
Dead space fraction
VD/VT = (PaCO2 - PECO2) / PaCO2
Dead space dilutes expired CO2 because it adds air without gas exchange.
Alveolar gas
PAO2 = FiO2 x (Patm - PH2O) - PaCO2/R
Explains high altitude, hypoventilation and oxygen therapy.
Rapid shallow breathing
Same VE, very different VA.
Normal depth
VT 500 x RR 12 -> VA 4.2 L/min
Rapid shallow
VT 250 x RR 24 -> VA 2.4 L/min
Dead space map
Anatomic
Conducting airways; about 150 mL in a typical adult.
Alveolar
Ventilated alveoli with inadequate perfusion.
Physiologic
Anatomic dead space plus alveolar dead space.
Extreme
Ventilation without perfusion: V/Q approaches infinity.
V/Q pattern board
Matching decides oxygenation
normal whole-lung V/Q about 0.8
0.8
Normal V/Q
Ventilation and perfusion are matched enough for efficient exchange.
Low
Perfusion > ventilation
Asthma, COPD, pneumonia, edema, mucus plugging, atelectasis.
High
Ventilation > perfusion
Pulmonary embolism, emphysema, low pulmonary blood flow.
0
Shunt
Perfusion without ventilation; blood remains deoxygenated.
Why low V/Q causes hypoxemia
High V/Q units cannot fully rescue low V/Q units.
Low V/Q blood loses a large amount of oxygen content.
High V/Q blood gains little extra oxygen because hemoglobin is already nearly saturated.
Mixing therefore lowers arterial oxygen content and PaO2.
Oxygen response
Low V/Q -> usually improves because alveoli still receive some ventilation.
Diffusion limitation -> improves when alveolar PO2 and DeltaP increase.
Hypoventilation -> PaO2 may improve, but PaCO2 stays high unless ventilation improves.
True shunt -> poor response because blood bypasses ventilated alveoli.
Hypoxemia differential
Low inspired O2
High altitude. Low PAO2. Normal A-a gradient.
Hypoventilation
Low VA -> high PaCO2 -> low PAO2. Normal A-a gradient.
Diffusion
Fibrosis, edema, exercise worsening. Increased A-a gradient.
V/Q mismatch
Most common in lung disease. Increased A-a gradient.
Shunt
Blood not oxygenated. Poor O2 response. Increased A-a gradient.
A-a gradient rule
A-a = PAO2 - PaO2
Normal A-a + hypoxemia: low inspired oxygen or hypoventilation.
Increased A-a + hypoxemia: V/Q mismatch, diffusion limitation, or shunt.
Fick diffusion logic
Vgas = (A / T) x D x DeltaP
A down: emphysema reduces surface area.
T up: fibrosis or edema thickens the barrier.
DeltaP down: high altitude lowers inspired PO2.
DeltaP up: supplemental O2 improves diffusion.
Clinical pattern map
Asthma
Airway narrowing -> low V/Q. Early low PaCO2 from hyperventilation; rising PaCO2 is ominous.
COPD
V/Q mismatch; emphysema adds dead space by destroying capillary surface area.
Pneumonia
Exudate fills alveoli -> low V/Q or shunt physiology.
PE
Blocked blood flow -> high V/Q or dead space; redistribution may create low V/Q elsewhere.
Fibrosis
Thick membrane -> diffusion limitation, worse during exercise.
Edema
Fluid increases diffusion distance and can produce low V/Q or shunt.
Atelectasis
Collapsed alveoli with persistent perfusion -> shunt physiology.
Anemia
Oxygen content falls even when PaO2 can remain normal.
Final diagnostic questions
If PaO2 is low
Is inspired oxygen low? Is VA low? Is there diffusion failure, V/Q mismatch, or shunt? Is hemoglobin limiting O2 content?
If PaCO2 is high
Is alveolar ventilation inadequate? Is dead space increased? Is the respiratory pump failing or too fatigued to maintain ventilation?