O2, CO2, Ventilation & PerfusionLogan & Jason

Haemoglobin Curve

Which of the following environments would NOT shift the haemoglobin dissociation curve to the right?

a) Muscle capillaries during exerciseb) Metabolic alkalosisc) Hyperthermiad) A patient with SIRSe) A patient with type II respiratory failure

O2 CO2

Storage location

Chemoreceptor

(Molecular weight)(Solubility)Speed of DiffusionNet Flux

Partial pressure controlled by

Bound to haemoglobin(Dissolved in plasma)

Peripheral

↓↓

Big gradient, big flux

Perfusion limited (rest)Diffusion limited (exercise)

(Dissolved)(Bound to αα)(Bound to haemoglobin)As bicarbonate

Peripheral Central (as H+)

↑ ↑ ↑21x faster than O2Small gradient, small flux

Diffusion/ventilation limited

NB Diffusion is ↑ by pressure gradient/ventilation eg 100% O2, ↓ by thicker alveolar walls eg fibrosis, oedema

Ventilation: Perfusion RatioIs there more ventilation at the base or apex of the lung?

APEXBASE

Perfusion

Ventilation

V/P or V

/Q

“aeration”

Ventilation and perfusion are both greatest at the BASE, but their ratio is most favourable at the APEX (V/Q>1), which is where most gas exchange happens “good aeration”.

Case 1A 57 year old woman presents with a 3 day history of a productive cough, fever and rib pain on inspiration.

What is your most likely diagnosis?Pneumonia (RLL)

Is the V/Q ratio changed initially?Perfusion normal, ventilation and diffusion reduced in RLL initially, so V/Q decreased

Therefore what would you expect to happen to the PaO2 and PaCO2 in an ABG?V/Q=0 so decreased O2 (diffusion) and mildly elevated CO2

How does it change with time?Lung shunts blood away from RLL, decreasing Q, therefore increasing V/Q overall

Best immediate treatment?100% O2, ABx

Case 2A 42 year old male presents with a swollen right calf, chest pain and shortness of breath, tachypnoea

What is your most likely diagnosis?Pulmonary embolus (from DVT)

Is the V/Q ratio changed initially?Perfusion REDUCED, ventilation and diffusion NORMAL, so V/Q increased

Therefore what would you expect to happen to the PaO2 and PaCO2 in an ABG?Hyperventilation REDUCES PACO2 (blow off), but poor perfusion means that PAO2 is REDUCED

Best immediate treatment?100% O2, thrombolyse (ie dissolve clot)

Case 3A 70 year old male, long term smoker, is brought into A&E via ambulance after falling at home. On examination he is short of breath, cyanosed and is noted to have a hyper-resonant chest.

What is the most likely cause of the examination findings?COPD (emphysema+ chronic bronchitis)

Why is he cyanosed?Elastic destroyed, so REDUCED ventilation LOW O2 (and HIGH CO2)

Would you give supplementary oxygen?Yes in an emergency setting. (hypoxia kills before hypoxic drive). Aim for sats 88-92%.

Case 4A 42 year old male potter presents with worsening shortness of breath over the last 6 months.

What is the most likely diagnosis?Silicosis/pulmonary fibrosis

In a healthy lung what limits oxygen transport into the blood?Perfusion limitied

In this case, what is limiting transport?Thickening of the alveolar walls, so diffusion limited. Hb saturation not achieved by the end

of the capillaryWill the transport be worse or better with exercise?

Worse, because Hb has even less time in capillary to uptake O2

Case 5Patient presents with pale complexion, shortness of breath on exertion, dizziness, palpitations.

What is your most likely diagnosis?Anaemia

You give 100% O2 through a mask to help with shortness of breath. What will this do to the O2 content of the blood?

Very little, since Hb is already saturated, and only a small amount can dissolve