Pulmonary

Question 0

Bohr measurement of Dead Space

Answer 0

Vd/Vt = (PaCO2 - PeCO2)/PaCO2

Vd = Volume of dead space
Vt = Tidal volume
PaCO2 = Arterial partial pressure CO2

The Vd/VT ratio is normally about 0.3, i.e. about 1/3rd of ventilation is not-participating in gas exchange (i.e. 150 ml anatomic dead space for 500 ml VT). This can increase significantly with disease.

PeCO2 = Partial pressure of exhaled CO2

Question 1

Physiologic Dead space

Answer 1

Anatomic dead space (bronchus, etc.) + Alveolar dead space (areas not exchanging gas)

~150ml

Question 2

Alveolar Ventilation

Answer 2

Alveolar ventilation = (tidal vol x freq) –(wasted ventilation x freq)

Alveolar ventilation = [Vol expired CO2 in 1 min. x Constant (0.863)] Divided by Partial pressure CO2 (arterial or alveolar)

Increase in dead space (COPD) requires an increase in tidal volume to maintain normal alveolar ventilation.

An increase in breath frequency and decrease in tidal volume => decreased alveolar ventilation.

Decrease in alveolar ventilation = Increased PCO2 (arterial)

Question 3

Respiratory Exchange Ratio

Answer 3

The respiratory exchange ratio is the ratio between oxygen supply and carbon dioxide production

R = VCO2/VO2

Normal values VCO2 ~ 200 ml/min and for VO2 of 250 ml/min giving an R = <1 (Normal = 0.8)

Question 4

Respiratory Quotient

Answer 4

Ratio of CO2 produced to O2 consumed at the cellular metabolic level and its values changes with different substrates.

Carbohydrate metabolism = 1 CO2 to 1 O2 RQ = 1
Fat metabolism RQ= 0.7
Protein metabolism RQ = 0.8

Mixed diet (assumed value) RQ = 0.8

Under steady state conditions the respiratory exchange ratio will be equal to the respiratory quotient.

Question 5

Alveolar gas equation

Answer 5

P(alv)O2 = PIO2 – (PaCO2/R)

Alveolar oxygen tension = inspired PO2 (PIO2) normally 159 mm Hg in dry air but only 149 mm Hg by the time the air reaches the alveolar surface – due to water saturation.

The PaCO2 can be determined by the PaCO2 from a blood sample, and R is assumed to be ~0.8 for a mixed diet.

Question 6

Restrictive Lung Disease

Answer 6

Decreased volume. Restriction of lung expansion.

Total lung volume decreased. FEV1/FVC is normal (80%)
Increased Elastic Recoil Pressure (ERP) => Increased Equal Pressure Point (EPP) (toward pharynx)

Ex. Obesity, Respiratory Distress Syndrome, Pulmonary Fobrosis Muscular Dystrophy, ALS

Question 7

Obstructive Lung Disease

Answer 7

Decrease in Flow, Obstruction of airway

Normal total lung volume, Reduced FEV1/FVC ratio. ( decreased equal pressure point (toward lungs)

Ex. Emphysema, COPD, Asthma, Cystic Fibrosis