Respiratory

Question 1

Pulmonary vascular resistance (PVR) = ?

Equation

Answer 1

PVR = (Pressure in pulmonary artery - pressure in left atrium)/CO

Remember:
Pressure in left atrium = pulmonary capillary wedge pressure

Question 2

Alveolar gas equation: Alveolar PO2 = ?

Answer 2

Alveolar PO2 (PAO2) = PIO2 - [PaCO2/R]

PAO2 = alveolar PO2
PIO2 = PO2 in inspired air = 150 mm Hg (sea level breathing room air)
PaCO2 = arterial PCO2
R = respiratory quotient = CO2 produced/O2 consumed = 0.8 (sea level breathing room air)

Question 3

A-a gradient = ?

Answer 3

PAO2 - PaO2 = alveolar PO2 - arterial PO2

Question 4

Normal range of A-a gradient? What happens with age?

Answer 4

10-15 mm Hg

Increases with age

Question 5

Define hypoxia vs. hypoxemia vs. ischemia

Answer 5

Hypoxia: decreased oxygen delivery to tissue
Hypoxemia: decreased PaO2
Ischemia: loss of blood flow

Question 6

List the causes of hypoxia.

Answer 6

Decreased CO
Hypoxemia
Anemia
CO poisoning

Question 7

List the causes of hypoxemia with a normal A-a gradient.

Answer 7

High altitude

Hypoventilation

Question 8

List the causes of hypoxemia with an increased A-a graidient

Answer 8

V/Q mismatch
Diffusion limitation (eg, fibrosis)
Right-to-left shunt

Question 9

List the causes of ischemia.

Answer 9

Impeded arterial flow

Decreased venous drainage

Question 10

V/Q at the apex of the lung (zone 1) vs. V/Q at the base of the lung (zone 3)?

Answer 10

V/Q at apex = 3 (wasted ventilation)

V/Q at base = 0.6 (wasted perfusion)

Question 11

Explain why ventilation and perfusion are greater at the base of the lung than at the apex of the lung.

Answer 11

At the base of the lung, the arterial pressure is greater than the venous and alveolar pressures. It drives the pressure gradient. Perfusion > > ventilation, so V/Q decreases.

At the apex of the lung, the alveolar pressure is greater than the arterial and venous pressures. It drives the pressure gradient and collapses the capillaries. Ventilation and perfusion both decrease, but perfusion decreases more, leading to an increase in V/Q. x

In zone 2, arterial pressure is greater than alveolar pressure, which is greater than venous pressure.

Question 12

What happens to the V/Q ratio during exercise and why?

Answer 12

It approaches 1 - cardiac output increases and apical capillaries vasodilate.

Question 13

What causes V/Q = 0?

Answer 13

Airway obstruction (no ventilation) - defined as a shunt

Question 14

100% oxygen does not improve PaO2 in this setting; 100% O2 improves PaO2 in this setting.

Answer 14

Does not improve - shunt (airway obstruction)

Does improve - physiologic dead space (blood flow obstruction)

Question 15

What causes V/Q = infinity?

Answer 15

Blood flow obstruction (physiologic dead space)

Question 16

List the three ways CO2 is transported from tissues to lungs.

Answer 16

1. HCO3- (70%)
2. HbCO2, aka carbaminohemoglobin (21-25%) - CO2 bound to Hb at N-terminus of globin; this favors the taut form, leading to O2 unloading
3. Dissolved CO2 (5-9%)

Question 17

What is the Haldane effect?

Answer 17

In the lungs, oxygenation of Hb promotes dissociation of H+ from Hb. This shifts equilibrium toward CO2 formation; therefore, CO2 is released from RBCs.

Question 18

What is the Bohr effect?

Answer 18

In peripheral tissue, increased H+ from tissue metabolism shifts the curve to the right, unloading oxygen

Question 19

Describe the mechanism of altitude sickness.

Answer 19

Decreased atmospheric oxygen (PO2) -> decreased PaO2 -> increased ventilation -> decreased PaCO2 -> respiratory alkalosis -> altitude sickness

Question 20

What chronic changes occur in response to high altitude?

Answer 20

1. Chronic increase in ventilation
2. Increased EPO to increase hematocrit and hemoglobin
3. Increase 2,3-BPG (binds to Hb so that Hb releases more O2)
4. Increased mitochondria
5. Increased renal excretion of HCO3- to compensate for respiratory alkalosis (can augment with acetazolamide)
6. Chronic hypoxic pulmonary vasoconstriction can lead to pulmonary hypertension and RVH

Question 21

Describe the response to exercise.

Answer 21

Increased CO2 production and O2 consumption
Increased ventilation rate to meet O2 demand
V/Q ratio from apex to base becomes more uniform
Increased pulmonary blood flow due to increased CO
Decreased pH during strenuous exercise (lactic acidosis)
No change in PaO2 and PaCO2, but venous CO2 increases and venous O2 decreasses