Case Studies and Equations

Question 1

What does PaO2 represent?

Answer 1

The partial pressure of O2 in the arteries

This means it is the amount of O2 that is dissolved in the blood

In the case study, the PaO2 was lower than normal, indicating that there is less dissolved O2 in the blood that would be expected in a patient experiencing optimal respiration

Question 2

What does SaO2 represent?

Answer 2

The percent of saturated oxygen

This means it is the amount of hemoglobin binding sites that have O2 bound to them divided by the total amount of hemoglobin binding sites

Question 3

What determines SaO2?

Answer 3

The PaO2

If PaO2 goes up, SaO2 goes up

Question 4

How do you calculate O2 content?

Answer 4

O2 content = O2 binding capacity x SaO2 + dissolved O2

Question 5

How do you calculate O2 binding capacity?

Answer 5

Hb x 1.34

Question 6

How do you calculate dissolved O2?

Answer 6

PaO2 x 0.003

Question 7

Sooo…. Putting that all together now, how do you calculate O2 content again?

Answer 7

Remember O2 content = O2 binding capacity x SaO2 x dissolved O2

O2 content = (Hg x 1.3) x SaO2 + (PaO2 x 0.003)

Question 8

How do you calculate minute ventilation?

Answer 8

Minute vent. = VT x RR

Tidal volume multiplied by respiratory rate

Question 9

How do you calculate alveolar ventilation rate?

Answer 9

Alveolar vent. = (TV-DS) x RR

Tidal volume MINUS dead space multiplied by respiratory rate

Alveolar ventilation accounts for dead space when calculation the ventilation rate

Question 10

What is hyperventialtion?

Answer 10

NOT the rate or volume of breathing

Hyperventilation is the rate of CO2 that is being expired or “blown off” therefore we use PaCO2 in order to determine the state of hyperventilation

Just because the respiratory rate is higher than normal, it may be appropriate for the condition of the patient, so instead we look at PaCO2

Question 11

How do you calculate the PaCO2? What determines it?

Answer 11

PaCO2 = (VCO2 x 0.863)/VA

PaCO2 is determined by the amount of CO2 expired and the alveolar ventilation

Question 12

How does exercise affect VCO2 and PaCO2?

Answer 12

Increase in VCO2 and therefore an increase in PaCO2

Question 13

How do you determine VA?

Answer 13

Alveolar ventilation is determined by respiratory rate, tidal volume and dead volume (VD?)

Question 14

What does hyperventilation do to RR, VA and PaCO2?

Answer 14

Hyperventilation increases respiratory rate, which increases the alveolar ventilation, which decreases PaCO2

Question 15

How can you calculate PaO2? The alveolar pressure of O2?

Answer 15

PaO2 = FiO2 x (PB-PH20)-(PaCO2/RQ)

FiO2 = percent of oxygen being breathed (100% for tx)
PB = barometric pressure (760 mm Hg at sea level)
PH20 =
RQ =

Question 16

How much higher is the Hb affinity for CO than for O2?

Answer 16

250 times higher

Question 17

What happens to the total lung capacity in the case of cardiogenic pulmonary edema?

Answer 17

Reduced because there is fluid in the lungs which will reduce the amount of space there is for air

Question 18

What will happen to the surface tension in the lungs in the case of cardiogenic pulmonary edema?

Answer 18

Increased

The excess fluid will result in stronger intermolecular forces that need to be overcome in order to expand the alveoli

Question 19

What will happen to lung compliance in the case of cardiogenic pulmonary edema?

Answer 19

Reduced

The increased surface tension will result in a decreased elasticity or stretch capacity

Question 20

What will happen to airway resistance in the case of cardiogenic pulmonary edema?

Answer 20

Increased

The increased surface tension and edema in both the alveoli and interstitium will result in airway resistance as well as J receptor function

Question 21

How can you explain the butterfly appearance of the chest x-ray in cardiogenic pulmonary edema?

Answer 21

The edema follows the distribution of the vasculature, so you will see two “wings” of the butterfly at the right pulmonary artery and the left pulmonary artery

Since the hydrostatic pressure is high in the capillaries during heart block/failure, fluid will accumulate in the lungs around the source

Question 22

How does cardiogenic pulmonary edema change the dynamics of the capillary-interstitium-alveoli fluid balance?

Answer 22

Fluid is pushed out of the capillaries and into the interstitium

The high hydrostatic pressure prevents fluid from being reabsorbed by the capillaries

Question 23

When would you see a more opaque x-ray?

Answer 23

If the fluid was pushed further out of the interstitium and into the actual alveoli

Question 24

What clinical sign do we use to determine whether or not fluid has reached into the alveoli?

Answer 24

The patient will be coughing up blood stained fluid