Hypercapnia

Question 1

Which of the following would decrease PaCO2?

a. An increase in CO2 production
b. A decrease in minute ventilation
c. An increase in the dead space to tidal volume ratio
d. An increase in alveolar ventilation

Answer 1

d. an increase in alveolar ventilation - PaCO2 is directly proportional to the amount of CO2 produced by metabolism and delivered to the lungs (V ̇CO2) and inversely proportional to alveolar ventilation (( V_A ) ̇).

Question 2

( V_A ) ̇ = ( V_E ) ̇- (V_D ) ̇ where( V_A ) ̇ is alveolar ventilation, ( V_E ) ̇ is minute ventilation, and (V_D ) ̇ is dead space or wasted ventilation. Note (V_D ) ̇ with a dot over the V = V_(D )×RR. A 72 year-old man with severe smoking-related emphysema presents with acute breathlessness from pulmonary embolism. He is anti-coagulated with low molecular weight heparin but is eventually intubated and mechanically ventilated for acute hypercapnic respiratory failure. He is sedated and unresponsive and afebrile. He has decreased breath sounds with expiratory phase prolongation and faint expiratory wheezes. PaO2 is 90 mmHg on 30% supplemental oxygen. PaCO2 is 60 mmHg with a respiratory rate of 20/min and a tidal volume of 600ml. These data suggest

a. Increased CO2 production
b. Increased dead space
c. Decreased dead space to tidal volume ratio
d. Increased alveolar ventilation

Answer 2

Despite a large minute ventilation (( V_E ) ̇) of 12 L/min (remember, normal at rest is roughly 5 L/min) this patient has an elevated PaCO2. Nothing in the question stem suggests increased CO2 production (he is sedated and afebrile). Therefore, he must have increased dead space (both emphysema and pulmonary embolism increase alveolar and therefore physiologic dead space).

Question 3

An elderly woman with a history of depression, anxiety and metastatic colon cancer is evaluated in hospital for tachypnea (respiratory rate, 30/min). The house officer concludes that the patient has a respiratory alkalosis and prescribes a benzodiazepine for perceived hyperventilation/anxiety. Using the PaCO2 equation, explain why this is potentially a critical error.

Answer 3

The error here is equating a respiratory rate with a patient’s overall ventilation (here an elevated RR is equated with hyperventilation and a low PaCO2). Look back at equation and note all of the other factors (V ̇CO_2, V_T, and V_D/V_T ) that impact a patient’s PaCO2. If, for example, this patient is febrile and has a new pulmonary embolism, V ̇CO_2 is increased, V_(D )is increased, and V_D/V_T might be quite high if that RR of 30 is paired with small shallow breaths. All of these would lead to an increased PaCO2. Dropping ( V_E ) ̇ with a CNS depressant like a benzodiazepine could lead to critical hypercapnia and a respiratory arrest. Do not assume you know someone’s PaCO2 based solely on their respiratory rate.

Question 4

What is the normal PaCO2 value?

Answer 4

35 to 45 mmHg

Question 5

CO2 delivery or ventilation is determined by what equation?

Answer 5

PACO2 x VA

Question 6

Considering that PACO2 x VA gives you the ventilation rate of CO2, how can you determine PACO2?

Answer 6

VCO2/VA

Question 7

When is the A-a difference equal to zero?

Answer 7

NEVER!

Question 8

Given that PA CO2 can be determined by the relationship VCO2/VA, and that VCO2 is proportional to metabolism, how can you determine PaCO2?

Answer 8

(VCO2 x 863)/VA

Question 9

What is the most LIKELY cause of hypocapnia or respiratory alkalosis?

Answer 9

increased minute ventilation (Ve) due to pain, anxiety, fever, sepsis, etc.

Question 10

What are three causes of hypercapnia (math wise)

Answer 10

Increased VCO2, decreased Ve** most common clinically**, and increased Vd/Vt

Question 11

Fevers, seizures, overfeeding and hyperthyroidism all:

a. increase VCO2
b. Decrease Ve
c. Increase Vd/Vt

Answer 11

a. increase VCO2

Question 12

Head trauma, encephalitis, depressant drugs, obesity, ALS, chest wall deforities, and COPD (severe) could all:

a. increase VCO2
b. Decrease VE
c. Increase Vd/Vt

Answer 12

Decrease Ve

Question 13

Pulmonary embolisms, emphysema, hyperinflation, low Vt during mechanical ventilation, and shallow breathing could all:

a. increase VCO2
b. Decrease Ve
c. Increase Vd/Vt

Answer 13

Increase Vd/Vt

Question 14

An imbalance between strength/drive and the load on the respiratory system is frequently reflected in:
hypercapnia
hypoxemia

Answer 14

hypercapnia

Question 15

Why must ventilation and over-oxygenation be very carefully controlled in severe cases of COPD?

Answer 15

it can cause hypercapnia, secondary to worsening ventilation and perfusion matching, which in turn is secondary to impaired hypoxic pulmonary vasoconstriction

Question 16

When you’re evaluating hypercapnia, what ‘s on your differential?

Answer 16

Decreased VCO2 (seizure, fever, exercise)
Decreased minute ventilation
increased dead space

Question 17

The hypecapnia differential (Drop in VCO2, VE, increase in dead space) comes from which equation for PaCO@?

Answer 17

PaCO2 = (VCO2 x 863 mmHg)/ ((RR x Vt (1-Vd/Vt))

Question 18

In the equation for PaCO2, what’s the numerator?

Answer 18

VCO2 x 863 mmHG

PaCO2 = (VCO2 x 863 mmHg)/ ((RR x Vt (1-Vd/Vt))

Question 19

In the equation for PaCO2, what’s the denominator?

Answer 19

((RR x Vt (1-Vd/Vt)) (I.e. alveolar ventilation)

PaCO2 = (VCO2 x 863 mmHg)/ ((RR x Vt (1-Vd/Vt))

Question 20

What causes increased Vd/Vt in hypercapnia?

Answer 20

PE and emphysema

Question 21

What is your differential for a shunt with increased A-a leading to hypoxemia? What’s the V/Q?

Answer 21

blood pus water atelectasis V/Q = 0

Question 22

What is the differential for low V/Q not zero as a cause of hypoxemia with increased A-a?

Answer 22

asthma and PE

Question 23

What is the differential for diffusion limitation as a cause of hypoxemia with increased A-a?

Answer 23

ILD, emphysema