Control of Breathing

Question 1

How to calculate PaCO2?

Answer 1

PaCO2 = VCO2/VA

So PaCO2 is governed by:
VCO2 = CO2 production
VA = alveolar ventilation

Question 2

What influences PaO2?

Answer 2

• Alveolar Ventilation (VA)
• Barometric Pressure
• Fraction of O2 Inspired
• Matching of Ventilation to Perfusion in the Lungs
• Diffusion membrane thickness in the Lungs

Question 3

Whats the equation for PaCO2?

Answer 3

VAPBFIO2*(V/Q)/DM Thickness

Question 4

Carotid bodies transfer electric impulse through which nerve?

Answer 4

IX Nerve

Question 5

Central chemoreceptors are activated by which molecule?

Answer 5

H+ (via CO2)

Question 6

What happens to to central chemoreceptors with increased PaCO2?

Answer 6

Increased PaCO2 stimulates the central chemoreceptors which, in turn, stimulate the respiratory centre to increase ventilation and decrease PaCO2.

Question 7

T or F: the carotid body can respond to hypercapnea

Answer 7

T

A small part of the ventilation response to hypercapnia originates from CO2 stimulating the carotid bodies.

Question 8

T or F: H+ in the blood directly stimulates the central chemoreceptors

Answer 8

F

Although H+ is what stimulates the central chemoreceptors, the blood brain barrier largely prevents H+ in the blood from entering the CSF. Blood H+ (metabolic acidosis ) stimulates ventilation mainly via a different mechanism – via the carotid bodies.

Question 9

What activates carotid bodies?

Answer 9

LOW PaO2, High PaCO2 and High H+

Question 10

What percentage of oxygen is used to assess sensitivity of carotid bodies?

Answer 10

21% O2

Question 11

At which point does PaO2 have an effect on ventilation?

Answer 11

Low PaO2 does not have much effect on ventilation

until it falls below about 60 mm Hg

Question 12

When would require to give supplemental oxygen?

Answer 12

A PaO2 of 60 mm Hg at rest is near that required for

funding of supplemental O2

Question 13

Where is the respiratory center located?

Answer 13

medulla

Question 14

Where is the central chemoreceptors located?

Answer 14

ventricles of the brain

Question 15

What happens to PaCO2 and PaO2 at higher altitude?

Answer 15

The typical arterial blood gases seen at high altitude are low PaO2 due to the low barometric pressure and hyperventilation and low PaCO2 due to the carotid bodies stimulation from hypoxia.

Question 16

What is the formula to calculate tissue oxygenation?

Answer 16

TO = O2-delivery / O2-consumption

Question 17

What is the formula for O2-delivery?

Answer 17

VO2= Q(CaO2)

Q is the cardiac output
CaO2 is the arterial content of oxygen

Question 18

What is the minimal capillary PO2 needed?

Answer 18

20 mmHG

Question 19

How can one tell if O2 delivery is seriously deficient?

Answer 19

When PvO2 approaches 20 mm Hg then O2 delivery is seriously deficient and tissue hypoxia exists.

Question 20

How to calculate Cardiac Output?

Answer 20

Q = VO2/(CaO2 - CvO2)

VO2 –> O2-delivery (minute ventilation?)

Question 21

What happens with decreased Hb on arterial and venous oxygen?

Answer 21

Both lowers, but CaO2 much drastic so that CaO2 - CvO2 is smaller –> Q is higher (cardiac output)

–>lower oxygen-dissociation curve

Question 22

What is normal arterial O2 content?

Answer 22

0.1954 mlO2/ml

Question 23

What is tissue hypoxia?

Answer 23

Tissue hypoxia: Capillary PO2 ~20mm Hg

Question 24

T or F: PaO2 (ABG) is an accurate indicator of tissue oxygenation

Answer 24

F:
depends on many things such as:
cardiac output, arterial content of O2 [Hb content, sat of O2, PaO2), O2 consumption

Question 25

The arterial content of O2 depends on what?

Answer 25

The arterial O2 content depends on the hemoglobin concentration, the saturation of the hemoglobin with O2 and the PaO2.

Question 26

What dictates the Hb O2 sat?

Answer 26

PaO2 and pH

Question 27

How can a low capillary PO2 achieved?

Answer 27

A capillary PO2 of 20 mmHg can be reached by having a low PaO2 (lung disease or high altitude), having a low blood flow or having a low hemoglobin concentration (or high COHb).