Block 4: 7 Alveolar Ventilation and Gas Composition

What is the respiratory quotient (R)?

0.8 Describes the ratio of # of CO2 molecules we generate for each O2 molecule consumed (80 CO2 / 100 O2)

What is the alveolar pressure of oxygen?

100 mmHg (150 of inspired drops as CO2 increases)

How much of the normally inhaled 500 ml stays in the conductance airways?

150 ml

What is the inspired conducting airways pressure of oxygen?

150 mmHg (160 of atmo partial O drops as H20 partial pressure increases)

How much of the normally inhaled 500 ml makes it to the gas exchange airways?

350 ml

What is the arterial pressure of CO2?

40 mmHg

What number should b used for PalvCO2 pressure in the alveolar gas equation unless otherwise noted?

40 mmHg

What number should b used for H20 pressure in the alveolar gas equation unless otherwise noted?

47 mmHg

What percent of the air we breath is available for gas exchange?

70% makes it to the exchange airways

What is the arterial pressure of oxygen?

99-100 mmHg Slightly lower than alveolar due to blood shunt

What is a normal alveolar ventilation value?

AV = (500 ml - 150 ml) * 12 breaths/min = 4200 ml per min

Describe how the ratio of CO2 and O2 in the alveoli is affected by decreased ventilation with unchanged metabolism

Alveolar O2 decreases, CO2 increases

Describe how the ratio of CO2 and O2 in the alveoli is affected by increased metabolism with unchanged ventilation

Alveolar O2 decreases, CO2 increases

Describe how the ratio of CO2 and O2 in the alveoli is affected by increased ventilation with unchanged metabolism

Alveolar O2 increases, CO2 decreases

What is alveolar ventilation?

Amount of air that makes it into the alveolar airways AV = (TV - Anatomical dead space) * Respiratory rate

How can the respiratory quotient change?

Can change with diet Glucose increases, palmatic acid deceases Mixed diet though is 0.8

PRACTICE: True or false; H2O does not exert any partial pressure in gas form.

False; H2O exerts 47 mm Hg of pressure at 37oC and at 100% relative humidity.

PRACTICE: True or false; a normal PaO2 value is 40 mm Hg.

False; a normal PaO2 value is 100 mm Hg, not 40 mm Hg.

PRACTICE: True or false; barometric pressure increases as we climb to higher altitudes.

False; barometric pressure decreases (not increases) as we climb to higher altitudes.

PRACTICE: True or false; the fraction of inspired O2 (FIO2) decreases at altitude.

False; the fraction of inspired O2 (FIO2) does not change with altitude. Rather, barometric pressure decreases with altitude.

PRACTICE: True or false; the partial pressure of O2 in arterial blood is higher than it is in inspired air.

False; the partial pressure of O2 in arterial blood is lower (not higher) than it is in inspired air.

PRACTICE: True or false; the respiratory quotient (R) will increase if we eat a diet that is 100% fat.

False; the respiratory quotient (R) will decrease (not increase) if we eat a diet that is 100% fat.

What two factors can change partial pressures of O2 inspired, alveolar, and arterial?

Fraction of inspired O2 and barometric pressure

What does FIO2 stand for? What is normal at 760 mmHg?

Fraction of inspired oxygen Normal at 760 mmHg is 21%

PRACTICE: If we increase ventilation without increasing metabolism, what will happen to arterial O2 (PaO2)?

If we increase ventilation without increasing metabolism, arterial O2 (PaO2) will increase.

How do we calculate inspired partial pressure of oxygen?

Inspired partial pressure O = (atmo pressure - 47 mmHg H20 pressure) * FIO2

What is normal minute ventilation?

MV = 500 ml per breath / 12 breaths per minute = 6000 ml

Why must ventilation and metabolism be matched?

Maintains normal balance of O2 and CO2 in the alveoli at 100 mmHg O2 and 40 mmHg CO2

What is the alveolar gas equation?

PalvO2 = PinspO2 - (PalvCO2 / 0.8)

How do we calculate alveolar partial pressure of oxygen?

PalvO2 = PinspO2 - (PalvCO2/R) R is constant of 0.8

What happens if you increase FIO2 from 21% (normal) to 100%?

PalvO2 increases to 663 mmHg, resulting in an arterial partial O pressure of about 660 = greatly helpful for hypoxic patients

Compare PinspO2 and PalvO2 at sea level (760 mmHg) to 10,000 feet (523 mmHg)

PinspO2 at sea level: 150 mmHg PalvO2 at sea level: 100 mmHg PinspO2 at 10,000: 100 mmHg PinspO2 at 10,000: 60 mmHg

What happens if ventilation and metabolism are not matched?

Ratio of CO2 and O2 in the alveoli will be skewed from normal (100 mmHg O2 and 40 mmHg CO2)

Which is more important for alveolar ventilation, TV (depth) or frequency (resp rate)?

TV is more important, as even with greatly decreased frequency an increased TV can still raise AV greatly (While decreased TV but increased freq just causes higher % of air to stay in conductance airways = lower AV)

What is minute ventilation?

The amount of air that we breath in and out each minute MV = Tidal volume * respiratory rate

PRACTICE: What is the arterial O2 (PaO2) when breathing a gas mixture that is 30% and you are in Gainesville?

The arterial O2 (PaO2) when breathing a gas mixture that is 30% and you are in Gainesville is ~164 mm Hg

PRACTICE: A tidal volume is 600 mls per breath and the respiratory rate is 10 breaths per minute. What is the minute ventilation of this individual?

The minute ventilation of this individual is 6,000 mls per minute.

PRACTICE: What is the partial pressure of O2 in atmospheric air?

The partial pressure of O2 in atmospheric air is 160 mm Hg.

Describe Dalton's Law

The total pressure of a gas mixture is the sum of the individual pressures

What is the atmospheric partial pressure of oxygen?

Total atmo pressure is 760 mmHg Partial oxygen pressure is 160 mmHg

Why is PaO2 decreased at higher altitudes?

While there is still 21% O2 in the air at higher altitude, the driving pressure bringing O2 into the lungs is reduced

MOCK EXAM: A patient begins to hyperventilate while sitting in a clinic on Miami Beach and breathing room air. Over the ensuing 1 minute, his PACO2 drops to 20 mm Hg. What is his PaO2 at that moment in time? a) 150 mm Hg b) 125 mm Hg c) 100 mm Hg d) 75 mm Hg e) 50 mm Hg

b) 125 mm Hg Palv O2 = PInsp O2 - (Palv CO2 / R) = 150 - (20 / 0.8) = 125 mm Hg

MOCK EXAM: When at sea level and breathing room air, the partial pressure of O2 in inspired air is a) 760 mm Hg b) 40 mm Hg c) 100 mm Hg d) 160 mm Hg e) 150 mm Hg

e) 150 mm Hg PInsp O2 = (Patm - 47 mm Hg) x FIO2 = (760 - 47) x 0.21 = 150 mm Hg