ALVEOLAR/ARTERIAL/VENOUS GAS TENSIONS

General
-each gas contributes separately to the total gas pressure
-partial pressure of each gas is directly proportional to its concentration
-air contains 21% oxygen at 760 barometric pressure (sea level)
-partial pressure of oxygen at sea level PO2=159.6 mmHg

Oxygen
alveolar oxygen tension
pulmonary end-capillary oxygen tension
arterial oxygen tension
mixed venous oxygen tension

Carbon Dioxide
mixed venous carbon dioxide tension
alveolar carbon dioxide tension
pulmonary end-capillary carbon dioxide tension
arterial carbon dioxide tension
end tidal carbon dioxide tension

Oxygen

Alveolar oxygen tension
-with every breathe the upper airway humidies oxygen to temp of 37 degree C
-with humidification of oxygen the added vapor pressure of water decreases the inspired concentration of oxygen

Inspired oxygen concentration before humidification of upper airways
PI02 = Pb x FI02 = 159.6 mm Hg
Pb = barometric pressure =760 mmHg at sea level
FI02=inspired oxygen concentration= 21% =.21 at sea level

Inspired oxygen concentration after humidification of upper airways
PI02 = (Pb-47mmHg) x FI02
(760-47)mmHg x 0.21
(713 mmHg) .21
149.7 mmHg

ALVEOLAR OXYGEN TENSION

PA02= PI02 - PaCO2/R
149.7 mmHg - ( 40mmHg/.8)
149.7 mmHg - 50 mmHg
99.7 mmHg
PaCo2 is measured by ABG
R= respiratory quotient. normal =0.8

Pumlonary end-capillary oxygen tension (Pc' 02)
-practical reference Pc' 02 is identical to PA02
-the PA02 - Pc' 02 gradient is miminal

Factors affecting Pc' 02
-rate of oxygen diffusion across the alveolar-capillary membrane (large capillary surface area promotes oxygen diffusion)
-pulmonary capillary blood flow
-transit time = pulmonary capillary blood volume x cardiac output (nl. =0.8 sec; 70ml/ 5000ml/min)
-enhanced oxygen binding to hemoglobin
* Rate Limiting Factor of Oxygen Transfer from Alveolar gas to blood is binding of oxygen to hemoglobin

Pulmonary diffusing capacity reflects
-capacity of oxygen to pass through the alveolar-capillary membrane
-permeablitliy of oxygen to pass through the alveolar-capillary membrane
-assesment of pulmonary blood flow
-oxygen uptake is generally limited by pulmonary blood flow rather than oxygen diffusion across the alv-cap membrane
ex. exercise is healthy individuals at high altitudes
ex. patients with extensive destruction of the alveolar-capillary membrane

DLCO = carbon monoxide uptake/ PACO
decreased DLCO: indicated imediment of gas transfer across the alveolar-cap. membrane

Causes:
-abnormal V/Q ratios
-destruction of the gas alveolar-capillary membrane
-extemely short capillary transit times
-can exacerbated with increases of oxygen consumption and cardiac output ex. exercise

Arterial oxygen tension (Pa02)
-not calculated
-measured at room air with ABG
-normal A-a gradient < 15 mmHg
-normal A-a gradient upto 20-30 mmHg with increasing age may be normal
estimated Pa02 = 102 - age/3
-nl Pa02 btn 60-100 mm Hg
-decreases in Pa02 may be from increases in closing capacity compared with FRC
Pa02< 60 mmHg = hypoxemia

Most common mechanism for hypoxemia: increased A-a gradient.

Factors affecting A-a gradient
-degree of right to left shunting: direct relationship
-amount of V/Q scatter
-mixed venous oxygen tension: indirect relationship
low cardiac outputs tends to accentuate the effects shunts have on Pa02
high cardiac outputs tends to increase venous admixture by increasing mixed venous oxygen tension

Mixed venous oxygen tension
-nl value Pv02 = 40 mmHg
-represents overall balance between oxygen consumption and oxygen delivery
-obtained from pulmonary artery cathetor
mixed venous sample contains blood drained from
-superior vena cava
-inferior vena cava
-right side of heart

Carbon Dioxide

metabolic byproduct of aerobic metabolism occuring in the mitochondria
continual small gradients of CO2 tensions
-from mitochondria to cell cytoplasm
-cell cytoplasm to extracellular fluid
-extracellular fluid to venous blood
-venous blood to alveoli
-alveoli to expired air for elimination

Mixed venous carbon dioxide tension (Pv'CO2)
-nl value = 46 mmHg
-consist of CO2 from various tissues each with varying metabolic activity
ex. skin tissue has lower metabolic activity therefore releases lower CO2
ex. heart tiss. has higher metabolic activity therefore releases more CO2

Alveolar carbon dioxide tension
-represents the balance between total C02 production and alveolar ventilation
-balance between production and elimination
-more dependent on alveolar ventilation than C02 production due to buffering capacity of increased C02 production

PaC02 = VC02/ VA
VC02 = C02 production
VA = Alveolar ventilation
steady-state: C02 production = C02 elimination

imbalances : C02 production > C02 elimination
increases total body C02 content
ex.hypoventilation
hypoperfusion

Pulmonary end-capillary carbon dioxide tension
-generally identical to PAC02
-diffusion rate of C02 across the alveolar-capillary membrane is 20 times greater than oxygen

Arterial carbon dioxide tension (PaC02)
-measured with ABG
-nl values 38+/- 4 mmHg
-nl value of 40 mmHg accepted for practical use
-even amongst moderate to severe disturabances usually fail to alter PaC02 due to buffering capacity
increased PaC02
-low V/Q ratios
decreased PaC02
-high V/Q ratios

Arterial-alveolar C02 gradients are not common but may occur in:
-presence of marked V/Q abnormalities
-venous admixture > 30 %

End-tidal carbon dioxide tension (ETC02)
-clinically used to estimate PaC02
-gradient represents dilution of alveolar gas with C02 free gas from alveolar dead space
PAC02-PETC02 gradient < 5 mmHg