Gas Transfer
Oxygen and carbon dioxide are easily dissolved in plasma, with other gases being less soluble. This normally allows complete equilibration across the alveoli, with the full partial pressure of oxygen, PaO2, ( 100 mmHg) reached in arterial capillaries. However, in situations of disease, this full saturation is often not reached.
Diffusion
Gas flux is determined by Fick's Law of Diffusion, which states that flow is proportional to the surface area (A), diffusibility (D), and partial pressure gradient (P1-P2), and inversely proportional to the membrane thickness (t).
The lung's surface are is ~85m2 per lung, while its thickness is 0.5 μm. The diffusibility, D, is determined by a molecule's solubility divided by the root of its molecular weight. While oxygen and carbon dioxide have similar molecular weights, carbon dioxide is 20 times more soluble than oxygen and therefore has a much higher D.
Partial Pressures
PAO2 (Alveolar O2)
The partial pressure of alveolar O2 cannot be measured. However, it can be measured using a few calculations.
PA CO2 (Alveolar CO2)
The partial pressure of CO2 can be derived from an equation taking into account CO2 production and alveolar ventilation. The first can be measured, while the second is deduced from breathing frequency fb), tidal volume (VT), and dead space (VD). Dead space can be estimated by the patient's weight in pounds, in ml.
In the following equation, 863 is the correction of pressure due to warm, moist air.
As can be seen from this equation, hyperventilation (increased VT) will decrease PACO2, while an increase in dead space will increase PACO2.
Alveolar -Arterial Difference in Pressure
An important measure of Normally, the difference in O2 (DO2) between PAO2 (calculated) and PaO2 (measured) is less than 10 mm Hg. If it is greater, this suggests diffusion impairment.
A PaO2 of less than 60 is a candidate for home O2.
| Alveolar | Arterial | Venous | |
|---|---|---|---|
| O2 | 150 | 150 | 40 |
| CO2 | 0.21 | 40 | 46 |
| H2O | 47 | 47 | 47 |
| N2 | 556 | 573 | 573 |
| total | 760 | 760 | 706 |
FIX THIS Atmospheric pressure is 760 mmHg and is composed of 21% O2 and 79% N2 . This produces a pO2 of 160 mm Hg ( .21 x 760 ) and a pN2 of 600 ( 0.79 x 760 ).
PaO2 and PaCO2 are determined by the degree of equilibration between alveoli and capillaries, and depends on: VQ matching, ventilation, shunting, and diffusion.
Matching of ventilation (V) and perfusion (Q) In normal lungs, according to rates of blood flow across regions of the lung, V/Q occurs within a narrow range of about 0.5 to 3.0. In lung disease, increasing mismatch can no longer be compensated for, and PaO2 will fall while PaCO2 rises.
Ventilation With hypoventilation, CO2 displaces O2. A reciprocal relationship exists between CO2 and O2.
As PaCO2 rises, PaO2 will fall. Increasing the FIO2 by giving supplemental oxygen will help reverse hypoventilation-induced hypoxia.
Shunting is blood traveling from the right to the left heart without being oxygenated. Anatlomic shunting can occur with cardiac structural problems. Physiologic shunt can occur when blood flows past unventilated alveoli, an extreme of V/Q mismatch.
Diffusion impairment occurs with increased alveolar-capillary thickening, changes in the lung architecture, hemoglobin concentration, or capillary volume. Blood spends an average of 0.75 seconds in pulmonary capillaries at rest, and this time decreases to 0.25 seconds with vigourous exercise. As such, with diffusion impairment, hypoxemia will occur first with exercise. Diffusion capacity can be measured using carbon monoxide diffusion.
Gas Circulation
The majority of oxygen is carried on hemoglobin, capable of carrying four molecules each.
Oxygen Transport
Oxygen is transported from the lungs both dissolved in the blood and bound to Hgb. Both are proportional to levels of PaO2, which is normally 100 mm Hg when leaving the lungs.
Normally there is 0.3 ml of O2 dissolved in 100 ml blood, a negligible amount. However, this amount can be increased if the atmospheric pressure is increased to 2-3 ATA and 100% oxygen is given, because of the PAO2 equation.
One gram of Hgb holds 1.39 ml O2, which means 15 gm of Hgb (a normal amount for 100 ml blood) holds 20.8 g of O2.
Oxygen Saturation of Hgb
The O2 saturation is a relative percentage of maximum O2 binding on a given amount of Hgb; it dos not tell you O2 concentration.
SaO2 refers to saturation measured by arterial blood, while SPO2 is the saturation measured with a pulse oximeter.
The oxygen dissociation curve is sigmoidal, due to cooperative binding to Hgb.
O2 extraction is CaO2- CvO2 and is facilitated by the steep part of the slope.
Venous PaO2 results in a saturation of 75% and a CaO2 of 15 ml O2/ 100 ml blood.
The O2 saturation curve will shift right in conditions such as increased temperature, PCO2, [2,3 BPG], and decreased pH. By shifting the curve to the right, these conditions assist in Hgb unloading of O2.
Exercise thus shifts all these to the right, and thereby at a given pO2
Someone who is anemic will still reach saturation but will have low levels of circulating O2.
Carbon Dioxide Saturation
The blood has a greater capacity for CO2 than for O2, I think because of the bicarbonate buffer. What this means is that the rise and fall in PO2 is greater than the rise and fall in PO2.
Venous Unsaturation
As can be observed from combined partial pressures, venous blood is unsaturated. This allows for gas to be absorbed from gas bubbles surgical air, pneumothorax, and spaces such as the sinus and middle ear.