The alveolar gas equation is used to calculate alveolar oxygen partial pressure as it is not possible to collect gases directly from the alveoli. The equation is helpful in calculating and closely estimating the PaO2 inside the alveoli. The variables in the equation can affect the PaO2 inside the alveoli in different physiological and pathophysiological states.
How does the alveolar gas equation help us understand gas exchange?
The alveolar gas equation helps us understand the pathophysiology of abnormal gas exchange by providing information on alveolar oxygen tension. According to the Fick equation, under steady state conditions, the quantity of O 2 taken up by the lungs equals the amount of O 2 removed from inhaled air:
Is the alveolar gas equation valid at low PiO2?
The alveolar gas equation predicts a relationship of inspired oxygen partial pressure ( Pio2) to the resulting alveolar partial pressure ( Pao2) that is linear. It also predicts a nonsensical negative Pao2 at low Pio2. The equation is valid only at values of Pio2 sufficient to allow normal oxygen uptake V̇o2.
Is Fenn’s alveolar gas equation hard to follow?
While the seminal paper by Fenn et al. (5) on the alveolar gas equation presents fascinating graphs that have been cited as a learning opportunity (4), their derivation uses antiquated notation, is difficult to follow for the typical medical student, and does not include the alveolar ventilation equation.
What is the gas composition of the alveolar capillaries?
Put another way, the gas composition of each alveolar-capillary unit depends on its individual V̇a/Q̇ ratio, and all units together compose overall gas exchange. This concept has major clinical significance because CO2and O2are transported differently in the blood.
What is the purpose of the alveolar arterial oxygen tension gradient?
The A-a gradient, or the alveolar-arterial gradient, measures the difference between the oxygen concentration in the alveoli and arterial system. The A-a gradient has important clinical utility as it can help narrow the differential diagnosis for hypoxemia.
What is the alveolar ventilation equation?
Alveolar ventilation is calculated by the formula: VA= R(VT-VD) where R is respiratory rate, VT is tidal volume, and VD is dead space volume.
What is the PCO2 equation?
In contrast, the equation pCO2 = 1.5 × HCO3 + 8, known as Winters' formula, exhibits larger errors.
What determines alveolar PO2?
0:029:15Determinants of Alveolar Partial Pressures of Oxygen and Carbon ...YouTubeStart of suggested clipEnd of suggested clipAnd carbon dioxide is important because this ultimately determines the systemic arterial pressure ofMoreAnd carbon dioxide is important because this ultimately determines the systemic arterial pressure of those blood gases. So what determines alveolar po2 and pco2 first let's talk about alveolar po2.
What does the alveolar gas equation tell you?
The alveolar gas equation is a formula used to approximate the partial pressure of oxygen in the alveolus (PAO2):PAO2=(PB−PH2O)FiO2−(PaCO2÷R)where PB is the barometric pressure, PH2O is the water vapor pressure (usually 47mmHg), FiO2 is the fractional concentration of inspired oxygen, and R is the gas exchange ratio.
Why is alveolar ventilation a more accurate indicator of breathing efficiency How is this volume calculated?
Alveolar ventilation rate provides a more accurate measure of ventilatory efficiency because it considers only the volume of air actually participating in gas exchange.
How is the alveolar gas equation derived?
2:3212:23Derivation of the Alveolar Ventilation and Alveolar Gas Equations (ABG ...YouTubeStart of suggested clipEnd of suggested clipBy the total pressure for example the partial pressure of carbon dioxide in alveolar gas is equal toMoreBy the total pressure for example the partial pressure of carbon dioxide in alveolar gas is equal to the fractional concentration of co2 in the alveoli.
What is the relationship between PaCO2 and alveolar ventilation?
It is the change in local PaCO2 as well as the change in pH that causes a change in minute ventilation. Under normal physiologic conditions, an increase in PCO2 causes a decrease in pH, which will increase minute ventilation and therefore increase alveolar ventilation to attempt to reach homeostasis.
What does PaCO2 measure?
Partial pressure of carbon dioxide (PaCO2). This measures the pressure of carbon dioxide dissolved in the blood and how well carbon dioxide is able to move out of the body. pH.
What is the significance of partial pressure in respiration?
The air in the lungs has a higher concentration of oxygen than that of oxygen-depleted blood and a lower concentration of carbon dioxide. This concentration gradient allows for gas exchange during respiration. Partial pressure is a measure of the concentration of the individual components in a mixture of gases.
Why does alveolar air pressure first decrease and then increase during inspiration?
During inhalation, the increased volume of alveoli as a result of lung expansion decreases the intra-alveolar pressure to a value below atmospheric pressure about -1 cmH2O. This slight negative pressure is enough to move 500 ml of air into the lungs in the 2 seconds required for inspiration.
What is the difference between PO2 and PAO2?
PO2 is just partial pressure of oxgen in a given environment, such as room air. 21% O2 in standard barometric pressure of 760mmHg means usual PO2 in room air is 760 x 0.21 = 160mmHg. PAO2 is partial pressure of oxygen in alveoli. PaO2 is partial pressure of oxygen dissolved in (arterial) blood.
Why is the equation useful?
The equation is helpful in calculating and closely estimating the PaO2 inside the alveoli. The variables in the equation can affect the PaO2 inside the alveoli in different physiological and pathophysiological states.
What are the three variables of the equation?
The 3 major variables of the equation are the atmospheric pressure, amount of inspired oxygen, and levels of carbon dioxide. Each has an important clinical significance and can help explain different physiological and pathophysiological states.
What does RQ mean in chemistry?
RQ = amount of CO2 produced/amount of oxygen consumed
What is the normal alveolar oxygen tension?
At sea level, the normal expected alveolar oxygen tension is 100 mm Hg. The alveolar-arterial oxygen difference (referred to as the A-a gradient) is a measure of how effectively air from the alveoli is transferred to the pulmonary capillaries.
What is alveolar hypoventilation?
Alveolar hypoventilation, a decrease in alveolar ventilation in relation to CO 2 production, elevates PACO 2 (PaCO 2) and decreases PAO 2. Alveolar hypoventilation is observed when tranquilizers, sedatives, anesthetic drugs, or injury depresses the central nervous system; when there is severe airway obstruction; or when there is damage to the thorax and respiratory muscles ( Figure 2-14 ). Use of inhalation anesthetics is routinely associated with alveolar hypoventilation in horses, and for this reason positive pressure ventilation should always be available. 70
What causes hypoxemia in high altitudes?
The first mechanism that can cause hypoxemia is reduced inspired oxygen tension. This most commonly occurs at high altitudes where the decrease in atmospheric pressure directly decreases the alveolar oxygen tension, as can be seen by the alveolar gas equation. In this case, the A-a gradient remains normal.
What is the normal oxygen level for hypoxemia?
Values below 15 mm Hg are generally considered normal. 16 If the (A − a) O 2 ratio is widened, a component of the hypoxemia results from ventilation-perfusion mismatching. It should be remembered that F io2 is dependent on barometric pressure and will be lower at higher altitudes. At an altitude of 500 m (approximately 1640 feet), P io2 = 140 mm Hg, whereas at 1000 m (3280 feet), P io2 = 130 mm Hg. Although it has long been thought that in the hypercapnic patient the alveolar-arterial oxygen difference differentiates hypoxemia caused by pure hypoventilation from hypoxemia in which other factors play a role, this idea has been seriously challenged, 26,33 because the (A − a) O 2 gradient may be increased in some patients with extrapulmonary disorders. Clinically, a normal gradient excludes pulmonary disease and suggests some form of central alveolar hypoventilation or an abnormality of the chest wall or inspiratory muscles. 67 To increase the specificity of the test to diagnose the ventilation/perfusion mismatch, only patients with (A − a) O 2 gradient values greater than 25 mm Hg should be considered abnormal. 16 These patients are likely to have primary pulmonary disease, but extrapulmonary disorders cannot be completely ruled out.
What causes hypoxemia in the lung?
Causes of hypoxemia include a low F i O 2, a low P B (high altitude), hypoventilation (increased PaCO 2 ), and incomplete oxygenation of the blood by the lung (ventilation-perfusion [] mismatch or shunt). In mismatch, some alveoli are underventilated for their blood flow (low ) and blood is incompletely oxygenated.
Why does Paco 2 decrease during hyperventilation?
The PACO 2 (and PaCO 2) decrease during alveolar hyperventilation because ventilation is greater than needed to eliminate CO 2 being produced by the body. Hyperventilation occurs when the drive to ventilate is increased by stimuli such as hypoxia, increased production of hydrogen ions, or an increase in body temperature. Animals kept at high altitude routinely hyperventilate to increase the delivery of oxygen to their lungs. For this reason it is usual for PaCO 2 to be less than the normal of 40 to 45 mm Hg in areas such as the Rocky Mountain States. Overly vigorous use of the ventilator also can cause hypoventilation in the anesthetized horse.
What is the path of diffusion in the alveolus?
In diffusing from the alveolus to the erythrocyte, an O 2 molecule must cross several layers before combining with hemoglobin (Hb). These include the alveolar gas, the surfactant/fluid layer, the alveolar epithelial membrane, the capillary endothelial membrane, the plasma, the erythrocyte membrane, and the intra-erythrocytic fluid. The diffusion path length thus outlined may be increased pathologically in several ways, e.g. by edema, fibrosis, and in theory may impair diffusion. The area of perfused alveoli available for gas exchange is reduced in human emphysema when alveolar walls are broken down, and O 2 diffusion may be limited by a decrease in the area available for gas exchange
Why is the alveolar air equation not widely used in clinical medicine?
The alveolar air equation is not widely used in clinical medicine, probably because of the complicated appearance of its classic forms.
Which gas obeys the ideal gas law?
Inspired and alveolar gases obey the ideal gas law. Carbon dioxide (CO 2) in the alveolar gas is in equilibrium with the arterial blood i.e. that the alveolar and arterial partial pressures are equal. The alveolar gas is saturated with water.
What are the assumptions of inspired gas?
The equation relies on the following assumptions: Inspired gas contains no carbon dioxide (CO 2) Nitrogen (and any other gases except oxygen) in the inspired gas are in equilibrium with their dissolved states in the blood. Inspired and alveolar gases obey the ideal gas law. Carbon dioxide (CO 2) in the alveolar gas is in equilibrium ...
What is the alveolar gas equation?
For background, the alveolar gas equation is a way of calculating what the level of oxygen is in the alveoli, given the atmospheric pressure (p ATM, usually 760mmHg), the fraction of inspired oxygen (F I O 2, which is 21% for room air and increases if they’re on supplemental oxygen), the pressure of water vapour in the lungs (pH 2 O, usually 47mmHg), the arterial CO 2 (p a CO 2, taken from your ABG), and the respiratory exchange ratio (RER, the amount of oxygen exchanged for carbon dioxide in one breath, usually 0.8). The equation is:
Which chapter of Harrison's Principles of Internal Medicine discusses ventilation?
Read more: Adequacy of Ventilation in Chapter 306e: Disturbances in Respiratory Function, Harrison’s Principles of Internal Medicine 19e.
How is the A-A gradient useful?
How is the A-a gradient useful? Well, there are only two things that cause hypoxia with a normal A-a gradient: hypoventilation (not moving enough air), and decreased P i O 2 (that is, high altitude). Since I’m rarely doing my ABGs on a mountaintop, the A-a gradient is basically a quick and easy way to rule out hypoventilation as the cause of their hypoxia.
What is the alveolar gas equation?
The alveolar gas equation (AGE) is well known and relates the alveolar concentration of oxygen Fao2 (or equivalently partial pressure Pao2) to three variables: Fio2, Faco2 and the respiratory quotient ( R ). However, the AGE predicts an absurdity: if we input a Fio2 sufficiently low, say 0.05 ( i.e. a Pio2 of about 5 kPa), into the equation
What is the extra term in the complete form of the alveolar gas equation (AGE), where R is less than?
The extra term in the complete form of the alveolar gas equation (AGE), where R is less than unity, adds a small amount to the predicted Pao2. This is interesting but clinically trivial and may be neglected for practical purposes.
How much does paco2 increase in apnea?
In apnoea, Paco2 increases by 0.3–0.7 kPa min −1, say 0.5 kPa min −1. If Paco2 increases at this rate, then, by our assumption that Paco2 = Paco2, so does Paco 0 2. Since this is in the gas phase, we can apply Dalton's law and know that carbon dioxide is entering the FRC at 0.05 × 2 litre min −1 ( i.e. about 10 ml of the 200 ml min −1V̇co2 ). The remainder of the V̇co2 remains in the aqueous phase in the body water. Provided Pao2 is sufficient to saturate the Hb (preoxygenation), then uptake V̇o2 (say 0.25 litre min −1) will proceed normally, and if the airway is filled with oxygen (flowing through catheter in trachea) this will be drawn into the FRC at a rate of 0.24 litre min −1 by bulk flow: this is apnoeic mass-transfer oxygenation. Under ideal circumstances (preoxygenation and normal gas exchange), this technique is limited by carbon dioxide toxicity and not hypoxia.
What gas is present in the alveolar space?
This states that an individual gas such as carbon dioxide in the mixture of gases in the alveolar space will be present in a concentration (or fraction) that is the same proportion as Paco2 is of the total pressure ( i.e. approximately Pi at the end of inspiration).
Is Pao2 a linear or nonlinear relationship?
The relationship of alveolar ventilation ( V̇a) to the resulting Pao2 for constant Pio2 is non-linear.
Is fao2 a variable?
Fao2 is dependent upon three variable quantities: Pio2, Paco2 and R. Although all are variables, they are not equally easily engineered. Pio2 is very easily manipulated, of course. Paco2, on the other hand, is quite easy to manipulate in the intubated, ventilated patient but much less so in the spontaneously breathing subject. R is alterable by manipulation of the diet but not in any rapid, simple and predictable manner.
Is oxygen input steady state?
The input of oxygen into the alveolar space, and the output from it, must, in steady-state conditions, be equal.
