1. Calculate and use the alveolar-arterial oxygen gradient.
alveolar PO2 (PAO2) and arterial O2 is clinical relevant to characterize diffusion across the lungs and is normally between 8-16 mmHG in young adults
2. Explain the 5 major mechanisms of hypoxemia reduce the arterial PaO2 tension. Include how these conditions occur.
1. decreased inspired oxygen tension (altitude reduces O2 partial pressure) 2. alveolar hypoventilation (a balance between CO2 production and washout) 3. diffusion impairment (usually apparent with exercise- caused by increased membrane thickness, decrease diffusion area or decreased blood transit time) 4. ventilation-perfusion mismatch 5. anatomic shunt (venous deoxygenated blood reaches the arterial circulation )
3. Illustrate in detail how ventilation-perfusion mismatches occur in health and disease.
the amount of oxygen carried in the blood is controlled by the amount of oxygen entering the alveoli (ventilation) and the amount of blood that can carry oxygen away from the alveoli (perfusion) disease causing low Va/Q ratios in regional areas of the lung will cause a decrease in the arterial PO2 if it is present in areas with higher blood flow
4. Explain the difference between shunt and dead space.
dead space is a ventilated area that does not get perfused; a shunt is perfused alveoli that are not ventilated (anatomical shunt)
4. Explain the difference between minute ventilation and alveolar ventilation.
minute ventilation is the rate of respiration times the tidal volume alveolar ventilation is the minute ventilation minus the dead space ventilation
5. Explain the concepts of alveolar ventilation and physiologic dead space and their importance in determining the arterial PCO2 tension.
physiologic dead space is a calculation of the anatomic dead space plus all the effects of Va/Q imbalances to estimate how much dead space is present to produce the observed lowering in PCO2
6. Interpret clinical and arterial blood gas information from a patient to determine which of the 5 major mechanisms of hypoxemia, singly or in combination, are responsible for causing hypoxemia in a given clinical situation.
1. VA: judged by PaCO2, higher than 42mmHg = hypoventilation and lower than 37 denotes hyperventilation 2. Alveolar-arterial O2 gradient 3. PaO2 and SaO2 (35% life threatening,75% cyanosis, serious; 90% shoulder of hemoglobin curve, 95% normal for older, 97% normal for young person. 4. Acid base status
7. Describe how to differentiate between the different mechanisms of hypoxemia.
mechanisms that respond to O2: decreased PIO2, alveolar hypoventilation, diffusion abnormatility and V/Q mismatch
PaO2 is not easily increased by increased FIO2 in teh case of true, anatomic shunt (ie. ARDS, lobar pneumonia, alveoalr pulmonary edema, lobar collapse, intrapulmonary AV fistula or shunts, extra pulmonary shunts
8. Explain how the alveolar-arterial oxygen gradient and the response to breathing 100% oxygen differ with different mechanisms of hypoxemia.
with shunt, alveoli is not perfused and it will not see an increase in the supplemental oxygen
9. Explain the principles of pulse oximetry and its application in different clinical situations.
pulse-ox measure oxygen saturation of hemoglobin in arterial blood
with each pulse of aterial blood, ther is a resulting increase in light absorption
light source emits two wavelengths 660 and 940, characteristic of oxyhemoglobin and reduced hemoglobin are very different at these wavelegths and therefore can be measured and compared in a relative sense.
carboxyhemoglobin (overestimate) and methoemoglobin (underestimate) can cause counfounding results (also dyes, anemia, low perfusion, increased venous pulse and external light sources)
1. What is the alveolar gas equation?
Used to calculate the partial pressure of oxygen (pO2) in the pulmonary alveoli is also required to calculate both the alveolar-arterial gradient of oxygen and the amount of right-to-left cardiac shunt, which are both clinically useful quantities. However it is not practical to take a sample of gas from the alveoli in order to directly measure the partial pressure of oxygen. The alveolar gas equation allows the calculation of the alveolar partial pressure of oxygen from data that is practically measurable.
Describe the effect of gravity on ventilation and perfusion in the normal lung.
alveoli at the top of the lung are under high distending pressure, operate on a flatter pressure-volume curve compared to the bottom region, even though ventilation is considered hither at the base of the lungs (greatest change during inspiration) hydrostatic pressure is greatest in the lower lung, blood flow is greatest in the base as compared to the apex Va/Q ratio is highest at the top of the lung
Why cant units with a high Va/Q compensate for the decrease in arterial PO2?
these unit operate on a relatively flat portion of the hemoglobin-O2 dissociation and therefore, the blood leaving these units have a relatively small increase in O2 content
What is the most frequent cause of reduced arterial PO2 in patients?
Va/Q inequalities or mismatches
Describe how PCO2 is effected differently than PO2 in Va/Q mismatch.
less CO2 exhaled from dysfunctional alveoli can help to offset high pCO2 from functioning alveoli increasing PCO2 stimulates central chemoreceptors, resulting in increased ventilation which significantly raises the CO2 excretion
Name several examples of conditions that would cause uneven ventilation or perfusion in the lung.
uneven ventilation: changes in elasticity, partial obstruction, dynamic compression, rigid lung/chest wall uneven perfusion: embolization, partial or complete occlusion, compression, resection (surgical or disease damage)
In most cases PaO2> _____ while on FiO of 100% rules out the presence of an abnormal shunt.
How is partial pressure of CO2, alveolar ventilation (VA) and CO2 production (VCO2) related?
PaCO2 is directly proportional to VCO2 and inversely proportional to alveolar ventilation (VA)
Pure diffussion abnormaility is occurs with the combined effects of these three conditions alveolar membrane is thickened, _____ and ___ _____ ____
exercise and fixed capillary bed
The most common ABG pattern in patients with mild or moderate lung disease is ____ and _____; hypercapnea is more likely in _____ lung disease.
hypoxemia and hypocapnea
severe lung disease
What is the source of Alveolar-arterial oxygen gradient in healthy lungs?
50% due to V/Q mismatch
50% duet o small true shunt (thebesian veins or bronchial veins)
Which mechanisms of hypoxemia preserve the A-a gradient.
decreased PIO2 and alveolar hypventilation
Shunt equation measures ____ shunt when FIO2=1.0 and _____ shunt when FIO >1.0.
True or false: Breaths that are shallow and rapid as compared to those that are deep and slow of the same volume will result in increased alveolar ventilation
False, dead space reduces actual alveolar ventilation with additional respiration frequency
PaCO2 increases when Ve (minute ventilation) decreases or Vd/Vt (dead space to tidal volume ratio) increases