What is the ideal V/Q ratio?
True or False: With a V/Q of 1, blood gases equilibrate with alveolar gases
When is V/Q equal to infinity and what does this mean for gas exchange?
V/Q is equal to infinity when ventilation is normal but perfusion is occluded (denominator approaches 0). In this situation, the alveolar gas equilibrates with inspired, humidified air and there is no oxygenation of blood.
When is V/Q equal to zero and what does this mean for gas exchange?
V/Q is equal to zero when the ventilation is zero but perfusion is normal. In this situation, alveolar gas equilibrates with blood gases but there is no oxygenation of blood.
Generally, what is a normal V/Q level? What is V/Q in the upper lobes? Lower lobes?
Normal V/Q ratio = 0.8 Upper lobes V/Q ratio = 2.5 (well ventilated but relatively under perfused) Lower lobes V/Q ratio = 0.6 (perfused but relatively under ventilated)
What happens with local regulation of V/Q when the V/Q ratio is too high?
In situations with high V/Q ratio, the ventilation of CO2 is happening faster than the blood (perfusion) can bring the CO2. This results in a drop in alveolar PCO2. The drop in alveolar PCO2 causes bronchoconstriction to increase local airway resistance. This decreases ventilation and lowers the V/Q ratio.
What happens with local regulation of V/Q when the V/Q ratio is too low?
In situations where the V/Q ratio is too low, alveolar PO2 drops because the blood is carrying oxygen away faster than the ventilation can bring oxygen into the alveoli. The drop in PO2 in the alveoli causes hypoxic vasoconstriction which decreases perfusion and increases V/Q.
What is normal PaCO2 at sea-level vs. denver?
Sea-level = 40 mmHg
Denver = 35 mmHg
What is normal O2 saturation at sea-level vs. Denver?
Sea-level = 97.5%
Denver = 95%
What is normal CaO2 at sea-level vs. denver?
Sea-level = 20.7 ml O2/100 ml blood
Denver = 19 ml O2/100 ml blood
What is normal CaCO2 at sea-level vs. Denver?
Sea-level = 44 ml CO2/100 ml blood
Denver = 42 ml CO2/100 ml blood
What is anatomic dead space?
This is the part of the respiratory system that doesn't participate in gas exchange (the first 16 branch points of the airway tree/trachea, bronchi, bronchioles). This makes up about a third of your whole airway.
What is alveolar dead space?
These are alveoli that aren't being perfused.
How do you calculate physiological dead space?
Physiological dead space is the total dead space. To calculate this, you just add together anatomical dead space and alveolar dead space
Why is dead space bad?
Dead space is bad because it is ventilation that isn't being used. Your body is working to cause ventilation but without any benefit.
True or False: Dead space is a common contributor to hypoxemia
False. Dead space doesn't generally cause hypoxemia unless very severe.
Does dead space increase or decrease with exercise?
Dead space decreases with exercise because exercise increases perfusion (Q).
PaCO2 is regulated by what 3 things?
What 5 things can increase dead space?
1. rapid shallow breathing
2. pulmonary embolus
3. decreased cardiac output
4. mechanical ventilation (e.g. snorkeling)
How can you differentiate between low V/Q and shunt in patients?
Low V/Q responds to increased FiO2 but shunts do not.
What are some common things that cause shunts? (name 3)
1. Transudate from heart failure
2. Exudate from pneumonia
3. Exudate from ARDS
Name 3 anatomic issues that can cause V/Q mismatch
1. congenital heart disease
2. pulmonary fistula
3. vascular lung tumor
What is the most commonly used calculation to estimate V/Q mismatch?
Blood gas is typically presented in this format. _ /_ /_ /_ /_ on _.
What does each space stand for?
pH/PaCO2/PaO2/SaO2/HCO3- on how much O2
In a normal patient, A-a gradient is typically due to ____ and ____
V/Q mismatch, shunt
How does pulse oximetry (SpO2) work?
Pulse oximeters measure the ratio of deoxy-Hb and oxy-Hb to give an estimate of % oxy-Hb as SpO2.
It is able to do this because deoxy-Hb absorbs maximally in visible red band and oxy-Hb absorbs maximally in infrared band.
What is the main issue with pulse oximetry? (1 main issue and 4 minor)
Pulse oximeters only measure Hb saturation but not what they are bound to. So, if the Hb is bound to CO or methylene blue, the pulse oximeter would still give a normal value. Thus, it's important to understand that SpO2 is different from PaO2 or SaO2. Clinical examples would be someone who was in a house fire may have CO poisoning and low O2 saturation but their pulse ox reading would falsely show that they are normal. Or, patients could be on topical anesthetics which would cause methylene blue to bind to Hg but the pulse oximeter would falsely show a normal reading.
Also, pulse oximeters may have falsely high or low measurements due to lighting, nail polish, movement, or temperature.
What is met-hemoglobin and how is it caused?
Met-hemoglobin is hemoglobin that is bound to methylene blue. Some anesthetics can cause this so hemoglobin is bound to methylene blue which causes a decrease in SaO2 but the SpO2 (from pulse ox) still reads normal. In clinic, you might increase FiO2 when the patient has low SpO2 but with Met-hemoglobin, the FiO2 increase won't help