Control of Carbon Dioxide and Oxygen (B2: W7) Flashcards
Which sensors in the blood control CO2 levels?
-
Central chemoreceptor in the medulla
- An increase in PaCO2 flowing to the medulla causes an increase in ventilation
- Peripheral chemoreceptor in carotid and aortic bodies
- Drives breathing when PaO2 falls below 60 mmHg
- Has little effect on the breathing of a normal person resting at sea level
What equation relates arterial CO2 to ventilation?
PaCO2 = (VCO2 • 0.86) / VA
VCO2 = Metabolic production of CO2
VA = Alveolar ventilation
- Buildup of CO2 in the blood is due to failure of some component of the respiratory system
- Not an increase in metabolic CO2 production

What is the Henderson Hasselbalch acid-base equation for relating arterial CO2 to pH?
pH = 6.1 + log [HCO3-] / (0.03)(PaCO2)
- A buildup of CO2 in the blood causes the pH to fall

What is the relationship between alveolar CO2 and arterial CO2?
PACO2 = PaCO2
- Both controlled by rate of alveolar ventilation (breathing) and rate of CO2 production (metabolism)

How does an increase in CO2 influence breathing?
Increases breathing
- Breathing maintains the PaCO2
- Blow off CO2 at a higher rate
- Higher rate of alveolar ventilation (VA)

What is the arterial CO2 in the event of hypercapnia, and what is the state ventilation?
Hypercapnia: PaCO2 > 45 mm Hg
- Caused by hypoventilation
What is the arterial CO2 in the event of eucapnia, and what is the state of ventilation?
Eucapnia: PaCO2 = 35-45 mm Hg
- Seen during normal ventilation
What is the arterial CO2 in the event of hypocapnia, and what is the state of ventilation?
Hypocapnia: PaCO2 < 35 mm Hg
- Seen in hyperventilation
How do the patient’s respiratory rate, depth of breathing, or breathing effort influence hyer- and hypoventilation?
These terms are unrelated to respiratory rate, depth, effort
- All dependent on PaCO2

How is a patient’s state of alveolar ventilation measured?
Can only be measured by arterial blood gas
- Measure PaCO2
What is hypercapnia?
Elevated PaCO2
- Failure of some component of the respiratory system
- The only physiologic reason for elevated PaCO2 is a level of alveolar ventilation inadequate for the amount of CO2 produced and deliverd to the lungs
- Sign of advanced organ system impairment
Why is hypercapnia potentially dangerous?
- As the PaCO2 increses, pH falls (unless compensated)
- The higher the PaCO2, the less defended the patient is against any further decline in alveolar ventilation (VA)
- As PaCO2 increases, PAO2 and PaO2 fall, unless inspired O2 is supplemented
How can the rate of alveolar ventilation be defined?
VA refers only to ventilation rate of ALIVE volume of lung
- Dead space does not count
- VA = VE - VD = total - dead

How is the total ventilation rate (VE) calculated?
VE (L/min) = respiratory rate • tidal volume

What is dead space ventilation (VD)?
Space that is ventilated but not perfused
- Gas entering and leaving
- No blood flow
- No gas exchange because it is not perfused
- Ex: trachea

What components are required for gas exchange in the lungs?
- Gas entering and leaving
- Blood flow = perfusion
- Diffusion of gase across capillary membrane

What is the difference between physiological and anatomical dead space?
- Anatomic dead space
- All the airways that are ventilated but not perfused
- Can never take part in gas exchange because of normal anatomy
- ~150 mL
- Physiologic dead space
- Anatomic + all other dead space
- Dead alveoli receive air but do not exchange gas
- No blood flow and no perfusion to these alveoli

When will a patient become hypercapnic?
- Inadequate total ventilation (VE)
- Anything that limits the rate or depth of breathing
- Massive obesity
- Respiratory muscle weakness
- Severe pulmonary fibrosis
- Central nervous system depression
- Anything that limits the rate or depth of breathing
- Increase in VD - alveoli lose perfusion
- Both of the above

What is the requirement for minute ventilation?
- The level of total ventilation (VE) needed to keep PaCO2 constant
- Tries to maintain CO2 homeostasis in the blood
- Body does this normally
- When dead space (VD) increases, minute ventilation requirement increases
- A rise in VE is needed to keep PaCO2 constant

What is the normal partial pressure for CO2 in the alveoli?
PACO2 = 40 mm Hg
PACO2 = PaCO2
Therefore,
PaCO2 = 40 mmHg

What is the normal partial pressure of CO2 in the veins?
PvCO2 = 45 mm Hg
- CO2 picked up by capillaries from tissues and delivered to veins

If PACO2 is equal to PaCO2, is the same true for O2?
PAO2 ≠ PaO2
- In a normal lung, these are not the same

What is the percent of O2 in the atmosphere, and how does that affect PO2?
- Atmosphere on earth contains 21% O2
- Inspired air is 21% O2
- PB = 760 mm Hg
- 0.21 • 760 mm Hg = 159 mm Hg = PO2 at sea level

What is the difference between PO2, inspired air (PIO2), and terminal respiratory units (PAO2)?
- Atmospheric air: PO2 = 159 mm Hg
- Inspired air with H2O: PIO2 = 150 mm Hg
- Terminal respiratory units: PAO2 = 100 mm Hg

What is the definition of a terminal respiratory unit?
- Physician’s working “alveolus” (not a single alveolus)
- Region in which gas phase diffusion is so rapid that the PAO2 and PACO2 are uniform throughout the unit
- All alveolar ducts and their alveoli from proximal (first) respiratory bronchiole
What is an acinus?
10-12 respiratory units
- Anatomic unit used by pathologists
Why is it good to know the PAO2 if we already know the PaO2 from an ABG?
- At the level of the lung, terminal respiratory unit, or patient, PAO2 ≠ PaO2
- PAO2 calculated from the alveolar gas equation is not the same as measured PaO2 from an ABG, even in a healthy patient with no gas exchange problems
- Prediction of PAO2 is important to assess if the measured PaO2 is within its normal limits for the patient
How is the PAO2 calculated from the PaCO2?
PAO2 = PIO2 - (PaCO2 • 1.2)

PIO2 = inspired O2
How is inspired partial pressure of O2 (PIO2) calculated to be used in the alveolar gas equation?
- Fraction of O2 in the air is always 0.21 at all altitudes
- PB = barometric pressure = 760 mm Hg at sea level
- Water vapor pressure in the trachea = 47 mm Hg
PIO2 = 0.21 • (760 - 47 mm Hg) = 150 mm Hg at sea level
What is the PAO2 in a healthy lung with no gas exchange problems?
PAO2 = 100 mm Hg
- Alveolar gas equation
PAO2 = PIO2 - 1.2(PaCO2)
PAO2 = 150 mmHg - 1.2(40 mmHg) ≈ 100 mm Hg
What is PAO2 dependent upon?
Depends on environment
- Depends on PIO2
- Barometric pressure
- Oxygen percent in air
- This value is close to what you would expect a normal patient to have with no gas exchange defect
- This value is what you expect if the lungs are properly transferring O2 into the blood
What is a normal P(A - a)O2 range?
Between 5 and 20 mm Hg
- PaO2 will always be less than PAO2
- Unless there is a gas exchange problem
How is normal PaO2 for a patient calculated based on age?
Normal PaO2 = 100 - (0.4 x age)
- One definition of hypoxemia: partial pressure of oxygen in the blood is less than normal for the patient’s age
- Normal PaO2 decreases with age
How does age affect P(A - a)O2?
Normal P(A - a)O2 difference = [Patient age / 4] + 4
- A high P(A - a)O2 indicates hypoxemia
What are the two definitions of hypoxemia?
- Partial pressure of oxygen in the blood is less than normal for the subject’s age
- Oxygen content in the blood is less than normal for the subject’s age
What is hypoxia?
Decreased oxygen supply to organs and tissues
- Oxygen delivery to cells is insufficient for the demand
- Does not require that the PO2 in the blood is low
- But hypoxia can be caused by hypoxemia (hypoxic hypoxemia)
What are some causes of hypoxia?
- Respiratory causes - leads to hypoxemia
- Chronic obstructive pulmonary disease
- Coking, asphyxiation, smoke inhalation, suffocation, carbon monoxide poisoning
- Circulatory causes
- Content of the blood
- Cyanize toxicity
- Accumulation of abnormal hemoglobin in the blood
- Anemia
- Poor blood circulation
- Very low blood pressure
- Ischemic heart disease
- Heart failure
- Shock
- Content of the blood
What are the three stages of diffusion of gases?
- In the aireways
- Across the alveolar-capillary membrane and RBC membrane
- RBC interior to Hb
What equation is used for categorizing all the things that affect diffusion rate across a barrier?
Fick’s law
Gass diffusion rate = Area/Thickness • Solubility/√mol wt • (P1 - P2)
- Property of barrier: area/thickness
- Rate is slower with thick barrier
- Property of gas: solubility/√mol wt
- Gas has to be soluble in a barrier before going through
- Gases with higher masses take longer
- Driving force: P1 - P2
- Partial pressure gradient
When using Fick’s law, most of the variables in a particular situation are fixed. Which is not?
Partial pressure gradient (P1 - P2) changes with time
- Initially as a gas reaches the alveolus the pressure difference will be the greatest
- With time, the pressures start to equalize and the net gas diffusion will eventually goe to zero
- P1 = P2
Using Fick’s law; does CO2 or O2 have a faster barrier diffusion rate?
CO2 is 20 times more soluble than O2

How quick is the RBC capillary transit time for O2?
About 0.75 seconds

What are capillary transit time and capillary reserve time?
Capillary transit time: The time during which an RBC is transiting a capillary - surrounding an alveolus
- Typically 0.75 sec
- Is shorter as the blood moves faster, e.g. exercise
- RBC loads all the O2 possible during that time
- Process of O2 loading is about 0.25 sec
- The rest is the capillary reserve time
- Needs this reserve time during exercise
- Abnormal transit time
- Gas exchange problem - exaggerated with exercise
- Potentially leading to hypoxemia

What is the limiting factor for net O2 transfer?
Net gas exchange is perfusion limited
- Diffusion is over in plenty of time
- If perusion was slower, then it would even have more PAO2 to be reached
- Diffusion is not rate-limiting
- Normal conditions: O2 diffusion from alveolar air into pulmonary capillary blood is perfusion limited
- Abnormal conditions: if the rate of perrusion starts getting faster (exercise) and at the same time diffusion is slower
- Possible that diffusion becomes rate limiting

When is the NET O2 transfer rate diffusion-limited versus perfusion-limited?
- Under normal conditions: Net O2 transfer rate from alveolar air into pulmonary capillary blood is perfusion-limited
-
Diffusion-limited net O2 transfer can occur
- Pathological conditions: emphysema, pulmonary fibrosis, edema
- Sever exercise - capillary transit time is shortened sufficiently
- Pressure differences across membrane are lower (high altitude)
- Combination of all
Why is net O2 transfer so important?
Anything that decreases net O2 transfer will decrease PaO2
- Hypoxemia
What is the capillary transit time for CO2?
0.75 seconds
- Gets shorter as blood moves faster (e.g. exercise)
- During the 0.75 sec, the RBC (which has picked up CO2 from the tissueshas PvCO2 = 45 mm Hg
- Objective is to unload all the CO2 possible during the time transiting the capillary
- P2 - P1 = 45 - 40 = 5 mmHg
- Unloading takes about 0.45 sec

Will PACO2 (and thus PaCO2) increase with exercise?
PaCO2 will not increase with exercise
- Ventilation rate will increase to maintain homeostasis and keep PaCO2 normal
- If there is a gas exchange problem
- It will take quite a while to get the full unloading of CO2
- In exercise there may not be full unloading - potentially leading to hypercapnia

Is CO2 net gas transfer diffusion-limited or perfusion limited under normal conditions?
Like O2, it is perfusion limited