Module 2: Resp Ax (Alveolar Gas Exchange) Flashcards
alveolar gas exchange assessment
- main determinants = V/Q matching and diffusion
- diffusion is dependent on thickness of a-c membrane, anatomical SA, driving pressure, and diffusion coefficient
- V/Q matching is determined by ventilation and blood flow provided by cardiac output
diffusion
movement of gases from area of higher concentration to an area of lower concentration
what are ways to determine effectiveness of alveolar gas exchange?
measuring SpO2 and doing ABGs as it provides a PaO2 measurement (amount of partial pressure dissolved in the blood and available for use by tissues)
ax cues that can indicate hypoxemia (decreased PaO2 levels)
- decreased SpO2/SaO2 levels
- increased RR and depth of resps
- complaints of dyspnea or SOB
- pallor (early sign)
- cyanosis (late sign)
- tachycardia (as SNS tries to compensate)
- anxiety, agitation, restlessness
what is key in determining effectiveness of alveolar gas exchange?
measuring PaO2
what are the four main factors influencing the diffusion of gases between alveoli and capillaries?
1) thickness of a-c membrane
2) anatomical SA
3) diffusion coefficient
4) driving pressure
how thick is the alveolar-capillary network?
<0.5mm thick, composed of 5 layers
how does thickness affect diffusion of gases?
the thicker the space between alveoli and capillary beds, the more difficult it is for gases to diffuse through
factors that increase thickness of a-c membrane
- inflammation
- pulmonary edema
- exudate/secretions
- interstitial lung disease
- fibrosis
ax cues that support conclusion of increased thickness of a-c membrane
- crackles/secretions
- xray/CT findings showing pulmonary edema, pneumonia
- medical hx
what can surface area be influenced by?
- lobectomy
- pneumonectomy
- pneumothorax
- pulmonary edema
- atelectasis
- consolidation
ax cues that could indicate decreased anatomical SA
- med/surg hx
- xray/CT findings
- lung auscultation
what is diffusion coefficient?
how readily a gas diffuses across a membrane
what is diffusion coefficient influenced by?
size of gas molecule, solubility of gas in surrounding fluid, thickness of membrane across which gas is diffusing
T or F: O2 and CO2 have the same diffusion coefficients
F. they are different due to differences in molecular size, shape and solubility; CO2 diffuses 20x more rapidly than O2 allowing it to easily pass from the pulmonary capillaries to the alveoli
driving pressure
force that drives the diffusion of a gas from an area of high concentration to an area of low concentration
what is true about the relationship between driving pressure and partial pressure?
the greater the difference in partial pressure, the greater the driving pressure and the faster the gas will diffuse
how can driving pressure be influenced?
- can increase driving pressure by giving supplemental O2
- hypoventilation decreases amount of C02 in alveoli, decreasing driving pressure from alveoli into capillary. it will also increase CO2 in alveoli, decreasing driving pressure from capillary into alveoli
- thickness of a-c membrane will influence amount of driving pressure required for O2 to diffuse across it
ax cues for driving pressure
- supplemental O2 requirements
- SpO2/SaO2/PaO2
V/Q Matching
process by which the amount of air (ventilation) that reaches the alveoli is matched with the amount of blood (perfusion) that is supplied to capillaries surrounding alveoli
when does a V/Q mismatch occur?
when there is an imbalance between the amount of ventilation and perfusion to a particular region of the lung; leads to decreased O2 of blood and increased CO2 levels
perfusion (Q)
flow of blood through blood vessels in the lung. in context of VQ matching, it is the delivery of blood to capillaries surrounding alveoli
when can impaired perfusion occur?
when there is decreased CO, impaired blood flow in the lung, and blockages in blood vessels
ie) PE, decreased CO/shock states, damage to pulmonary capillaries, microthrombi, hypoxic vasoconstriction in the pulmonary circulation
V/Q Match: Shunt
alveoli are not ventilated but are fully perfused = gas exchange can’t occur
V/Q Mismatch: Shunt
shunt will occur in patients where patho processes result in alveoli being completely non-ventilated (ie. lobar pneumonia or atelectasis). degree of hypoxemia caused by a shunt depends on what portion of the lungs is involved
V/Q Match: Shunt-Like
V/Q Mismatch: shunt-like alveoli are partially ventilated and fully perfused. Gas exchange will occur but will be less than normal. Pts with overall decreased ventilation or ventilation decreased to specific alveoli will experience shunt-like V/Q mismatches. Because some gas exchange occurs, the degree of hypoxemia will be less than severe in a shunt
V/Q Match: Normal
alveoli are ventilated and perfused. optimal ventilation and perfusion matching is present
V/Q Match: dead space-like
Mismatch: alveoli are fully ventilated but poorly perfused. Gas exchange will occur but be less than normal. In a dead space-like unit, O2 is available via the fully ventilated alveoli, but low blood flow in capillary beds results in diminished gas exchange, and some degree of hypoxemia. Dead space-like units will occur in patients with low CO.
V/Q Match: Dead Space
Mismatch: alveoli are ventilated but perfusion is absent. Gas exchange cannot occur. Dead space will occur if something completely disrupts blood flow to a portion of the pulmonary capillary network. Degree of hypoxemia that results depends on proportion of blood flow that is interrupted
how can you promote V/Q matching?
by positioning patient. ie) when pt is upright, gravity pulls blood and fluid toward bottom of the lungs, improving perfusion to lower lung regions and eliminating trapped air
how would you position someone with L lung consolidation to promote V/Q matching?
position with good lung (R lung) down to redistribute blood flow to alveoli with greatest O2 content
