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