Respiratory Physiology and Mechanics Flashcards
Define physiologic dead space
anatomic dead space - conducting airways not involved in gas exchange
alveolar dead space - airways not involved in gas exchange with vasculature
Lung Measurements
What happens in neonate with RDS to…
- total lung capacity
- vital capacity
- residual volume
- inspiratory capacity
- tidal volume
- FRC
- ERV
Is dead space greater or lower in neonate with RDS?
Lung capacities and volume - all lower
Dead space - greater
note: bony chest wall limits exhalation to a min volume (FRC)
Compliance
Elastance
Changes in compliance and impact on P-V curve
Resistance
What determines change in pressure in a laminar flow system
Laminar flow
**
Laminar - smaller airways, ETT is laminar flow
Turbulent flow
Turbulent - large conducting airways
Understanding gas flow and why heliox works in upper airway obstruction
Helium has similar (slightly higher actually) viscosity as air, but about 7-fold LOWER density. Hence, Heliox(He + O2) is a common treatment for diseases with high large airway resistance
Breakdown of total respiratory system resistance
Where is majority of airway resistance?
What does resistance do during inspiration? exhalation?
nasal resistance; remainder in 1st few branches of bronchi
Inspiration - bronchi dilate - decrease resistance
Exhalation - less tethering - increase resistance
If length of ETT is doubled, what happens to ETT resistance?
If internal diameter of ETT increased from 2 mm to 4 mm, how is resistance impacted?
Length - doubles
Diameter smaller - 16 times less (radius^4)
Define time constant
Time required for a lung compartment to fill or empty by ~63% following a step-change in pressure
Increased time constant - risk of incomplete emptying of previously inspired breath / gas trapping
1 time constant healthy term ~ 0.15 sec SO 95% of last tidal volume emptied from lung in about 0.45 seconds (3 time constants).
can help determine your iT
Time constant in RDS
Time constant in BPD
RDS - low resistance (chest wall very compliant, lungs more compliant), low compliance - short time constant - can use faster RR
BPD - high resistance, high compliance - long time constant - need slower RR
Minute ventilation
what is normal MV in infant?
250-350 mL/kg/min
Alveolar ventilation
similar MV but corrects for phyiologic dead/disease space (and lack of gas exchange in those areas)
Dynamic Volume-Pressure Curve
work of breathing = P X V
Describe hysteresis
Energy applied during inspiration not returned during exhalation.
Hysteresis in the lungs is related to alveolar air-liquid surface forces and the opening and closing of alveoli [19, 21, 22]. Changes in resistance to air flow will affect the hysteresis, with the curve appearing wider with increasing resistance
How to evaluate compliance on a dynamic P-V curve
Dynamic lung volume-pressure curves in different disease states (low FRC, high FRC)
low FRC eg RDS (stiff lungs, lead to atelectasis, lung collapse), greater change in P required for a given change in volume
high FRC eg MAS, BPD, excessive vent pressures (trap air) - decreased compliance BUT this loop exists at higher lung volumes (trap air)
Dynamic lung volume-pressure curve in RDS
Pressure volume curve in newborn vs adult
- look at static and dynamic curves
static - chest wall
dynamic - lung
Describe this P-V curve state
atelectasis. can increase PEEP (but also keep in mind your delta p, may need to also increase PIP)
Describe this P-V state
beaking, air trapping, over-distention
Flow volume loops
During inspiration, the flow ascends rapidly (phase I) followed by a slower phase (phase II) until the end of inspiration. After opening the expiratory valve, the (negative) expiratory flow drops rapidly (phase III), followed by a slower drop until the end of expiration (phase IV). The 4 phases follow each other in a clockwise direction, ending the curve at the point where it started.
Flow–volume loops are particularly important in the assessment of excessive airway resistance and in alerting for the presence of copious airway secretions or circuit leaks. The shape of the curve varies with various conditions that alter the air flow, making it an important curve in situations of obstruction, that is, of increased resistance.
Give examples of this flow-volume loop type
chronic lung disease (prominent deficit in exhalation)
Give examples of this flow-volume loop type
RDS, MAS, low tidal volume, rapid decrease in inspiratory flow rate, high peak expiratory flow rate
Give examples of this flow-volume loop type
vocal cord paralysis, laryngomalacia
note: flattened inspiratory phase, obstructed inspiration, normal exhalation
Give examples of this flow-volume loop type
tracheomalacia, vascular ring
note: normal inspiration, obstructed exhalation leads to air trapping
Give examples of this flow-volume loop type
tracheal stenosis
note: obstruction of inspiration and exhalation (both flattened)
Total CO2 content equation
What is determined by alveolar PO2 and alveolar-capillary interface?
PaO2
What happens to PaO2 in..
- severe anemia
- V/Q mismatch
- hypoventilation
- CO poisoning, methemoglobinemia
- R to L cardiac shunt
- no change
- low
- low
- no change
- low
NOTE: PaO2 independent of available hemoglobin
Define O2 sat , SpO2
oxygen bound to hemoglobin (heme sites “saturated” with oxygen)
percentage of all AVAILABLE heme binding sites in arterial blood saturated with O2.
Does O2 sat depend on hemoglobin? Does it depend on PaO2?
Not depend on hemoglobin
Yes depends on PaO2 (if more dissolved oxygen, then would lead to more available heme sites bound with oxygen).
NOTE: PaO2 and O2 hemoglobin dissociation curve determine SpO2.
Oxygen content
- define
- equation
how much oxygen is in the blood (mL O2/dL)
Oxygen delivery definition
Use definition to think about what factors would infuence decreased oxyg
If there is low CO, a low [Hb], low O2 sat, THEN O2 delivery will be inadequate.
note: CO affects how O2 binds hemoglobin so CO can decrease O2 content but can give FP on pulse ox.
note: with higher altitude, have decrease PaO2 (partial pressure of oxygen is lower) so affects O2 binding and O2 content as a result.
REMEMBER: PaO2 affects SpO2 (if you have less dissolved O2, thn less O2 available to bind Hb)
What does methemoglobin due to oxygen dissociation curve?
Causes a LEFT shift
oxygen dissociation curve consequently gets shifted to the left because, interestingly, the oxygen binding affinity of normal hemoglobin is increased in the presence of an abnormal increase of methemoglobin, resulting in a decreased delivery of oxygen to the tissues [12,13].
a full term infant has PaO2 = 75 mm Hg, O2 sat 99% and Hb 14 g/dL. Hb down to 7 due to A/O incompatibility. Assuming no lung disease, how will her PaO2, O2 sat and O2 content be altered?
PaO2, O2 sat unchanged. O2 content decreased.
which infant is more hypoxemic.
A: PaO2 85 mmHg, O2 sat 95%, Hb 7 g/dL
B: PaO2 55 mmHg, O2 sat 85%, Hb 15 g/dL
A because lower O2 content (due to lower Hb)
what shifts hemoglobin dissociation curve…
- to the left
- to the right
2,3 DPG - phosphate compound in blood. It stabilizes T formation and decrease T-state conformation and decreases hemoglobin affinity for oxygen.
Bohr effect
Hb lower affinity for oxygen secondary to increase in partial pressure of CO2 and/or decrease in blood pH. It ENHANCES unloading of oxygen into tissues to meet oxygen demand.
Shifts oxyhemoglobin dissociation curve to the R.
SUMMARY: Acidosis -> helps blood be better at delivery oxygen at tissue level.
Haldane Effect
increased capacity of blood to carry CO2 under conditions of decreased hemoglobin oxygen saturation.
What happens to pH and CO2 binding because of oxygen.
More CO2 bind hemoglobin at lower oxygen saturations because deoxygenated hemoglobin has a higher affinity for CO2 - to facilitate REMOVAL of CO2 from the tissues.
SUMMARY: Facilitates CO2 removal from tissues.
What is Fick’s Law and its clinical correlation
partial pressure of oxygen along the pulmonary capillary is alinear and limited by PERFUSION.
partial pressure of CO2 also alinear but equilibrates over a VERY short distance (20x more soluble than oxygen).
partial pressure of CO is LINEAR and depends on time in the capillary - diffusion limited
Fick’s law and diffusion of gas across the alveolar-capillary membrane - depends on thickness of membrane and partial pressure.
What perfusion zone of the lung does neonatal lung operate?
III
Infant with a PDA and subsequent pulmonary edema, what zone does lung change to?
Infant with MSAF - what zone does lung change to?
IV (because increased lung fluid), increases vascular resistance SO Pa > Pv
I or II because PA > Pa
Name causes of HYPOXEMIA
V/Q mismatch
Hypoventilation
Diffusion abnormality
R to L cardiac shunt
Decrease in partial pressure of oxygen eg high altitude
What happens to PaO2 and PaCO2 in areas of:
V/Q < 1 (no ventilation, perfusion OK)
V/Q > 1 (dead space, ventilate, poor perfusion)
Clinical reality of high V/Q regions - what happens to PaO2 and pCO2
Describe effect of altitude on PaO2
Infant requires 90% FiO2 in Denver (Pb 687).
What FiO2 will be needed in Seattle (sea level, Pb 760)
81% oxygen
Review the differences in modes of conventional mechanical ventilation
How does pressure time curve in mech ventilation change with changes in..
- increase PIP
- increases PEEP
- increased iT
- increased rate
Factors that increase MAP (in order of impact)
MAP equation
increase PEEP, PIP, rate, iT
HFJV vs HFOV
OI equation
When to consider ECMO?
> 10 - mod resp distress
15 - severe resp distress
25 - severe hypoxemic resp failure
40 - fulminant, ECMO
A-a gradient
- what does this reflect
Are the lungs transferring oxygen properly from the atmosphere to the pulmonary circulation?
The larger the gradient, the worse the transfer. A-a gradient should increase with increase levels of inspired oxygen. A-a gradient about 10 – 15.
Impact of disease states on A-a gradient
- In RA, A-a increases with increasing V/Q mismatch.
- If A-a > 600 with FiO2 = 1 for 8 – 12 hours -> consider ECMO.
A-a gradient equation
C
