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SaO2 <90% lasting for more than 30-60 seconds
Can we deliver a precise FIO2 with a venti mask?
No, this can only occur with an air-tight seal and separation of expiration and inspiration
What are some reasons that post op pt's are at risk for desaturation?
- 1. reduction in FRC
- 2. decreased clearance of secretions
- 3. increased oxygen consumption
- 4. inhibition of hypoxic pulmonary vasoconstriction
- 5. post-op hypoventilation - narcotics
what % of pt's experienced severe hypoxemia if not provided with O2 post-op?
Why might pt's shiver post-op?
the GA temporarily resets the hypothalamus temp even pt's with normal temp may shiver
How much can shivering increase O2 consumption?
What is the phenomenon that describes the closure of alveoli in relation to low volumes and 100% O2?
What are some causes of decreased mucosciliary clearance?
- gasses that are inhaled are dry
- ET Tube
- Pain can inhibit coughing
What is the normal way the body deals with areas of lowered O2 in the blood, and how does anesthesia effect this mechanism?
hypoxic pulmonary vasoconstriction, anesthesia inhibits the normal response causing V/Q mismatch
What causes hypoventilation postoperatively?
- residual anesthetics
- incompletely antagonized muscle relaxants
Does a substance like levophed affect the pulmonary vasculature?
Yes, it would inhibit the ability of the pulmonary vessels to redirect flow
What patients would be most at risk for hypoxemia?
smokers, obese, pre-existing lung disease, pre-existing heart disease, thoracic and high abdominal surgery
What age groups would have an increase risk for hypoxemia?
- the very old- increased closing capacity
- the very young i.e. premature- lack of surfactant
- and babies have a small FRC-small alveoli more prone to closure
What three divisions can we place on O2 delivery devices?
- complete control over inspiration and expiration
What are some examples of low flow devices?
nasal prongs, face tent, face masks
Why do low flow devices have a limited ability to raise the FIO2?
they depend on entrained air to make up the balance of tidal volume
What are some example of high flow O2 delivery devices?
- Venti masks
- O2 nebulizers
- Cpap devices
What do High flow devices attempt to deliver?
entire inspired volume at a controlled FIO2
What are the specific approximate FIO2's of the NC that correspond to 1liter - 6 liters?
What effect can entrained air have on the the overall % FIO2?
dilutes the total FIO2, total FIO2 will change as the minute ventilation changes
NC volumes over ____ can cause what uncomfortable feelings in the pt?
4L/min, nasal mucosal drying.
What FIO2 can be delivered with a simple face mask?
What is the difference between the non-rebreathing mask and the partial re-breathing mask?
nothing, same mask, the determining difference in FIO2 is the liter flow going into the mask
What is the FIO2 of a non-rebreathing mask?
What gas does a pt rebreath when he inspires using a partial re-breather?
dead space gas
Where is the dead space gas caught during the use of a partial rebreather mask, how many liters are typical of a partial rebreather?
in the reservoir and is inspired at the beginning of inspiration along with the additional O2, 8 liters
What is the typical flow of a non-rebreather mask?
why does the pt not breath in the dead space gas in a rebreathing mask when the flow is high?
the flow is high enough that the O2 pushes out the dead space gas
Does the dead space gas that is "rebreathed" contain CO2?
no CO2 and a higher FIO2 than ambient air
What principal do high flow devices use?
the venturi principle
What is the purpose of the valve that is used with venturi systems?
It sets the ratio of entrained gas to O2 flow to regulate FIO2
Why are venturi systems termed "high flow?"
the total flow of the device is high, not the liter flow coming from source gas
What situations would we be using a high flow O2 delivery device?
- Avoid O2 tox
- pt with COPD
- in situations that we would like to have more precise control over FIO2
What ventilation methods increase alveolar volume?
PEEP, Cpap, bipap
In a normal pt adding 10 cm/H2O increases the FRC by how much, what condition would cause a lesser increase in FRC?
500 ml, stiff lungs
Why does an increase in peep raise FRC?
cumulative effects of global distention of alveoli
as peep increases what happens to the cappillaries?
they are smashed
What is the difference between peep and Cpap?
Cpap is continuous pos pressure
What patients would cpap be used on?
someone who is spontaneously breathing, can't isolate inspiration from expiration
What pt's would we use peep on?
pt's on ventilators, can isolate expiration
What is bipap?
differing pressure on inspiration and expiration, higher level of support on inspiration
What effect do peep/cpap have on extravascular water?
Redistribution away from the tight junctions
What are the deleterious effects of peep/cpap?
- possible barrotruama
- rupture of pulm capillaries leading to pulm edema
- decrease in cardiac output- seen as a progressive decreases as peep increases - this decreases atrial filling time
What are the two beneficial effects of peep/cpap?
- increase in alveolar volume
- redistribution of extra-alveolar fluid
What pressure of peep is considered normal and to have minimal effect on cardiac filling?
what are some contraindications for peep?
- barrotrauma - seen in high levels of peep or fragile lungs
- may also cause decrease in CO in pt's
What is peep indicated for?
improvement of pt's Po2
What is auto peep?
pressure at the end of expiration due to pt's normal breathing, not by the ventilator
What causes auto peep?
hyperventilation-breathing too quickly to adequately empty the alveoli
What pt's would be more likely to auto peep?
In what situations is autopeep good?
COPD, seen as pursed lip breathing
how is a small degree of auto peep helpful?
it gives a small bump up to the FRC
What are the 4 types of respiratoy failure?
- pure hypoventilation
- restrictive lung disease
- ARDS low po2 and high resp rate
What is the relationship between the PO2 and PCO2 what we would in pure hypoventilation?
as the PO2 increases the CO2 decreases in a predictable fashion
when is ventillatory support most used?
- resuscitation following acute apnea
- anesthesia with paralysis
- intensive care
- treatment of chronic ventilatory failure
When lungs do not fill as one unit how do we want the ventilation pattern to be?
give more time for inspiration, other wise ventilation will favor the alveoli with short time constants
What are the recommendations for ventilation in adult with normal or mildly diseased lungs?
10-15 ml/kg over 1 sec at rates of 8-12 per minute
What is the advantage of large tidal volumes?
- compliance is near normal
- maintains oxygenation
- reduces incidence of atalectasis
- in pt's with spontaneous breathing it is tolerated better
What is inspiratory hold?
ventilator maneuver where expiration is delayed for a specific period after the volume has been delivered
What is happening to the alveoli during inspiratory hold?
the alveoli are opening
what are the characteristics of the alveoli at plateau pressure?
they are open and this pressure is a reflection of true alveolar pressure
What is the difference between recruitment maneuver and inspiratory hold?
the time during which the pressure is held and the frequency
What increases PIP?
- increased flow rates
- kinked tube
Does PIP have a relationship to plateau pressure?
no, predictable relationship
What pressure should plateau pressures not exceed?
A pressure that exceeds 40 cm/h20 put the pt at risk for?
Where do we want to keep the plateau pressure most of the time?
30 cm/h20 or below
What pressure is an indicator of possible trauma to the lungs?
what two means would be effective to open up the alveoli?
- increasing inspiratory hold time
- increasing peep
Is there a limit to how much we should extend inspiration?
there is a chance that we could decrease CO
What are the three modes of ventilation?
- volume control
- pressure control
- dual control
What are the two triggers for ventilation?
What is the usual trigger for anesthesia?
What is the limit in pressure vs time ventilation?
Why is pressure ventilation tidal volume dependent on compliance?
the ventilator delivers tidal volume to a certain pressure. of there is a sharp increase in resistance a the pressure may be reached while only delivering a small tidal volume
what is the difference between pressure control and volume control int terms of the changes in respiratory pressure?
- with volume control the flow continues until the set volume is reached and then the pressure drops to the plateau pressure
- with pressure control the tidal volume is set to deliver a breath at a constant pressure for a set time
Is there a peak in pressure control ventilation?
What type of ventilatory cycling would be preferred with a pt whom has altered time constants?
- pressure control, because it allows time for all the alveoli to fill, longer plateau pressure
- with volume control the alveoli that open will open early in the inspiration but when the pressure falls to plateau all of the alveoli may not have had sufficient time to open
In what mode is peak pressure variable?
In anesthesia are the pt's generally able to take their own breaths?
no, we are the controllers because we paralyze our pt's
Would we typically use pressure support in the OR?
not likely because they are usually paralyzed
what is pressure support?
the patient has to make a ventilatory effort and when the vent senses this will assist them in their inspiration
What is the typical I/E ratio?
1-1.5 to 1-3
why are one second inspirations recommended?
they are closer to physiologically normal
In what mode can you have a reverse I/E ratio?
what types of pts would you want an inverse I/E ratio?
pt's with restrictive lung disease, pt's who have a hard time filling their lungs. ARDS pt's
What mode can you set up the tidal volume to be forced in quicker to allow for more plateau pressure hold?
If a pt has low lung compliance what would you do?
in VC reduce the I/E ratio from 1:3 to 1:2 effectively lengthening out the inspiratory time relative to expiration. If compliance was still an issue, we could switch the pt to pressure control to ensure that we were opening as many alveoli as possible. If additional support is needed in pressure control we could reverse the I/E ratio.
What is the danger of using revers I/E?
build up of intrathoracic pressure
What are some potential complications of increased plateau time?
increased intrathoracic pressure-
What is an additional parameter that we will have to monitor when using pressure control?
the tidal volumes
What is the normal preset pressure on most vents?
What are some situations that would cause the pressure limit to be reached?
- coughing on the tube
- surgical retractors
What is high frequency ventillation?
provides less tidal volumes than the dead space at much higher frequencies
what are some of the risks associated with high frequency ventilation?
- risk of barrotrauma
- decreased cardiac output
- over-distention of the alveoli
What type of ventilation is auto peep common in?
high frequency ventilation
what are the manifestations of pulmonary barrotrauma?
- subQ emphysema
- brochopleural fistula
What mode is the lowest risk for barrotrauma?
pressure control ventilation
What are the parameters for discontinuance of the ventilator?
- underlying indication is reversed or sig improved
- cardiopulmonary reserve are adequate
- general clinical evaluation suggest nothing that increases ventilatory demand
What is a reason that we consider not weaning the pt from the ventilator at surgery termination?
If the pt has lost a lot of blood and we feel that DO2 might be impaired. we also might consider this if the pt is hypothermic
Why would we keep a pt on the ventilator if they are hypothermic?
we are expecting that they will shiver and need the vent until properly rewarmed
What are some common weaning parameter for weaning from the ventilator during anesthesia?
- Insp pressure <25 cm h20
- tidal volume >5ml/kg
- vital capacity >10 ml/kg
- RSBI <100
What is the RSBI?
- RR/Vt in liters
- if <100 its ok
What are aside from parameters what other considerations are there that would indicate continued need for vent support?
- hemoglobin levels
- ventiliatory demand