General Anesthesia Maintenance Flashcards
what are the objectives of Anesthesia
AMNESIA,
ANALGESIA
, MUSCLE RELAXATION,
control of SNS during noxious stimulation
what equation is used for maintaining oxygen requirement?
Bordy equation
VO2 = 10 x kg0.75 mL O2/min
what are normal values of ETCO2
35 mmHg
PaCO2 (40) - PA-aCO2 (5)
convert ETCO2 to a %
ETCO2 (mmHg)/(PB - PH2O) = %
how to calculate eucapnic ventilation (example 70 Kg pt)
VaCO2 = VCO2/ % in lungs
VaCO2 + anatomic deadspace = eucapnic ventilation
what is formula for ventilation of CO2
VCO2 = 8 x kg.075
How do you calculate anatomical dead space?
2 ml x kg x 10
what is the alveolar gas equation?
PAO2 = [(FiO2)(PB-PH20)] - (PaCO2/R)
what is Alveolar-arterial partial pressure gradient (AaO2) formula?
also expressed as (PA-aO2)
AaO2 = PAO2 - PaO2
(normal < 10 mmHg)
what causes Abnormal alveolar arterial O2 gradient?
- Anatomic shunting: R → L (thebesian, bronchiole, etc), Pulmonary AVM, etc
- Diffusion impairment: thickended capillary membrane, Low V/Q areas (intrapulmonary shunt), High V/Q areas (alveolar dead space)
which respiratory limb is peep valve placed on ?
expiratory
what are benefits of PEEP? disadvantages?
a) Increase FRC (recruitment)
Improved lung compliance
VQ corrections
b) barotrauma
what are drawbacks of inhalation agents?
PONV d/t ethers
no analgesia (N2O exception)
what is value of MACBAR
BAR = blocks anatomic receptors
1.5
what is MACawake value
(10% of MAC)
0.3 - 0.5
What is value of MACamnestic
0.25
what are 3 major factors that effect MAC?
extremes in age (increases and decreases MAC)
acute intoxication (decreases MAC)
pregnancy (decreases MAC)
which way do anesthetic gases move in regards to pressure
down the pressure gradient
what does Rise of FA/Fi depend on ?
solubility of agent
Agents Rise FA/Fi
From greater to lesser ratio (most insoluble to most soluble)
N2O
Des
Sevo
ISO
Halothane
what are factors that affect inspiratory concentration (Fi)?
Fresh gas flow (increases)
Circuit Volume (decreases)
Any machine/circuit absorption (decreases)
formula for time constant of agent Fi
Time constant = Vc/FGF
FGF = fresh gas flow
Vc = volume in circuit
what is important about time constants inregards to wash in
instead of looking how much is left in a system (i.e. 1 t = 37%), you look at how much change has occured (i.e. 1 t = 63 %)
in regards to Rate of Rise of FA/Fi, how do you speed up equilibrium?
decrease FRC
increase volume in Alveoli
what is Most important factor in rate of rise FA/Fi
uptake
what is relationship of uptake to rise of FA/Fi
increase in uptake = decrease in FA/Fi
why does increased uptake slows the rate of induction
less remains in the lung to increase concentration
when will FA = Fi
when uptake is 0
what how does an agents blood gas partition coefficient effect uptake
the higher the partition coefficien
the greater the solubility which means more of the agent is taken up into the pulmonary circulation
this causes the alveolar concentration to rise more slowly, and prolong induction
how does alveolar blood flow effect uptake?
less blood flow means less agent is taken up, which speeds induction…
but at what cost…the same things that determine pulmonary blood flow, help to determine cerebral blood flow…if we hyperventilate the patient to decrease pulmonary flow and speed induction, we decrease delivery of the agent to the brain…
which partition coefficient effects how fast patient goes to sleep?
blood:gas
Des 0.42
N20 0.47
Sevo 0.65
Iso 1.4
Halo 2.4
which partition coefficient effects how fast patient wakes up
muscle:blood
N20 1.2
Des 2.0
Sevo 3.1
Halo 3.5
Iso 4.0
How do we overcome the issue of decreasing alveolar concentration by uptake
increasing alveolar concentration (hyperventilation)…by doing this we are constantly replacing the anesthetic, and maintaining a more constant alveolar concentration
Concentration of the inhaled agent can also be increased…turn up the dial…to increase the rate of rise of agent concentration
hyperventilation more effective for highly soluble agents
How does a large FRC slow the rate of Rise FA/Fi
slows the rate
what happens to FA (alveolar partial pressure/concentration) with increased Cardiac output
slower uptake with an increaes flow (Cardiac Output)
what are advantages of using N2O
Powerful analgesic properties
Decreases MAC of other inhal. Agents
Safe in MH patients
Rapid induction/recovery
What are some disadvantages of N2O
Decreases myocardial contractility
Increase risk PONV
Increase ICP by inc CBF
Teratogenicity
what are contraindications for N2O
Air embolism
Pneumothorax (75% 2-3x 10 min)
Acute intestinal obstruction
Intracranial air (tension pneumocephalus)
Pulmonary air cyst
Intraoccular air bubbles
Tympanic membrane grafting
what is a concern regarding Sevo
Compound A (low gas flows, high temperatures, disiccated absorbent)
advantages of Sevo
Non-irritant sweet odor
Rapid induction/recovery
Does not sensitize myocardium to catecholamine
No CO production with Na lime
disadvantages of sevo
Compound A production (< 2L/min)
Contraindicated with Baralyme
Post op agitation in children
advantages of Des
Rapid onset/recovery
Stable with CO2 absorbers
disadvantages of des
Requires special vaporizer
Low potency
Pungency
Rapid increase >1.25 MAC SNS stimulation
what are agent effects on MAP
Iso, Des, and Sevo (all produce dose dependent) decrease in MAP
Result from sympathectomy causing peripheral vasodilation.
Nitrous oxide No Change or increases MAP
what are agent effect on HR
iso, des, sevo increase HR
(d/t stimulation of carotid baroreceptors– decrease MAP cause increase HR)
what is effect of agent on Cardiac Output
Most agents cause some reduction of CO (Isoflurane exception)
CO mildly increased or NC by N2O
Sympathomimetic
