Inhalation Agents Part 3 Flashcards
What effects do inhaled agents have on MAP?
At equipotent doses all agents will have similar effects
- MAP decreases with increased concentration of Sevo, Iso and Des in a dose dependent manner
- SVR also decreases (vasodilation)
- Exceptions *
- Halothane decreases MAP by decreasing CO
- N2O has either no change or mildly increases MAP
How are halogenated agents dosed compared to N2O?
- N2O is based on flow (% x L/min)
- Halogenated agent based on MAC (% x MAC value)
How do halogenated agents effect HR?
Dose dependent and different for each agent
- Iso—> starting at .25 MAC, linear dose dependent increase in HR, especially with rapid increase in concentration
* Des—> ≥ 1 MAC- linear dose dependent increase in HR, especially with rapid increase in concentration
<1 MAC- min increase in HR
- Sevo—> no increase in HR until > 1.5 MAC
What is the rule of 24?
Total combined flow of Des x O2 flow if >24 will see tachycardia
- keep <24 to keep HR under control (have Des ~8 and flows ~3)
What type of patient must you be cautious with increasing agent slowly to prevent tachycardia?
Cardiac compromised
Is CI influenced by inhalation agents?
Minimally
- TEE shows Des to produce minor increase in EF compared to awake measurements
Rapidly increase the dose will rapidly increase HR with ________ and ________ when > _____MAC.
Iso and Des
1 MAC
Abrupt increase in HR is not observed with ______, ______, or _______ up to MAC 2.
Sevo, halo or enflurane
Do the newer agents predispose the heart to PVCs?
Not unless they are given in extremely high concentrations
—> halothane does d/t catecholamines and hypercarbia
How do the agents effect QT interval and what should be avoided?
Inhalation agents prolong QT interval
* avoid SEVO in patients with known congenital long QT syndrome (LQTS)
(Safe with beta blocker therapy)
How do inhalation agents effect CAD?
Data shows no difference between inhaled and IV opioid anesthetic
—> volatile anesthetics exert protective effect on the heart
- limit the area of myocardial injury and preserving function after exposure to ischemic insult
What is coronary steal?
ISO’s ability to dilate healthy coronary arteries and not stenotic arteries, thus stealing from diseased vessels
What is ischemic preconditioning?
Like inoculation or vaccination against ischemia
- protective mechanism present in all tissues
- exposure to brief episodes of ischemia confers protective effect on myocardium agains subsequent prolonged ischemic insult (reversible or irreversible)
What are the 2 periods of protection with ischemic preconditioning?
1st period: 1-2 hours after conditioning episode
2nd period: benefits reappear 24 hours later and can last as long as 3 days
What is the crucial event that confers protective activity with ischemia preconditioning?
Opening of mitochondrial ATP sensitive K+ channels
What effect do the agents have on chronic HTN?
BP will drop even lower with anesthesia
- vessels are always clamped down —> less blood volume- when gas vasodilates vessels are hypovolemia
—> IVF boluses help
What are the pulmonary effects of inhaled agents?
- increase in RR and decrease in tidal volume (minute ventilation preserved)
- as anesthesia deepens- gas exchange is less efficient (shallow breathing)
- PaCO2 increases proportionately to depth of anesthesia
- dose depended blunted response to hypercarbia
- apnea threshold increased- higher CO2 level needed to stimulate breathing
- hypoxic drive altered- O2 must be lower to stimulate breathing
How do the agents effect the chest wall?
Gas relaxes everything
- cephalad displacement of diaphragm
- inward displacement of rib cage from enhanced expiratory muscle activity—> decreases FRC
- atelactasis in dependent lung zones
- worse with spontaneous ventilation
- increasing PEEP can help
- limit HPV
- Bronchodilation: greatest effect when bronchoconstriction present
- will help with bronchospasm
What are the effects of pungency?
- Coughing- worse in asthma and smokers- use inhaler before (gas is irritating)
- breath holding
- laryngospasm
- arterial desaturation
- Des and Iso are pungent —> Des is the most pungent
(Non-pungent gases are Sevo, Halo, Nitrous Oxide)
What are the CNS effects of the agents?
- decrease CMRO2
- with normal CO2 levels- cerebral vasodilation with MAC > 0.6
≥1MAC vasodilation that increases flood flow -> be careful with increased ICP
How does N2O effect CNS differently?
- causes cerebral vasodilation
- increases CMRO2
- to counter these effects give opioids, barbs, or propofol. (Not ketamine—> bad for brains)
What happens to ICP with ≥1MAC of all agents?
Increases ICP
In head trauma keep gasses < 1 MAC
What happens to autoregulation of blood flow in the brain <1 MAC?
Impaired, worsens as MAC increases
How do the agents effect evoked potentials?
- depress amplitude and increase latency of SSEP in a dose dependent manner
- both of these are signs of nerve damage…..
- EPS abolished at 1 MAC
- or N2O + o.5 MAC
- even low concentrations (0.2-0.3) decrease reliability of MEPs
Which gas do you avoid when your patient has a history of seizures?
SEVO— can induce seizures
So can ethrane
How is the EEG effected by the agents?
Increases amplitude and synchrony as depth increases
Eventual burst suppression
- 1.5-2.0 MAC —> isoelectric EEG, stage 4, no brain activity
How do the agents effect neuromuscular characteristics?
- dose dependent skeletal muscle relaxation
- enhances activity of NMB drugs
- stopping agent enhances recovery from NMB
Do all inhalation agents trigger MH?
Except N2O
Halo>iso>sevo>des
How do the agents effect the liver?
Immune mediated liver injury
—> caused by trifluoroacetate metabolite
- mild liver injury
- more likely to occur after decreased blood flow and O2 to liver
- halothane > iso> des as far as metabolism is concerned
- from TFA and inorganic fluoride ions
What should you do if patient has had previous liver dysfunction after receiving inhalation anesthesia?
Low risk, but avoid agents
How do the agents effect the kidneys?
Sevo—> compound A—> nephrotoxic after prolonged exposure with O2 <1L/min
- causes transient proteinurea, enzymurea, glycosurea
- to avoid Compound A—> limit low FGF (<2L/min) to <2 MAC hrs of sevo anesthetic
What is something to be aware of with N2O?
- it inactivates methionine synthase—> enzyme that regulates B12 and folate metabolism
- N2O contraindicated with pre existing B12 deficiency or underlying critical illness
What is homocysteine?
Requires methionine synthase for conversion
- increased levels associated with adverse coronary events
- since N2O inactivates methionine synthase, homocysteine levels may increase
What does N2O do to closed gas spaces?
Transfers to them rapidly, 34 x more than nitrogen diffuses out
- compliant wall =increased volume
- noncompliant wall= increased pressure
What happens when the CO2 absorbent is fully dedicated?
Causes degradation and carbon monoxide production from all agents regardless of the temperature
What accelerates desiccation of CO2 absorbants?
High FGF (exceeding normal minute ventilation)
Degradation of soda lime is an exothermic process. What implications does this have?
May elevate temperature so high that it leads to explosion or fire in canister
*especially SEVO
With desiccated soda lime:
- Sevo—> compound A production is sped up
- Iso and Des—> produce carbon monoxide
How do you prevent desiccated soda lime?
- change CO2 absorbent frequently (q 1 week here)
- limit FGF during cases
- turn FGF off between cases
- when in doubt change absorbent
T/F
Immune response is depressed for a while after exposure to gas?
True
How does a variable bypass vaporizer work?
(Seen with Sevo, Iso, halo)
- 2 streams of inflowing fresh gas
- 1 with reservoir to liquid anesthetic
- 1 bypassing reservoir
- concentration of gas leaving the vaporizer determined by relative flow to gas through reservoir channel vs bypass channel
- controlled by adjusting dial
What is true regarding a variable bypass vaporizer?
- temperature compensated
- calibrated for individual anesthetic d/t different vapor pressures
- tilting/overfilling may lead to overdose
What is true about the tec 6 heated vaporizer?
Used for des since the vapor pressure of des at 20C and 1 atm is 700mmHg (boils are room temp) and using a variable bypass vaporizer will deliver inaccurate amounts
- heats des to 2 atms—> for accurate delivery
- at high altitudes partial pressure of des is lower—> need to give higher dose to account for altitude
What is the equation to account for altitude ?
Required vaporizer setting= (MAC desired x 760mmHg)/local barometeric pressure
