Page 51 - TNFlipTest

Toronto Notes 2019
Induction
Anesthesia A17
Table 9. Volatile Inhalational Agents
Sevoflurane
MAC 2.0 (% gas in O2)
CNS Increased ICP
Desflurane
6.0 Increased ICP
Isoflurane
1.2
Decreased cerebral metabolic rate Increased ICP
Enflurane
1.7
ECG seizure-like activity Increased ICP
Halothane
0.8
Increased ICP and cerebral blood flow
Nitrous oxide (N2O)*
104 —
—
Can cause decreased HR in pediatric patients with existing heart disease
Resp Respiratory depression (severely decreased TV, increased RR), decreased response to respiratory CO2 reflexes, bronchodilation
CVS Less decrease of contractility,
stable HR
Tachycardia with rapid increase in concentration
Decreased BP and CO, increased HR, theoretical chance of coronary steal**
Stable HR, decreased contractility
Decreased BP, CO, HR, and conduction Sensitizes myocardium to epinephrine- induced arrhythmias
MSK Muscle relaxation, potentiation of other muscle relaxants, uterine relaxation
*Properties and Adverse Effects of N20
Due to its high MAC, nitrous oxide is combined with other anesthetic gases to attain surgical anesthesia. A MAC of 104% is possible in a pressurized chamber only Second Gas Effect
Expansion of closed spaces: closed spaces such as a pneumothorax, the middle ear, bowel lumen and ETT cuff will markedly enlarge if N20 is administered
Diffusion hypoxia: during anesthesia, the washout of N20 from body stores into alveoli can dilute the alveolar [02], creating a hypoxic mixture if the original [02] is low **Coronary steal: isoflurane causes small vessel dilation which may compromise blood flow to areas of the heart with fixed perfusion (e.g. stents, atherosclerosis)
MAC (Minimum Alveolar Concentration)
• thealveolarconcentrationofavolatileanestheticatoneatmosphere(atm)ofpressurethatwillprevent movement in 50% of patients in response to a surgical stimulus (e.g. abdominal incision)
• potencyofinhalationalagentsiscomparedusingMAC
• MACofhalogenatedvolatileanestheticsdecreasebyapproximately6%peradditionaldecadeofagein
adults
• 1.2-1.3timesMACwilloftenablateresponsetostimuliinthegeneralpopulation
• MACvaluesareroughlyadditivewhenmixingN2Owithanothervolatileagent;however,thisonly
applies to movement, not other effects such as BP changes (e.g. 0.5 MAC of a potent agent + 0.5 MAC of
N2O = 1 MAC of potent agent)
• MAC-intubation: the MAC of anesthetic that will inhibit movement and coughing during endotracheal
intubation, generally 1.3 MAC
• MAC-blockadrenergicresponse(MAC-BAR):theMACnecessarytobluntthesympatheticresponseto
noxious stimuli, generally 1.5 MAC
• MAC-awake:theMACofagivenvolatileanestheticatwhichapatientwillopentheireyestocommand,
generally 0.3-0.4 of the usual MAC
Muscle Relaxants and Reversing Agents
Factors increasing MAC: chronic alcohol use, hyperthyroidism, hyperthermia, stimulant (amphetamines), young age
Factors decreasing MAC: acute alcohol intoxication, hypothermia, sedating
drugs, advanced age, drugs (opioids, benzodiazepines)
1 Action potential
Prejunctional motor nerve ending
Vesicles with ACh
2
K+
345
6
Lower [K+]
Post synaptic ACh receptor
Depolarization and muscle contraction
Higher [Na+]
muscle membrane
AChE Na+
1. Action potential arrives
2. Release of ACh into cleft
3. ACh binds to ACh receptor, ion channels open
4. Change in membrane permeability
5. AChE hydrolyzes ACh
6. Action potential spreads across muscle membrane
Figure 10. Review of anatomy and physiology of the neuromuscular junction (NMJ)
© Julie Man 2012