Inhalant Anesthesia: Pharmacodynamics Part 1 and 2

What is minimum alveolar concentration?

Measure of potency where 50% of patients at that dose will not have gross movement

The lower the MAC value means what about the inhalant?

More potent

How can you compare inhalants?

MAC

What is the ED95?

MAC x 1.5

What is the ED95 with isoflurane?

About 2

What drugs are on board when they calculate MAC?

Nothing else

T/F MAC is based on alveolar concentration, not vaporizer setting?

True

What is the MAC for nitrous oxide?

222%

Why can nitrous oxide not be used as a sole inhalant?

MAC is 222%, so patient will die before not being able to move

What is the MAC in cats?

Usually slightly higher

What are factors that increase MAC?

Stimulants (cocaine, amphetamines), hyperthermia, young patients

What are factors that decrease MAC?

Opioids, alpha-2s, benzo, neuromuscular blockers, ketamine, propofol, hypothermia, older, pregnant, hyponatremia, acidosis

What happens if you have 0.5 MAC of halothane and 0.5 MAC of iso?

You have 1 MAC (additive relationship)

What are some of the limitations of MAC?

Different stimuli can produce different results

Movement is not a great endpoint. We want to know when unconsciousness begins

Species differences

How do you dose inhalants?

Dose to effect as other drugs and temp will alter the necessary dose

What does the vaporizer control?

Only depth, not HR, RR, or BP

How much does it take to make them unconscious?

0.5 MAC

What receptors are modified by inhalants?

Inhibit NMDA, AMPA, nicotinic, Na+

Enhance GABA, Glycine, and K+ channels

Details are not known but they are all modified

What do all inhalants do to the CNS?

Increase cerebral blood flow increasing intracranial pressure

What inhalant has a higher increase in cerebral blood flow compared to sevo?

Isoflurane

What happens to cerebral blood flow and PaCO2 increases?

It increases

When does an increase in PaCO2 causing an increase in cerebral blood flow matter?

If the patient already has elevated intracranial pressure form a tumor or something

Inhalants have what effect on CMRO2 (cerebral metabolic rate of oxygen)

Dose-dependent decrease

Inhalants have what effect on ICP (intracranial pressure)?

Dose-dependent incrreases in ICP

What can blunt an increase in ICP?

Hyperventilation

What two things can exacerbate elevated ICP?

Head down position and prolonged anesthesia

How do you calculate BP?

Cardiac output x SVR

How do you calculate CO?

HR x SV

What effect do inhalants have on arterial blood pressure?

Dose-dependent decrease

When does myocardial depression due to decreased contractility occur?

larger than 2 MAC

What do inhalants do to HR?

Increase it, but other predications typically reduce HR

What effect do inhalants have on pulmonary vascular resistance?

Little to no effect (will not improve pulmonary hypertension or change hypoxic pulmonary vasoconstriction)

What effect do ether inhalants have on an arrythmia?

Do not increase likelihood of arrhythmia

What inhalant are arrhythmogenic?

Halothane

What do all volatile inhalants do to sensitivity of PaCO2?

Decrease it. Ethers will reduce rate more than tidal volume (but do reduce both)

What is the benefit of inhalants causing bronchodilation?

Good when a patient has increased airway resistance like feline asthma

T/F inhalants can cause upper airway irritation?

True (usually bypassed with opioids or IV induction agents)

What do inhalants do to lung volumes?

Reduce

What do inhalants do to functional residual capacity?

Reduce

When does atelectasis start after giving an inhalant?

Right away causing relaxation of respiratory muscles and absorption atelectasis

How can you fix inhalant induced hypoventilation?

Positive pressure ventilation

What is the problem with positive pressure ventilation?

Causes a reduction in venous return and sympathetic tone

What effect do inhalants have on the kidney?

Dose dependent decrease in renal blood flow, GFR, and urine output

T/F if you have kidney blood flow everything else has enough blood?

True

What are inhalant effects on the liver?

Reduce blood flow and O2 delivery but are not likely to cause enough issues for damage

What are inhalant effects on the GI?

Decreased gastric motility and delayed emptying

Reduced lower esophageal tone (increases risk of emesis, regurg, and aspiration)