Physiology/Pathophysiology

Question 1

Describe the pathways of nociception.

Answer 1

• Transduction
  
  • Transduction of noxious stimulus into an electrical stimulus occurs in the nociceptors that detect tissue injuring stimuli
• Transmission
  
  • Transmission of the nervous impulse occurs along the primary afferent fibers (a-delta and c-polymodal fibers) from the periphery to the somatosensory cortex
• Modulation
  
  • As the signal travels through the dorsal horn, modulation (amplification/inhibition) helps determine the strenght of the signal reaching higher centers in the brain
• Perception
  
  • Integration of theses processes with the psychology of the individual

Question 2

List the ASA categorization.

Answer 2

Question 3

Describe 2 methods of pre-oxygenating a patient, the benefits of pre-oxygenation and how long a patient should be pre-oxygenated.

Answer 3

• Preoxygenation reduces the risk of Hgb desaturation and hypoxemia during the induction process by increasing the PaO2. It is particularly beneficial if a prolonged or difficult intubation is expected or if the patient is already dependent on supplemental oxygen.
• Pre-oxygenation should occur for 3-75 minutes. Various devices may be used, including flow-by oxygen, mask oxygen, or nasal prongs.
• Two main techniques are utilized—tidal volume breathing versus deep breathing techniques.
  
  • With the traditional tidal volume breathing technique, the duration should be 3 minutes or longer and ideally, an FiO2 near 1 should be maintained. Studies have shown that a fresh gas flow rate of 5L/min is just as effective as higher rates at maintaining FiO2.
  • Deep breathing techniques were developed on the basis of the assumption that alveolar dinitrogenation can be achieved rapidly by deep breathing with varying numbers of deep breaths (ideally, full inspiratory capacity breaths) taken within a short period of time

Question 4

Define Total Intravenous Anesthesia (TIVA) and in basic terms describe the principle/s upon which this practice is based.

Answer 4

• TIVA is defined as a technique of general anesthesia using a combination of agents given solely by the IV route and in the absence of all inhalant agents.
• It is based on the principle that plasma drug concentration needed to produce anesthesia has to be reached quickly and maintained throughout the anesthetic period, therefore, a loading dose followed by a CRI is utilized.
• The infusion rate is determined by clearance of the drug and the drug concentration in plasma.

Question 5

Define partition coefficient (PC) with regards to inhalant anesthetics.

Answer 5

The partition coefficient describes the solubility of inhaled general anesthetics in the blood. The more soluble the anesthetic is in blood compared to air, the more it binds to plasma proteins and the higher the partition coefficient<ins> after equilibration</ins>.

Question 6

Your patient is critically ill. Would you like a low or high blood:gas PC in this patient and explain why?

Answer 6

A lower B:G PC is desired, as the lower the PC, the faster the onset of anesthesia and the faster the emergence from anesthesia (it predicts more precise control over the anesthetic state). A high coefficient would mean that a large amount of the anesthetic will be taken up and protein bound in the blood prior to being passed into the fatty tissues for metabolism.

Question 7

List two patient populations for which particular consideration should be given to the different oil:gas PCs of various inhalant anesthetics and explain why this is.

Answer 7

• The MAC is inversely related to the oil:gas PC—thus a very potent anesthetic will have a very low MAC and a high oil:gas PC.
• Oil:gas PC should be considered when evaluating the length of time of emergence from the anesthetic, particularly following long procedures.
• When the intraoperative/anesthetic phase is of long duration, an agent with a high oil:gas PC redistributes into the adipose tissue. Because the vascular supply to adipose tissue is sparse, the release of the agent to the blood is slow and anesthetic emergence is prolonged.
• Highly potent anesthetics should be avoided in patients with neurologic or cardiovascular compromise, as prolonged emergence from anesthesia may further compromise their stability.

Question 8

Define MAC (minimum alveolar concentration).

Answer 8

The MAC is the concentration of an anesthetic agent at 1 atmosphere that abolishes movement in response to noxious stimuli in 50% of patients.

Question 9

List the MAC of:

Which is the most potent?

Agent

Methoxyflurane

Halothane

Isoflurane

Sevoflurane

Desflurane

Nitric Oxide

Answer 9

Methoxyflurane

The lower the MAC, the more potent…

Question 10

Differentiate between type 1 and type 2 fluid shifts out of the intravascular space toward the interstitium in the perioperative period.

Answer 10

• Type 1 fluid shifts represent the physiologic, nearly protein free, shift out of the vasculature. This occurs even if the vascular barrier is intact.
• Type 2 fluid shifts are considered pathologic, containing proteins close to plasma concentrations. This type of shift occurs for 3 reasons:
  
  • surgical manipulation leading to excessive increases in capillary protein permeability,
  • reperfusion injury and inflammatory mediators compromising the vascular barrier
  • iatrogenic hypovolemia that can lead to glycocalyx degradation, causing an extensive shift of fluid and proteins towards the tissue.