2014A04 Describe the physiological basis of the methods used to prevent hypoxemia prior to
intubation in a rapid sequence induction. Include any adverse effects of these methods.

 

List:

·     Principles

·     Oxygen storage

·     Apnoeic oxygenation

·     Other

 

Principles:

Aim

·  Minimise time between induction and intubation

Reason

·  Prevent hypoxia

·  Prevent aspiration

Problem

·  Apnoea between loss of consciousness and intubation

·  Ongoing O2 consumption at 3-4mL/kg/min

Time to gross hypoxia

·  (See Nunn’s Applied Respiratory Physiology – numbers reflect ideal conditions)

·  Apnoeic, obstructed: <1 minute

·  Apnoeic, patent to room air: 1-2 minutes

·  Apnoeic, patent to 100% O2: several minutes

·  Apnoeic, patent to 100% O2, pre-oxygenated: 100 minutes

 

Oxygen storage:

Relevant physiology

·  Functional residual capacity

o Volume in lungs at end of tidal expiration

o Reservoir for oxygenation during apnoea

·  Alveolar gas equation: PAO2 = FiO2(Patm – SVPH2O) – PACO2/R

o Room air -> 100mmHg -> 275mL O2 in FRC

o 100% O2 -> 660mmHg -> 1800mL O2 in FRC

·  Alveolar ventilation equation: PACO2 = VCO2 / VA x k (where PACO2 ≈ PaCO2)

Problem

·  Supine position -> diaphragm displacement -> ↓FRC to 30mL/kg

·  Induction -> chest wall relaxation -> ↓FRC to 25mL/kg

Solution

(1)Positioning:

o Head up -> ↓Diaphragm displacement -> ↑FRC

(2)Pre-oxygenation

o 100% O2 3-5 mins tidal ventilation

o Replace alveolar N2 with O2

o ↑O2 in lungs > blood, tissues, myoglobin

o Room air 275mL O2, 100% O2 -> 1800mL O2

(3)Hyperventilation

o ↓PACO2 -> ↑PAO2

(4)Small amount CPAP e.g. 5cmH2O

o Splint and distend alveoli

o Limits ↓FRC on induction

o Advantage lost at intubation

Adverse effects

·  Head up: may impair CNS perfusion upon induction

·  ↑O2: absorption atelectasis, O2 toxicity in neonates

·  ↓CO2: symptoms e.g. paraesthesia

 

Apnoeic oxygenation:

Relevant physiology

Alveolar gas flux during apnoea:

·  O2 uptake 250mL/min

·  CO2 output 20mL/min (PaCO2 and P­ACO2 equilibrate at 1 minute)

·  Net uptake by diffusion 230mL/min

Problems

·  Apnoea

·  Airway obstruction (general anaesthetic + relaxant -> ↓pharyngeal dilator tone)

Solution

Prior to intubation:

·  Airway patency: jaw thrust, head tilt, chin lift

·  Face mask e.g. 10L/min of 100% O2

·  CPAP -> splint + distend alveoli

During intubation:

·  High flow nasal prongs e.g. 60L/min of 100% O2

Adverse effects

·  Discomfort

·  Epistaxis

 

Other things that may not attract marks:

 

↓O2 consumption:

Problem

·  Anxiety -> ↑SNS output

·  Fever

Solution

·  Pre-medication e.g. benzodiazepines

·  Human factors

 

Fast drugs -> ↓apnoeic time: (rapid sequence by definition)

Rapid onset hypnosis

 

Factors ↑C1:

·   IV route

·   ↑Dose

·   Priming dose

·   Rapid injection

·   Proximal injection

·   Pre-medication to reduce cardiac output

·   ↑Unionised fraction (e.g. propofol)

Factors ↑diffusion coefficient: (e.g. propofol)

·   ↑Lipid solubility

·   ↓Molecular weight

Rapid onset paralysis

·   Similar factors

·   Bowman principle: ↓potency -> ↑safe dose -> ↑(C1-C2)

·   Hence fastest drugs: suxamethonium 4x ED95, rocuronium 2-4x ED95

Exploit synergy

·  Midazolam: reduces propofol dose for hypnosis by 30%

·  Fentanyl 1mcg/kg: reduces propofol dose for hypnosis by 20%, for immobility by 50%

 

 

Feedback welcome at ketaminenightmares@gmail.com