Chapter 21 Anaesthesia Principles and Monitoring Flashcards
What two ‘bodily drug’ factors determine the requirement for loading dose and maintenance dose
- Re-distribution ot other areas
- Metabolism and excretion of the drug
From the perspective of GA action, the body is divided into three groups. What are they and give an example of each:
- Vessel rish e.g. heart, brain, kidney
- Muscle
- Vessel-poor
List 4 factors related to time necessary to achieve steady state anaestehtic for inhalant drugs:
- Minute ventilation (i.e. rate of delivery)
- Cardiac output
- Speed of redistribution
- Elimination
Speed between different inhalants dependent on solubbility. Less soluble –> more rapid changes
What is the approximate metabolic demand of oxygen (ml/kg/min)
5 ml/kg/min
Or
10 x (BW)0.75
What are the two types of vapourizer outputs and which is most common?
- Variable bypass (splits incoming gas so some directed through vapourizing chamber) = most common
- Measured flow
What are the three methods of vapourization? Which is most commonly used?
- Flow over = most common
- Bubble through
- Direct injection
What is the boiling point of desflourane?
23.5°C
Therefore has specific externally warmed vapourizer
What is soda lime a combination of?
Sodium hydroxide, potassiu hydroxide, calcium hydroxide, water
What is the indicator dye in soda lime?
Ethyl oxide
Label the diagram
What is the lower BW limit for use for rebreathing system?
5kg
What is the formula for recommended fresh gas flow in a non-rebreathing circuit
x3 minute volume
MV = RR x tidal volume
Tidal volume = 15 ml/kg
What is the side hoel at bottom of ET tube called and what is it for?
Murphy eye
In case bottom of tube becomes blocked
How long does it take for a non-pre-oxygenated vs pre-oxygenated animal to be come hypoxic (with apnea/obstruction?)
30s vs 5 mins
What is an adequate oxygen flow rate for pre-oxygenation with a mask?
4-5 L/min
What colour are the canisters for:
Oxygen
Air
Nitrous oxide
CO2
Nitrogen
Oxygen: White
Air: White/black
Nitrous oxide: Blue
CO2: Grey
Nitrogen: Black
What does absorbtion atelectasis refer to?
Gradual collapse of alveoli as oxygen is removed by pulmonary blood flow (i.e. better to have oxygen:air mix)
List 3 safety systems to ensure correct gas attachment
- Cylinder colour
- Pin index safety system
- Diameter index safety system
What is the inspiratory:expiratory ratio range for use with ventilators?
1:2 to 1:4
What is the recommended peak inspiratory pressure?
12 mmHg
What is the anaesthetic triad?
Narcosis, analgesia and muscle relaxation
How does hypoxia lead to cell death?
- Hypoxia –> anaerobic metabolism
- Insufficient ATP
- Enzymes e.g. NA-K-ATPase unable to function –> loss of maintenance of electrochemical gradient
- Cell oedema and death
Which patients (5) are at high risk for hypoglycaemia?
- Young
- Receiving insulin
- Insulin secreting tumour
- Sepsis
- Liver failure
What is the formula for calculating MAP (from systolic and diastolic)?
MAP = DAP + ((SAP-DAP)/3)
What are normal SAP/DAP values for dog and cat (the same!)
125/85 mmHg
At what BP does coronary perfusion become compromised?
Diastolic BP <40mm Hg (coronary perfusion occurs during diastole)
What is recommended cuff width for BP measurement
40-60%
In cats doppler tends to undersetimate SAP (i.e. we say clsoer to MAP). How much does it underestimate SAP by?
25 mmHg
HOw does oscillometric results differ from actual results in dogs and cats?
Oscillometric underestimates all (SAP, MAP and DAP) in dogs
Underestimates SAP in cats
Trends well correlated with actual trend though
What is electromechanical dissociation?
Plausible ECG trace not correlated with any muscular contraction of heart i.e. during arrest
What is normal expiratory carbon dioxide level?
35-45 mmHg
Significant adverse effects only if over 65 mmHg in anaesthetised patient
Identify cause of capnogram chnages
(A) normal
(B) hypoventilating (note progressive elevation of the plateau)
(C) rebreathing carbon dioxide (CO2) (note the progressive elevation in the baseline, as it fails to return to 0 with each inspiration)
(D) experiencing a leak in the system due to inadequate cuff inflation or other causes
(E) hyperventilating (note progressive decrease in plateau amplitude)
(F) experiencing cardiac arrest, a sudden decrease in cardiac output, or a circuit disconnect.
How does pulse ox work?
Emits 2 light frequencies (specific for oxyheamoglobin and deoxyhaemoglobin) through tissue and into a sensor.
Calculates % of oxy- vs deoxy-haemoglobin.
Relies on only the pulsatile portion of blood (absorbance spectrum of pulsatile component)
N.B. Carboxy- or Methaemogloib may confuse thigns!
What is normal PaO2 in patient at sea level breathing room air?
and 100% oxygen?
110 mmHg (=100% Hb saturation)
500 mmHg on 100% oxygen
What is PaO2 of patient with Pulse ox 90% (on room air)
At what pulse ox level should causes of hypoxaemia be investigated in anaesthetised patient breathing 100% oxygen?
60 mmHg i.e. hypoxaemia
Investigate if <98%
Breifly, how does a nerve stimulator work?
- Two electrodes + generator. Electrodes placed along path of motor nerve (usually peroneal or ulnar).
- Electrical current applied –> twitch. In NM block –> progressively weaker twitch (train of four)
Dont attempt reversal of NM blockade before all 4 twitches returned (i.e. can reverse when there but not all full magnitude)
What is normal central venous pressure in small animals?
0-5 mmHg
List 4 methods of determining cardiac output (non-invasive cardiac output monitoring)
- Thermodilution
- Lithium Dilution
- Pulse Contour Analysis
- Noninvasive Cardiac Output (NiCO device)
How is brainwava activity measured?
Bispectral Index.
0-100 (100 being awake). Good in humans - poor in animals
What are the three opioid receptors
µ (mu), κ (kappa), and δ (delta)
Mu: Regulates majority of clinical effects inc analgesia and side effects
Kappa: Role in alnalgesia but hard to distinguish from Mu.
Delta: Primarily modulating role on Mu.
What happens (on cellular level) with activation of opioid receptor?
- Influx of K+ and efflux of Ca2+
- –> decreased release of neurotransmitter (eg substance P or glutamate) in pre-synaptic cell
- Hyperpolarization on post-synaptic cell
- –> Net decreased neuronal activity and transmission of pain signals
Analgesia supplied by opioids can reduce inhalant anaesthesia requirement by how much?
40-60%
What drug and dose is used to reverse opioids?
Naloxone, 0.04mg/kg
List 5 potential side effects of opioid administration
- Vomiting
- Ileus
- Nausea
- Bradycardia
- Histamine release
- Increased pyloric sphincter and sphincter of Oddie (bile duct) tone
- Respiratory depression (dogs and cats relatively resistant)
- (Urinary retention when given epidurally)
List relative potency of the following drugs (c/f/ morphine which has arbitrary potency of 1):
Methadone
Hydromorphone
Oxymorphone
Fentanyl
Buprenorphine
What is the term used for sedation + analgesia?
Neuroleptanalgesia
Broadly speaking, how do benzodiazepines work?
Enhance effects of GABA (gamma-aminobutyric acid = inhibitory neurotransmitter in CNS).
Activated GABA receptor allows entry of chloride into neuron –> hyperpolarization and prevention of action potential propagation
What is the reversal agent and dose for benzodiazepines?
Flumazenil, 0.01-0.02 mg/kg
What is the difference between diazepam and midazolam metabolism?
Diazepam –> nor-diazepam + oxazepam –> sedative actvity and slower clearance.
Midazolam –> 1-hydroxymethyl midazolam = minimal biologic activity
What are 2 storage particulars re diazepam?
Light senstive and adsorbs to plastic
How does ACP work?
Depressed dopamine in reticular activating system.
Alpha 1 receptor antagonist
Tranquilizer, anti-ematic and anti-histamine
How do alpha-2 agonists work?
Decrease norepinephrine release in CNS.
Hypertension –> reflex bradycardia
What is the alpa-2:alpha-2 receptor sleectivity of xylazine vs medetomedine?
- Xylazine 160:1
- Medetomedine 1600:1
How does propofol work?
GABA agonist –> CNS inhibition
What is meant by dissociative anaesthesia?
Dissociation between higher brain functions and unconscious functions i.e. body systems not under conscius control (e.g. airway) maintain basal function c.f. other anaesthetic agents
What it the active metabolite of ketamine?
How is ketamine excreted?
Norketamine
Renal excretion
Define MAC.
MAC: Concentration of anaesthetic required to prevent purposeful movement in response to a standard painful stimulus in 50% of normal patients.
What is the MAC of Iso, Sevo and Desflourane in cats and dogs?
Typically need 1.2-1.5x MAC to prevent movement during surgery.
What is the MAC- blockade of adrenergic response (MAC-BAR)?
Concentartion of inhalant that prevents a CVresponse after painful standard stimulus.
In cats MAC-BAR usually 10% higher than MAC.
Where are inhalant anaesthetics metabolized?
Liver
Order iso, sevo and desflourance in order of solubility (Most to least soluble)
( i.e. less soluble –> faster induction/change/recovery)
Iso > Sevo >Desflourane
How do localanaesthetics work?
Block fast Na+ channels on afferent nerves –> inhibit afferent nociceptive transmission
What is the time until onset and duration of action of lidocaine?
5 min onset
60 min duration
What is the time until onset and duration of action for bupivacaine (higher lipid solubility cf lidocaine)?
45 min onset
6-8 hour duration
What is a Bier block?
IV regional anaesthesia
What are two reversal agents for non-depolarizing NM blockade agents (e.g. atracurium)
Edrophonium or neostigmine (they inhibit actions of acetylcholinesterase at nerve terminals
Lost 2 anti-cholinergic agents
Atropine or glycopyrrolate (parasympatholytic)
Which prostaglandin is primarily responsible for inflammatory pain?
PGE2
What sign on aretrial BP waveform can indicate hypovolaemia?
“Delta down” = attenuation of waveform with positive-pressure breath caused by collapse of great vessels in thorax.
