Anaesthesia 2 Flashcards
Why is monitoring and support required in anaesthesia?
- Influences the outcome
- Ethical and moral obligation
- Maintain oxygen delivery
What physiological parameters are monitored in anaesthesia?
- Oxygen delivery (affected by multiple parameters)
- CaO2 (carriage of oxygen in blood)
- Mean arterial pressure
- Respiratory function
- Cardiovascular function
What affects the carriage of oxygen in the blood?
- Saturation of Hb with oxygen
- Amount of oxygen dissolved in plasma
- Huffner’s constant (1.34)
What is Huffner’s constant?
The number of molecules of oxygen that attach to a haemoglobin molecule
How is respiratory function measured?
- Oesophageal stethoscope
- Capnograph
- Pulse oximeter
How is cardiovascular function measured?
- ECG
- Blood pressure
- Pulse
When is oxygen support delivered to a patient?
- During all anaesthetics
- Pre-induction
- In recovery
How can oxygen be delivered to a patient?
- Mask if tolerated
- Flow by: oxygen source next to nose and allowing them to breathe this
- Intranasal prongs
- Intratracheal tube attached to oxygen
- Tracheostomy
What are the limitations of pulse oximeters?
- Low or high heart rates alter results
- Alpha2 agonists lower heart rate
- Not all probes are designed for veterinary use
- Only shows early warning sign of when patient is about to become cyanotic
- Do not compensate for anaemic or hypovolaemic patients
Explain how pulse oximeters work
- Probe has transmitter and receiver of IR and red light, transilluminates pulsatile arteriolar bed
- Computer software analyses absorption of ight
- Oxyhaemoglobin absorbs more IR and reduced Hb absorbs more red light
- Ratio calculated corresponding to % haemoglobin saturated with oxygen
- Can be pulsatile due to arteriolar flow
Explain how pulseoximeters are used in monitoring anaesthesia
- SPO2% given by pulse oz
- Oxygen content = (1.39xHbxSPO2%) +(0.003xPaO2)
- Used to give early warning signs for potential cyanosis
How can oxygen content in the blood be assessed during anaesthesia?
- Blood gas analysis (pH, HCO3, PCO2, PO2), most accurate
- Capnography
- Oesophageal manometry (rare in veterinary)
Which arteries are commonly used during anaesthesia to assess pulse?
- Femoral
- Dorsal metatarsal
- Lingual
- Auricular
Describe the measurement of pulse during anaesthesia
- Compare dorsal metatarsal artery and femoral
- Femoral will beat even after death, therefore not good for monitoring subtle changes
- Dorsal metatarsal will show changes much sooner, will disappear with hypertension-
How can arterial blood pressure be monitored during anaesthesia?
- Non-invasive pressure monitoring (NIBP) e.g. sphygmomanometry, oscillometry (and HDO), Doppler
- Invasive blood pressure monitoring
- Pulse cannot be used
Outline the use of a Doppler to assess blood pressure during anaesthesia
- Piezoelectric crystal placed over artery (clipped)
- Locate artery with distinct noise of arterial pulse (whoosh)
- Cuff placed proximal to probe
- Give systolic pressure
- Tape in place, leave for length of anaesthetic
Outline the use of oscillometry to assess blood pressure during anaesthesia
- Unreliable in cats and small dogs
- Expensive
- Gives systolic, mean and diastolic pressures
- More accurate methods are available
Give examples of invasive blood pressure monitoring methods
- Artery cannulation
- Central venous pressure
Outline the use of artery cannulation to assess blood pressure during anaesthesia
- Auricular, dorsal pedal, facial arteries most commonly used
- Gives systolic, mean and diastolic arterial pressures, beat to beat wave forms of all 3 values
- Gold standard
Must label catheter, line and flush regularly - Never inject drugs
- Tubing must be narrow bore and non-compliant to amplify signal
- Difficult to place
Outline the use of central venous pressure to assess blood pressure during anaesthesia
- Long jugular catheter
- Indicates filling pressure of heart
- Affected by contractility and circulating blood volume
- Useful for fluid therapy
- Need several readings to discern trend, gives trace
- Normal: 0-10cm H2O in dog, 0-5cm H2O in cat
What is indicated by an increasing central venous pressure?
Failing heart or volume overload
What is indicated by a decreasing central venous pressure?
Haemorrhage, blood pooling, inadequate fluid therapy
How can heart rhythm be assessed during anaesthesia?
- Stethoscope
- Continuous ECG
Outline the use of a continuous ECG during anaesthesia
- Indicates trends, good for spotting changes
- Often distorted due to patient position and placement of electrodes
- Does not give heart rate, indicates electrical activity of heart, not performance
Outline the assessment of cardiac output during anaesthesia
- Flow parameter, indication of perfusion rather than pressure
- Implied when preload parameters (CVP, PA occlusion pressure, jugular vein distension, post-caval distension) are high
- ANd when afterload parameters (cardiac output, arterial blood pressure, physical and lab measures of tissue perfusion) are low or abnormal
- CO can be low with normal arterial blood pressure
- Example: lithium, measures rate of passage from venous to arterial blood system
What is included in basic anaesthetic monitoring?
- Muscle relaxation
- Neck muscle tone
- Eye rotation
- Jaw tone
- Whisker, pedal reflex
- Anal tone
- Mucus membrane colour
- Capillary refill time
- Toe-web/core temperature comparison
Describe the anaesthetic monitoring record
- Legal document
- Record exact mg od drugs given
- ASA grading
- Record as many parameters as possible
- Note any events e.g. moving from prep to theatre
- Assess recovery and analgesia
What is included in pulmonary monitoring during anaesthesia?
- Breathing rate, rhythm, nature, and effort
- Observe bag and chest excursions
- Ventilatometer or respirometer if available
- Mucus membrane colour
- Spirometry shows if ventilation is within acceptable limits
- Blood gas analysis and capnography
- respiratory flow and airway pressure, airway compliance
How is carbon dioxide monitored during anaesthesia?
- Capnometer and plotted onto a capnograph
- Blood gas analysis
Describe the 2 types of capnometer
- Mainstream: IR electrodes at junction between ET tube and breathing system
- Side stream: carbon dioxide measured in the machine
Describe the graph plotted by a capnograph
- 5 phases
- A-E
- Inspiration is phases E to A
- B is the start of expiration, sharp increase to C
- Alveolar plateau between C to D
- Sharp drop to E at beginning of inspiration
What may cause increased CO2 readings on a capnograph?
- Rebreathing
- Poor fresh gas flow
- Exhaustion of sada lime
- Too much dead space
- Too deep anaesthesia
- Hypoventilation
- Pyrexia
What may cause decreased CO2 readings on a capnograph?
- Disconnection of circuit
- MIs-intubation
- Circulatory failure
- Hypotension
- Cardiac arrest
- Hyperventilation
What may be indicated by a “shark fin” capnograph trace?
- Difficulty breathing off CO2, more effort required for expiration
- E.g. kink in ET tube, obstruction in tube, narrowing of tube
What causes cardiogenic oscillations on capnograph traces?
- Heart beating against lungs
- Normal
What is indicated by a curare cleft on a capnograph trace?
Neuromuscular blockage wearing off
Describe the appearance and cause of a “bucking ventilator” trace on a capnograph?
- Irregular insipiration and expiration
- Small peaks between normal peaks
- Suggests patient is a little too light and fighting against ventilator, taking own breaths
When are neuromuscular blocking drugs used?
- In referral practice for ocular surgery, thoracic surgery and facilitating IPPC
- Do not make the patient unconscious
What is peripheral nerve stimulation used for?
To assess neuromuscular transmission when neuromuscular blocking drugs are used
What are the different types of peripheral nerve stimulator tests that can be used?
- Train of Four (TOF)
- Double Burst Stimulation (DBS)
- Tetanic stimulus
Describe Train of Four peripheral nerve stimulator tests
- Administer 4 super maximal stimuli over a 2 second period, half a second apart
- Should lead to 4 muscle twitches where no blockade used
- With partial block get an initial larger twitch, then decreasing twitches
- As blocking is increased, decrease the strength and number of twitches
What laboratory tests can be used for anaesthetic monitoring?
- PCV and TTP
- Hb
- Platelets and coagulation
- Albumin
- Lactate
What is the significance of hypothermia during anaesthesia?
- Leads to prolonged recovery
- Decreased metabolism, decreased circulation and decreased mentation
- Increased analgesia needed
Why does hypothermia occur under anaesthesia?
- Patient unable to regulate temperature
- Reduced shivering
- Vasodilation
- Anaesthetic agents may reset thermoneutral point
- Open body cavity
- Cold, dry gases intrduced
- Wetting and prep
How can body temperature be supported during anaesthesia?
- Bubble wrap, socks, hot water beds
- Low flow anaesthesia
- Warm theatre
- Circle systems better as air is warm and humidified
What is the purpose of IV fluids in the peri-anaesthetic period?
Support renal function
Explain the volume of fluids that is administered during anaesthesia
- 5x maintenance, 10ml/kg/hr for dogs, less for cats
- Estimated output should be 1-2ml/kg/hr
List the inhalation anaesthetics
- Nitrous oxide
- Halothane
- Isoflurane
- Sevoflurane
- Desflurane
List the ideal properties of an agent for the production and maintenance of general anaesthesia
- Stable at room temp
- No preservatives
- Non-inflammable
- Cheap
- Ozone friendly
- Non-metabolised
- Non-toxic
- No CVS effects
- Some analgesic effects
- Pleasant to inhale
- Induce bronchodilation
- Non-irritant
- Low blood:gas solubility, high oil:water solubility
What factors affect the rate at which the concentration of inhalational anaesthetics rises in plasma?
- Ventilation rate
- Concentration of agent in carrier gas
- Cardiac output (inversely)
- Solubility of the agent in the body (inversely)
Describe the relationship between rate of uptake of an inhalation anaesthetic and rate of induction
Faster rate of uptake = faster induction
Explain the effect of cardiac output on the induction of anaesthesia using inhalational agents
- Higher cardiac output requires more agent, as is quickly pumped to organs and metabolised so more of the agent is required to reach steady state
- Low cardiac output means there is more time for the agent to be taken up into the blood in the lungs, more agent in the blood, slower metabolism, less agent needed and quicker induction
Explain how solubility affects the rate of induction of anaesthesia using inhalational anaesthetics
- Solubility determines induction and recovery
- Solubility coefficient = blood: gas partition coefficient
- What occurs in the alveoli and tissues affects potency and uptake of the drug
Explain the blood:gas partition coefficient with regards to induction of anaesthesia using inhalational agents
- Less soluble agents (low coefficients) are removed from lungs less quickly
- Alveolar concentration rises faster
- Faster induction of anaesthesia (transported to brain more readily)
Outline the pharmacokinetics of anaesthetic recovery
- Reverse of induction
- Solubility affects degree of redistribution into fat
- Fat acts as a depot of anaesthetic hence a fat animal will recover slower
- Recovery is faster with less soluble agents
What is the mechanism of action of inhalational anaesthetics?
- Unknown
- Many theories e.g. ligand gated ion channels
- Different sites on GABA, Ach, NMDA receptors
Describe the physiochemical properties of inhalation anaesthetics
- Exist as vapours
- Part liquid, part gas
- This is due to increasing the pressure in the cannister, but without lowering the temperature
Explain how vaporisers for inhalational anaesthetics work
- Molecules of agent liberated from the surface of a liquid, by a carrier gas
- Modern vaporisers are temperature conmpensated and flow compensated
What are the 2 types of temperature compensation mechanisms in vaporisers?
- Bi-metallic valve
- Metal rod
Explain how the delivery concentration of inhalational anaesthetics is produced
- If vaporiser is off (0%), then fresh gas will not go through the vaporising chamber
- If the vaporiser is set to 3% for example, 3% of the gas volume will go through the vaporising chamber and pick up the inhalational agent
What is the function of the folded material wick inside vaporisers?
- Increases surface area
- Maintains a fully inhalation agent saturated environment
- Route for gas to flow through is windy, to equalise pressure by the time it reaches the vaporising chamber
Describe nitrous oxide as an inhalational anaesthetic
- Health and safety issues (toxic)
- Expensive (need separate port and flow meter on anaesthetic machine)
- Minimal CVS and respiratory effects and analgesic effect
- Very high MAC >100%
- Long term side effects in abusers
- May have role in chronic pain management
Describe halothane
- Toxicity
- Oxidative (forms a hapten)
- Can lead to reversible hepatic hypoxia
- In guinea pigs causes emtabolite hepatic toxicity
- Not licensed in UK
Describe isoflurane
- Lower stability vs halothane
- Different CVS depression vs halothane
- Safer in dogs
- Licensed in dogs, cats, horses
- MAC: 1.28 in dogs, 1.63 in cats, 1.3 in horses
Describe sevoflurane
- Licensed for dogs and cats
- Anaesthetic induction, recovery and intraoperative modulation of depth of anaesthesia faster than halothane and iso
- More expensive
- MAC: 2.2 dogs, 2.58 cats, 2.3 horses
- Dose dependent CVS depression similar to iso
- Less noxious and potent
Why is anaesthesia a higher risk for horses?
- Larger, flight creatures
- Cardiopulmonary depression is more significant
- Loss of perfusion to muscle leads to inability to control limbs during recovery
- Prone to hypotension where inhalants are used
- Post-op myopathy may occur
What is the main benefit of TIVA protocols?
- Less risky
- Shorter
Outline hypoxaemia in equine anaesthesia
- Hypoxaemia common in equine anaesthesia
- Inhalants inhibit pulmonary vasoconstriction, may worsen V/Q mismatch, increase shunting in lungs
- However position, pathophysiology and low pulse pressure likely to be of more significance
Outline hypercapnia in equine anaesthesia
- Common
- Defined as >45mmHg
- Diagnosis with capnograph or blood gas analysis
- Most commonly caused by respiratory depression
- Caused by TIVA and inhalants (less with TIVA)
Compare recovery in horses with TIVA or gas anaesthetics
- Calmer recovery with TIVA
- Ataxia can be worse following TIVA
What is TIVA?
Total intravenous anaesthesia
What is PIVA?
Partial intravenous anaesthesia
How can the risks of equine GA be minimised?
- Minimise iso give
- Use infusions, local regional analgesia, or combination of all of above
- PIVA is best (iso plus something IV)
What are the benefits of PIVA?
- Reduces MAC
- Reduces cardiopulmonary depression
- Provides additional analgesia
- Improves plane of anaesthesia
- Less pollution
- Potentially better outcome
What are the different options of PIVA?
- Inhalant + lidocaine
- Inhalant + ketamine
- Inhalant + A2A
- Inhalant + opioids
What is the benefit of lidocaine and ketamine during isoflurane anaesthesia?
Improve cardiovascular stability
What is the effect of opioids on MAC in horses?
Does not decrease MAC< may increase it
What is MAC?
Minimal alveolar concentration The concentration (at 1 atm) producing immobility in 50%of patients in response to a noxious stimulus i.e. potency
What factors have no effect on MAC?
- Stimulation
- Duration
- Species
- Sex
- CO2
- NSAIDs
Outline the relationship between fat solubility of a drug and its MAC
- High fat solubility gives low MAC (slow induction and recovery, e.g. isoflurane, halothane)
- Low fat solubility gives higher MACH (faster induction and recover e.g. sevoflurane, desflurane, nitrous oxide)
How should MAC be applied in practice?
- Never exceed 1.5-2xMAC
- At 2-2.5xMAC the patient stops breathing, temperature drops and get lots of cardiovascular problem
List the factors that affect MAC
- Age
- Nitrous oxide
- Hypotension
- Hypoxia
- Anaemia
- Opioids
- Sedatives
- Local anaestheticss
- Pregnancy
Describe the potential risks of inhalational anaesthetics to animals
- Cardiorespiratory depression (vasodilation, arrhythmias, respiratory depression, halothane, hepatitis)
- Formation of carbon monoxide with soda lime
- Formation of other toxic gases
Describe the potential risks of inhalational anaesthetics to anaesthetists
- Little or no evidence except those for nitrous nitrous oxide
- Bone marrow suppression, teratogenesis
What is the importance of scavenging/COSHH protocols?
Minimise exposure of staff to inhalational anaesthetics and ensure safe removal of the agent from the air
Give examples of passive anaesthetic scavenging
- Tube out of the window
- Charcoal
Give examples of active scavening
Pumps that aid extraction into the environment
What is COSHH?
Control of Substances Hazardous to Health
Give examples of COSHH protocols for inhalational anaesthetics
- Use of scavenging mechansims
- Filling of vaporisers at the end of the day
- Flushing circuits before disconnecting
- Monitoring systems to assess exposure of staff over busy 8 hour period
- Fluosorbers can be used
Outline the importance of monitoring patients in the recovery period
- 60% of fatalities occur in the recovery period
- Half of those within 3 hours of disconnection
What are the risk factors for recovery?
- Higher ASA category
- Inhalant anaesthesia (TIVA safer)
- Breed
- Age
- Weight (riskier in obese patients)
- Duration of anaesthesia
- Drugs used in pre-med and during anaesthesia
- Temperature (cold=poor recovery)
Describe what should ideally be available in the anaesthetic recovery area
- Oxygen and delivery system
- Anaesthetic induction agents and analgesia
- Fluid therapy equipment
- Crash box/trolley for CPR
- Suctioin
- Monitoring equipment and warming devices
- PPE
- Bedding
Describe the sequence of events with regards to the breathing system at the end of the anaesthetic period
- Vaporiser switched off once all procedures are over
- If using NO2, switch off slowly and increase oxygen to deliver adeqaute FGF and minimise diffusion hypoxia
- “Dump” reservoir bag on the circuit and fill with fresh gas by increasing flow meter
Describe the process of disconnection of the patient from the breathing circuit at the end of surgery
- Disconnect and switch off oxygen
- Scavenge circuit gases
- Move patient to recovery area
- Deflate cuff on ET tube, loosen ties
- Place patient in sternal recumbency
Describe the ET tube removal after surgery
- Removed when gag reflex returns in most species
- Leave in longer in brachycephalic dogs
- In cats remove earlier to prevent laryngospasm
- In horses, remove when there is swallowing on tweaking of the tube
Why might the ET tube be left in with the cuff inflated?
- Following dental/oral surgery
- Prevents debris or blood being inhaled
Describe the signs of airway obstruction
- Increased respiratory noise and effort
- Abdominal effort, nares flaring
- “Air hunger” posture (head and neck extended)
- Cyanosis
- Restlessness and agitation
- Agonal breathing (terminal sign)
Outline the steps following identification of airway obstruction
- Call for help
- Open mouth, use laryngoscope
- Pull tongue forward and suction blood/mucus, or use swab on a stick to dry mucus
- Re-intubate if possible (may need drugs)
Describe the management of breathing in recovery
- Monitor rate, character and effort
- Pulse oximeter to assess SPO2%
- Supplement with oxygen if SPO2%<93%
- Oxygen cages/incubators/Buster collar with cling film can also be used (care re. overheating)
- Minimise stress and consider analgesia
Describe the monitoring of circulation in anaesthetic recovery
- Monitor pulse rate and quality every 5 mins in first stage
- Pulse depending on where can get access to vein
- May also monitor blood pressure, mucus membrane colour, CRT< ECG, temperature
