Therapeutics and anaesthesia of the dam and neonates Flashcards
Anatomical changes during pregnancy
- increased size of uterus
- cranial displacement of the stomach or altered position
- diaphragm pushed cranially
Changes during pregnancy - stomach
- increased gastrin production -> increased HCl secretion -> decreased stomach pH
- decreased gastric motility
- reduced oesophageal sphincter tone
- this with increased pressure in the abdomen -> increased risk of regurgitation + aspiration pneumonia
— this becomes worse the closer the animal is to term
Changes during pregnancy - respiratory system
- increased oxygen consumption and basal metabolic rate due to increased cardiac workload to meet the needs of the foetuses
- to meet this, minute volume (MV) is increased due to increased tidal volume (TV)
- in most cases resp rate doesn’t increase significantly
- progesterone sensitises the respiratory centre to the effects of CO2
- increased ventilation causes a respiratory alkalosis but blood pH remains normal due to renal compensation
- therefore there will be decreased CO2 and the kidneys will excrete more bicarbonate to maintain normal pH
- the increased size of the uterus pushes the diaphragm cranially
- negative pleural pressures helps maintain inflation of the lungs, but this pressure is increased
- therefore, the small airways close early (atelectasis) -> V/Q mismatch due to shunting (adequate blood flow with reduced ventilation)
- this means that pregnant animals more readily develop atelectasis, and are therefore more prone to desaturation
Changes during pregnancy - CV system
- dorsal recumbency may result in compression of the aorta and caudal vena cava
- therefore care re positioning for surgery
– however this is potentially less significant in animals cf women - the autonomic nervous system is affected by GA and volatile anaesthetic agents can cause exaggerated hypotension because of the loss of compensatory mechanisms
- the institution of positive pressure ventilation may profoundly decreased CO and worsen the hypotension
Increased cardiac output -> decreased reserve due to increased workload, oxygen demand and tachycardia
50% increase in blood volume
- initial decrease in PCV -> normal at term
- decreased plasma proteins
- thrombocytopenia (& increased turnover)
- increase in clotting factors (VII, VIII, IX, X, XIII), fibrinogen, vWF -> hypercoagulable
Uterine blood flow = 10% of CO
- depends on maternal CO and uterine perfusion pressure
- minimal autoregulation
Increased maternal 2,3-DPG
- right shift on the oxygen-haemoglobin saturation curve -> reduced affinity of haemoglobin for oxygen
- facilitates movement of oxygen across the placenta
Changes during pregnancy - effects of progesterone and neuroinhibitors (e.g. beta-endorphins)
- sedative and anti-nociceptive
- therefore requirements of anaesthetic and CNS depressant drugs are reduced (up to 60%)
C-section pt - preparation
- make a plan
- place IV cannula
- start IVFT
- get everything ready before premed/induction (including emergency drugs)
- also consider resuscitation of the pups i.e. enough people and a warm/dry box
C-section pt - premed, including which drugs to and NOT to use
- use low end of the dose range (need up to 60% less) and short acting drugs
- consider an opioid: methadone
- currently no drugs licensed for premed of pregnant animals
- drugs that cross the BBB (i.e. sedative and anaesthetic drugs) will also cross the placental barrier
- remember progesterone is sedative and antinociceptive, therefore the requirements of anaesthetic and CNS depressant drugs are reduced (up to 60%)
- ensure premed including analgesia: an opioid alone (e.g. methadone) is a sensible choice in many pts
OTHER PRE-MED DRUGS
Alpha-2 agonists:
- could be used in very fractious or feral pts
- but they cause vasoconstriction and reduced CO which can negatively impact both the dam and foetuses
- xylazine is associated with increased neonatal mortality but the same effects have not been seen in medetomidine/dexmedetomidine
- alternative: use low dose alpha-2 agonist in premed and antagonise in post-induction
Acepromazine:
- should probs be avoided or used extremely cautiously
- long acting, irreversible, may result in hypotension through alpha-1 antagonism
- hypotension associated can be difficult to treat
- may make things significantly worse if e.g. dehydration have been underestimated, there’s significant blood loss during sx, or it may exacerbate positional hypotension
Midazolam/ketamine
- combinations often recommended in compromised pts because they have minimal effects on the CV system but should probs be avoided in animals undergoing c-section
- neurological depression has been observed in pups
- benzodiazepines also rapidly cross the placenta and neonates don’t possess functional hepatic enzymes to metabolise them
- in humans they’ve been associated with low Apgar scores, spells of apnoea, cyanosis, sedation and reduced suckling
Fentanyl:
- very very lipid soluble, so crosses placenta readily and may persist in neonates for longer, but also very short acting??
C-section pt - preoxygenation
Pregnant animals have a higher oxygen requirement and increase in MV
- pre-oxygenate to prevent hypoxaemia (use a mask or the end of the ET tube in the mouth of panting pts) for at least 5 mins before induction
- supplement oxygen during delivery
C-section pt - induction
Risk of regurgitation and aspiration
- prior to induction reduce risk by considering omeprazole or maropitant or ondansetron
— maropitant is an anti-emetic licensed in dogs and cats, give with pre-med or earlier as it can take 30-60mins to effect
— ondansetron is not licensed but has a quicker onset of action (5-15mins) so is more suitable in an emergency
— omeprazole is common in anaesthetised pts, studies suggest that administration is a good idea in at risk animals
- prompt induction (propofol or alfaxalone NOT ketamine)
— ket causes less CV depression in dams but may have significant depressant effects on neonates, associated with a decreased likelihood of all pups breathing spontaneously at birth and greater neonatal depression cf with use of other induction agents
- raise head
- intubate swiftly
- cuff tube with head raised
- (extubate only once laryngeal reflexes have returned)
- having suction ready as a precaution is good idea (as well as for pups)
C-section pt - positioning
Tilt table and position carefully
- but not too much as to compromise venous return
- avoids exacerbating reduction in FRC and associated atelectasis and V/Q mismatch
- may reduce aortovenous compression
- reduces the risk of regurgitation
C-section pt - maintenance
Maintain with iso or servo but be aware that
- sevo has a lower blood gas solubility so diffuses into tissues more quickly and will result in quicker changes in depth
- MAC is reduced
- higher CO slows changes in depth initially
- decreased FRC (functional residual capacity) and increased Va results in quicker changes in depth
- inhalation anaesthetics cause significant CV depression in both dam and neonates
Reduce the CV depression using a balanced technique
- opioid in premed
- LA (lidocaine/bupivacaine line blocks)
– lidocaine is good choice as has rapid onset of action
– bupivacaine lasts longer but has a slower onset of action
- epidural (opioid/LA?)
– don’t use in pts with sepsis or hypotension
C-section pt - monitoring
Capnography
- normal ETCO2 may be lower due to progesterone causing sensitivity to CO2
- may indicate hypoventilation caused by cranial diaphragm -> assist ventilation
– but don’t hyperventilate as can cause placental vasoconstriction
- try to maintain ETCO2 normal/near normal
Pulse oximetry
- peripheral oxygen saturation
- if low consider positioning/alveolar
Blood pressure
- risk of positional hypotension, sepsis, blood loss
- placental blood flow is dependent on BP
- provide IVFT and treat hypotension
C-section pt - post-op
- provide NSAID post-op
— in humans NSAIDs are considered safe due to low excretion in breast milk, same is likely to apply to our spp
– can be started in the immediate postop period and continue as required for 1-2d after discharge - pain score and top-up opioid / give buprenorphine if needed
- discharge as soon as reasonable
— allows them to bond with pups/kittens
Neonates - post-op care
What is a suitable choice of induction agent for a dog undergoing a c-section?
- either propofol or alfaxalone
In a pregnant animal, how might the reduction in FRC affect tissue oxygen delivery?
- reduces oxygen delivery by causing ventilation/perfusion mismatch
