WSAVA Nov 2021 Proceedings - Flipbook - Page 40
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CAESAREAN ANAESTHESIA
T. Farry
Brisbane/Australia
Qualifications:
Trish Farry
RVN AVN VTS (ECC) (Anesthesia and Analgesia) GCHEd
t.farry@uq.edu.au
Maternal physiological changes
Cardiovascular
Increased demands on the cardiovascular system and hormonal changes
result in decreased cardiac reserve. Heart rate, stroke volume and plasma
volume all increase during pregnancy. Cardiac output and stroke volume
increase up to 50% and 35% respectively mainly in response to increased
plasma volume. Increases in circulating hormones cause vasodilation
and decreases in peripheral vascular resistance. Heart rate increases as a
result. Increases in plasma volume(approximately 40%), lead to dilutional
anaemia as red cell production is not proportional to that of the plasma.
Due to decreased reserve capacity and cardiovascular responses maternal
hypotension can occur rapidly. This can be exacerbated by depressant
effects of anaesthetic agents. A gravid uterus may compress the inferior
vena cava and lower aorta. Compression of the vena cava reduces venous
return resulting in decreases in preload and cardiac output. Compression
of the aorta may cause decreases in uteroplacental blood flow. As placental perfusion is directly proportional to systemic perfusion pressures,
a drop in blood pressure will affect placental perfusion and foetal oxygen
delivery. Vigilant monitoring of blood pressure with early treatment of
hypotension is essential.
Respiratory
Oxygen consumption increases by 20% due to development of the foetuses, uterus, placenta, and mammary tissue. To meet increased O2 demand,
both tidal volume and respiratory rate increase resulting in increases in
alveolar ventilation. Due to increases in minute volume, PaCO2 decreases
during gestation. Functional residual capacity (FRC) decreases due to
displacement of the diaphragm. During labour FRC decreases further as
pulmonary blood volume increases during uterine contraction. Pregnant
patients may be more prone to atelectasis.
Decreased FRC increases risk of hypoventilation and hypoxaemia which
may result in foetal hypoxaemia and acidosis. Hypoxaemia can occur
rapidly should hypoventilation or apnoea occur. Preoxygenation is recommended prior to induction.
Pregnant patients have decreased anaesthetic requirements (25 – 40%)
due to decreased FRC and the increase in progesterone and endorphin
levels within the CNS. Anaesthetic overdose is likely unless injectable and
inhalant doses are appropriately reduced.
Gastrointestinal
Decreased progesterone levels, gastric motility and physical displacement
of the stomach may result in delayed gastric emptying. Gastric secretions
are more acidic and lower oesophageal sphincter tone is decreased, and
intragastric pressure increased. Gastric motility is further decreased
during labour. Due to the altered gastric function these patients are at
increased risk of regurgitation and aspiration. When undergoing anaesthesia, the pregnant patient should always be assumed to have food in
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the stomach and induction techniques should be aimed at rapid airway
protection. Antacids and/or gastric protectants may be appropriate.
Premedication/Induction
Use of premedication will be dependent on condition and demeanour of
the patient. Most agents have profound effects on the cardiorespiratory
system which may compromise foetal perfusion.
Mu opioid receptor agonists can be administered to provide sedation and
perioperative analgesia. They have minimal cardiovascular effects but
may cause maternal respiratory depression and bradycardia. Fentanyl may
be preferable due to less accumulation. If bradycardia occurs it can be
treated with an anticholinergic. Glycopyrrolate does not cross the placental barrier whereas atropine does. Due to incomplete cardiac autonomic
development atropine is unlikely to influence foetal heart rate. Bradycardia
in the neonate is not vagally mediated and may be a response to hypoxaemia. To reverse opioid depression, naloxone can be administered to the
neonate post-delivery.
Phenothiazines are not recommended, due to the inability for underdeveloped neonatal livers to metabolise most drugs. It has a long duration
of action, causes vasodilation, and impairs thermoregulation in both
the mother and the neonate. If used, they should be at the lowest dose
possible.
The use of alpha-2 agonists is controversial. Previous research demonstrated higher doses of alpha-2’s caused 50% reduction in uterine blood
flow resulting in foetal hypoxaemia and acidosis. In 2017 a study was published where medetomidine(7ug/kg) was used as a premedicant, propofol
for induction, and sevoflurane as maintenance. The medetomidine was reversed in the puppy upon delivery and the dam post surgery. The induction
dose of propofol was reduced by 50% and Apgar scores and maternal and
foetal survival rate compared well to other common protocols.
Induction agents commonly used are propofol and alfaxalone. A 2013
study demonstrated alfaxalone had similar or better outcomes for mother
and offspring when compared to propofol. Using modified Apgar scoring,
vitality was found to be better within the first 60 minutes post delivery
compared to propofol. Both agents had similar puppy survival rates.
Shorter maternal recoveries were noted in in the alfaxalone group. In a
2016 study alfaxalone was compared as a CRI maintenance agent to isoflurane. Bitches in the CRI group recovered more slowly to the isoflurane
maintenance group and puppy Apgar scores were also lower in the CRI
group. Induction with either propofol or alfaxalone and maintenance with
an inhalant would be a reasonable protocol. It is optimal to wait at least
10 minutes before surgical delivery to enable foetal concentrations of
injectable drugs to decrease.
Maintenance of general anesthesia is commonly achieved using inhalational agents. All inhalational agents produce dose-dependent cardiovascular and respiratory depression. Isoflurane and sevoflurane are both
appropriate choices. These agents require minimal hepatic metabolism
and renal elimination. The lowest effective concentration should be used
remembering that pregnant patients have significant decreases in MAC
requirement(25 – 40%).
Locoregional anaesthetic techniques can be utilised. The use of local
anaesthetic techniques will enhance the anaesthesia plan, but care
must be taken not to delay delivery of neonates once the animal is under
anaesthesia. The epidural and CSF space are decreased by 30 - 50% due
to venous engorgement due to increased blood flow. Volumes of drugs
injected epidurally should be decreased. The use of intraoperative opioids
is controversial due to the increased risk of respiratory depression and
bradycardia in both dam and foetuses. It is common practice to administer opioids once neonates have been delivered.
Patient monitoring should include SpO2, ABP, CO2 and ECG at regular