Therapeutics and anaesthesia of the dam and neonates Flashcards
What anatomical changes occur during pregnancy?
- increased size of uterus
- cranial displacement of the stomach or altered position
- diaphragm pushed cranially
How is the stomach affected by pregnancy? What risk dis associated?
- Increased gastrin production -> increased HCl secretion -> decreased stomach pH
- Decreased gastric motility
- Reduced oesophageal sphincter tone
INCREASED RISK OF REGURGITATION + ASPIRATION PNEUMONIA
How is the respiratory system affected by pregnancy?
- Increased oxygen consumption and basal metabolic rate due to increased cardiac workload to meet the needs of the fetuses
- Minute volume (MV = RR x TV) is increased due to increased tidal volume (TV)
- Increased ventilation causes a respiratory alkalosis BUT blood pH remains normal due to renal compensation (metabolic acidosis - decreased carbon diaoxide and the kidneys will excrete more bicarb to maintain normal pH)
- Cranial diaphragm due to increased size of the uterus
- Increase in negative pleural pressure
- Reduced total lung capacity (TLC) = Reduction in residual volume (RV), expiratory reserve volume (ERV) and functional residual capacity (FRC)
- Early closure of the small airways – atelectasis (V/Q mismatch due to shunting - adequate blood flow with reduced ventilation)
- Reduced FRC in face of increased oxygen consumption – reduced oxygen reserve
= more prone to desaturation
What is normal blood pH?
7.35-7.45
How does pregnancy affect the cardiovascular system? How does it affect considerations for surgery?
- Positional compression of the vena cava in dorsal recumbency.
- Positioning for surgery! Less significant in animals cf. women (probably!)
- The autonomic nervous system is affected by general anaesthesia and volatile anaesthetic agents can cause exaggerated hypotension because of the loss of compensatory mechanisms. The institution of positive pressure ventilation may profoundly decrease cardiac output and worsen the hypotension.
- Increased cardiac output (CO):
- Increased stroke volume (SV) and heart rate (HR) [CO = SV x HR]
- Increase in venous capacitance d/t decreased systemic vascular resistance (SVR)
- Decreased reserve due to increased workload, oxygen demand and tachycardia
- 50% increase in blood volume
- Initial decrease in packed cell volume (PCV) -> normal at term
- Decreased plasma proteins
- Thrombocytopaenia (and increased turnover)
- Increase in clotting factors (VII, VIII, IX, X, XII), fibrinogen, vWF -> hypercoagulable
- Uterine blood flow = 10% of cardiac output (CO)
- Depends upon maternal CO
and uterine perfusion pressure - Minimal autoregulation
- Therefore, a reduction in maternal cardiac output affects blood flow to the uterus, placenta and fetuses
- Depends upon maternal CO
- Increased maternal 2,3-DPG
- Reduced affinity of haemoglobin for oxygen
- Right shift on the oxygen-haemoglobin saturation curve and so reduces the affinity of haemoglobin for oxygen.
- Fetal haemoglobin binds less tightly to 2,3-DPG and so has greater affinity for oxygen and is shifted to the left.
- There is a significant oxygen demand during pregnancy and a left shift i.e. alkalosis will cause reduced oxygen delivery to the fetus
What may affect how much anaesthetic is required?
Increased progesterone and neuroinhibitors (e.g. beta-endorphins) are sedative and antinociceptive
How should you prepare for a C section?
- Make a plan
- Place an IV cannula
- Start IVFT
- Get everything ready before premed/ induction! (including emergency drugs)
What should you use to premedicate before a C section?
- Currently no drugs licensed for the premedication of pregnant animals, so drug we use are off license and used following the cascade. Drugs that cross the blood-brain barrier (and therefore sedative and anaesthetic drugs) will also cross the placental barrier. Therefore, these drugs will affect the neonate as well as the dam.
- Use low end of the dose range (need up to 60% less)
- Consider an opioid: methadone
- Other premedicant drugs:
- Alpha-2 agonists could be used in very fractious or feral patients. However, they cause vasoconstriction and reduced CO which could negatively impact both the dam and the fetuses (by reducing uterine blood flow as this is reliant on maternal CO). Note that xylazine is associated with increased neonatal mortality but the same effects have not been seen with medetomidine/ dexmedetomindine. An option could also be to use a low dose of an alpha-2 agonist in the premed and antagonize it post-induction
- Acepromazine should probably be avoided or used extremely cautiously. It is long acting, irreversible and may result in hypotension through alpha 1 antagonism. The hypotension associated with acepromazine can sometimes be difficult to treat. It may make things significantly worse if for example, dehydration has been underestimated, there is significant blood loss during surgery, or it may exacerbate positional hypotension.
- Midazolam/ ketamine combinations are often recommended in compromised patients because they have minimal effects on the cardiovascular system but should probably be avoided in animals undergoing C-section. Neurological depression has been observed in puppies delivered following midazolam/ketamine anaesthesia
Benzodazepines also rapidly cross the placenta and neonates do not possess functional hepatic enzymes to metabolise them. In humans they have been associated with low Apgar scores, spells of apnoea, cyanosis, sedation and reduced suckling. - Fentanyl? Very, very lipid solubility so crosses placenta readily and may persist in neonates for longer but also very short acting??
What should you do with regard to oxygenation of a C section patient?
Pregnant animals have a higher oxygen requirement and increase in minute ventilation (MV)
- Pre-oxygenate (at least 5 minutes) to prevent hypoxaemia (use a mask or the end of the ET tube in the mouth of panting patients)
- Supplement oxygen during surgery
What should you do before inducing a patient prior to a C section? What agent should you use to induce?
Risk of regurgitation and aspiration
Prior to induction reduce risk by considering omeprazole and maropitant or ondansetron
- Prompt induction (propofol or alfaxalone NOT ketamine)
- Raise head
- Intubate swiftly
- Cuff tube with head raised
- Extubate only once laryngeal reflexes have returned
How should you maintain anaesthesia during a C section?
Maintain with isoflurane or sevoflurane but be aware that
- Minimum alveolar concentration (MAC) is reduced
- Higher cardiac output slows changes in depth initially
- Decreased FRC and increased VA results in quicker changes in depth
- Inhalation anaesthetics cause significant cardiovascular depression in both dam and the neonates
Reduce the cardiovascular depression using a balanced technique
- Opioid in the premed
- Local anaesthesia (lidocaine has rapid onset/ bupivacaine lasts longer line blocks)
- Epidural (opioid/local anaesthetic)? - appropriate only if experienced and quick
How should you monitor C section patients during surgery?
Capnography:
- Normal ETCO2 may be lower due to progesterone causing sensitivity to CO2
- May indicate hypoventilation caused by cranial diaphragm -> assist ventilation.
Pulse oximetry:
- Peripheral oxygen saturation
- If low consider positioning/ alveolar recruitment
Blood pressure: (MAP>60)
- Risk of positional hypotension, sepsis, blood loss
- Placental blood flow is dependent on BP (no autoregulation)
- Provide IVFT and treat hypotension!
What care should be given post op to a C section patient?
- Provide NSAID post-operatively
- Pain score and top-up opioid/ give buprenorphine if needed
- Discharge as soon as reasonable
How should you care for neonates post C section?
- Prepare a warm box/ cage or incubator
- Clear away any membranes and fluid from the mouth and nose
- Suction, cotton buds, bulb syringes
- Rub vigorously (avoid swinging)
- Supplement oxygen and assist ventilation
- Mouth-to-mouth, intubate and start IPPV, flow by oxygen
- Slow HR and RR indicates myocardial hypoxia (or hypothermia)
- GV26 acupuncture point (nasal philtrum)
- Consider naloxone if concerns re. opioid administration (IM, PO-sublingual, umbilical vein)
- AVOID doxapram!!
What is Apgar scoring? What does it include?
- Used to objectively assess neonatal vitality.
- Heart rate, respiratory effort (respiratory rate and type of crying), reflex irritability, motility and mucous membrane colour are measured.
- Each parameter is rated from 0 (absent) to 2 (detectable, strong).
- The sum of all parameters provides the total Apgar score.
- Apgar scoring helps to determine instantaneous neonatal health in new-born pups and kittens and could be a helpful tool in preventing and reducing neonatal death.
- A - activity P - Pulse G - grimace A - appearance R - respiration
