In the first instalment of this two-part article, we look at the physiological changes that occur in the pregnant animal, with emphasis on the bitch, and how those changes determine the planning of, and protocols for, anaesthesia of the Caesarean section. In addition to the physiology of pregnancy we will examine the goals of management of the C-section and the associated pre-anaesthetic procedures. In part 2, to be published shortly, we will explore premedication, induction, maintenance & monitoring, recovery and analgesia.
In the United Kingdom brachycephalic breeds have the highest requirement for C-section1,2 with Boston Terriers, Bulldogs and French Bulldogs accounting for over 80% of cases.2 Fewer studies have been performed examining the incidence of Caesarean sections in cats, but Robertson (2016) reported that approximately 8% of purebred kittens are delivered by C-section.
58% of canine Caesarean sections are performed as emergency procedures, and if parturition has been in progress for some time, hypovolaemia, dehydration, hypotension, exhaustion, hypothermia, hypoxia, haemorrhage, shock and toxaemia may all be present to some degree.1 In a study performed by Moon et al (1998), mortality rates amongst 808 dams requiring C-section was calculated to be approximately 1 %, with 7 of the 9 deaths in that study occurring in animals undergoing emergency C-section. 5 of the 9 deaths were attributable to pneumonia, possibly as a consequence of aspiration.
Amongst the offspring, Moon et al (2000) reported higher survival rates in non-brachycephalic breeds and puppies delivered to dams with 4 or fewer foetuses.
THE PHYSIOLOGY OF THE PREGNANT BITCH
Cardiovascular and haematological changes
During pregnancy blood volume can expand by 40% and although maternal erythrocyte production increases it does not match the elevation in plasma volume.5 This results in a “dilution anaemia” or “relative anaemia of pregnancy”.3,6 The anaemia develops between days 25 and 30 of the average 63 day pregnancy, and is most severe at full-term, when haematocrit (packed cell volume, PCV) values may be as low as 30%-35%.5 The severity of anaemia becomes more pronounced with increasing number of foetuses.3,5,6
A 30-50% increase in cardiac output, proportional to the rise in blood volume, occurs via elevations in heart rate and stroke volume. In the healthy animal a hormone-mediated reduction in peripheral vascular resistance (increased capacity of blood vessels in the uterus, mammary glands, kidneys, striated muscle and cutaneous tissues) maintains mean arterial blood pressure and uterine blood flow, despite the increased cardiac output.3,5,6,7 During pregnancy 25% of cardiac output is diverted to the uterus and placenta.3
There is no autoregulation of foetal blood flow and uteroplacental perfusion is pressure dependent.
As pregnancy proceeds, cardiac output increases and cardiac contractility reaches a maximum. In the face of hypotension or hypovolaemia, the compensatory baroreceptor mechanisms, which can be attenuated during pregnancy, may be delayed or fail.5 Patients with pre-existing heart disease and reduced cardiac reserve can rapidly decompensate and careful management of such cases is necessary.5
In women the supine position can cause mechanical compression of the aorta and caudal vena cava resulting in a reduced venous return and cardiac output with a consequential decrease in blood flow to the placenta and foetus.8 Although Probst & Webb (1983) and Probst et al (1987) did not demonstrate similar physiological changes in dorsally positioned dogs, it must be assumed that supine hypotension can occur, particularly in giant breeds or dams with large litters/oversized foetuses, and steps to reduce the incidence should be taken (see the Caesarean Section Survival Guide, Part 2).3
Based on the information mentioned above, it is essential to take steps to maintain the blood pressure of the dam, and therefore the foetuses, during elective or emergency C-sections, or any other anaesthetic procedure on a pregnant animal.
Increased collateral circulation causes the epidural veins to become engorged during pregnancy. This reduces the epidural and cerebrospinal fluid spaces by 30-50%. If extradural (epidural) anaesthesia is being considered then a smaller volume of drug will be required to achieve the same cranial extension of the block when compared to the non-pregnant animal (see the Caesarean Section Survival Guide, Part 2).6,7,11,12
Changes to the respiratory system
Metabolic rate and oxygen consumption are increased during pregnancy (oxygen consumption by approx. 20%),3,6 necessitating an increase in tidal volume (by 40%) and respiratory rate (by 10%).6 Additionally, the respiratory centre becomes more sensitive to carbon dioxide, and normal PaCO2 in pregnant animals may be as low as 30-33mm Hg.5
The gravid uterus displaces the diaphragm cranially reducing total lung volume, producing compression atelectasis7 and an approximate 20% reduction in functional residual capacity (FRC).
The decreased FRC, together with the increase in metabolic rate and oxygen demand (exacerbated by stress and/or pain), increases the risk of hypoxaemia, and supplemental oxygen should be provided to the Caesarean section patient if possible (see later under Pre-anaesthetic Stabilisation and Preparation).3,6
Gastrointestinal effects of pregnancy
The high levels of circulating progesterone during pregnancy lead to increases in gastric volume and acidity together with reduced gastric and lower oesophageal sphincter tone. Combined with the physical displacement of the pylorus by the gravid uterus, there is an increased risk of regurgitation, oesophagitis and aspiration during anaesthesia.5,6
During pregnancy, as a consequence of raised blood volume and cardiac output, renal blood flow and glomerular filtration rate increase. As a result, serum BUN and creatinine are lower than in the non-pregnant animal. Endogenous insulin resistance is also a possibility during pregnancy due to mammary secretion of growth hormone. Hyperglycaemia may occur in normal, healthy pregnant bitches and resistance to insulin therapy may also be observed in gravid diabetic animals.5
Physiological effects of pregnancy on the foetus
The haemoglobin of the foetus has a greater affinity for oxygen than maternal haemoglobin, and small reductions in the haemoglobin saturation of the dam can have considerable effects on the foetus. Maternal hypoxaemia can result in significant foetal hypoxaemia and acidosis. Vasodilation of the foetal heart and brain with vasoconstriction of the pulmonary vasculature, gut, kidneys and skeletal tissue occur as a consequence.5
Anaesthetic drug requirements
Increased progesterone and progesterone metabolites are positive modulators of the gamma-amino-butyric acid A (GABAA) receptor. Additionally, progesterone and oestrogen have antinociceptive activity in the pregnant patient. Together, these changes reduce the requirement of anaesthetic and CNS depressant drugs by 25-40% when compared to non-pregnant animal.6
SUMMARY OF THE PHYSIOLOGY OF THE PREGNANT BITCH
- Blood volume increases by approximately 40%
- Relative anaemia of pregnancy (plasma volume increases more than erythrocyte production)
- PCV 30-35% at full term.
- Cardiac output increases by 30-50% (increased heart rate and stroke volume)
- 25% cardiac output diverted to the uterus and placenta
- Pressure dependent uteroplacental perfusion
- Compensatory mechanisms following hypovolaemia or hypotension may be delayed or fail.
- Pre-existing heart disease and/or reduced cardiac reserve may rapidly decompensate.
- The supine position may compromise cardiac output
- Engorgement of vertebral canal vessels mean less volume of drug required for epidural anaesthesia
- Increased metabolic rate and oxygen consumption (approx. 20%) as pregnancy progresses.
- Increased tidal volume (approx. 40%) and respiratory rate (approx. 10%)
- Mechanical pressure on the diaphragm leading to reduced lung volume, atelectasis and reduced FRC (approx. 20%) as a result
- Increased risk of hypoxaemia
- Increased risk of regurgitation, oesophagitis and aspiration
- Lower BUN & creatinine than in a non-pregnant animal
- Potential for hyperglycaemia and insulin resistance
- Maternal hypoxia will lead to foetal hypoxia and acidosis
- Reduced anaesthetic drug requirements by 25-40%
PRE-ANAESTHETIC MANAGEMENT OF THE C-SECTION PATIENT
The goals of the C-section should include:3
- No maternal deaths
- No neonatal deaths
- Offspring with good vigour and high Apgar scores
- An alert dam willing to suckle the offspring immediately after surgery
Communication with the owner is essential as there may be controversies regarding the use of some drugs, and informed consent will be required for any “off-licence” use of products. The potential risks to both the dam and the neonates should also be communicated with the owner.
The availability of anaesthetists, surgeons, nurses and their experience of working as a team are essential for a successful outcome. Planning should include simple checklists and should also include resuscitation protocols and emergency drug availability for the offspring (and the dam).
A standard anaesthesia checklist should be in place before a Caesarean section is performed (e.g. the Association of Veterinary Anaesthetists checklist, which can be downloaded here). For further information on checklists please follow this link to access “Are you using safety checklists in your practice?” by Veterinary Anaesthetist Carl Bradbrook BVSc CertVA DipECVAA MRCVS. This article includes several downloadable resources and is a mine of information.
In addition, a C-section-specific checklist should be utilised, an example of which may viewed below.
C-SECTION SPECIFIC CHECKLIST
(Broadly based on a checklist published by Robertson, 2016)
- Mask (ideally) or other facilities for preoxygenation of the dam
- Number of offspring expected (ultrasound/x-ray if time permits)
- Laboratory tests: PCV, BUN, creatinine
- Intravenous cannula placed
- Fluid therapy initiated
- Doses of emergency drugs, including fluids and vasopressors calculated
- Surgeon(s) ready and scrubbed before induction
- All surgical materials and equipment prepared and ready for use
- Resuscitation team, materials and drugs prepared
PRE-ANAESTHETIC STABILISATION AND PREPARATION
Physical examination & clinical history
As with all patients, a thorough physical examination should be undertaken, and a comprehensive clinical history obtained.
The dam should be handled calmly and in a considerate manner to minimise excitement. In animals experiencing stress the catecholamine release can reduce blood flow to the uterus and foetus. It is particularly important to handle patients with pre-existing heart disease carefully as their cardiac reserve is reduced and stress or pain can result in rapid cardiac decompensation.5
Imaging (radiography or ultrasonography) should be performed, if time permits, determine the number of foetuses. Ultrasonography will give an indication of neonatal viability.
If time permits, simple, rapid laboratory tests should be considered. As a minimum:
Packed cell volume. This can provide information on the hydration status of the dam. As mentioned above, there is a relative anaemia during pregnancy with a haematocrit of 30-35% at full term in the dog. A normal canine PCV would be in the range 39-55%.13 If the PCV approaches non-pregnant values then dehydration must be considered.
BUN, creatinine: Expect lower values than in the non-pregnant animal. Levels approaching normal may, as for the PCV, suggest dehydration and possible renal compromise.
An intravenous cannula should be placed. This will allow the administration of fluids, anaesthetic and other agents “to effect”, and will also permit rapid delivery of emergency drugs.
Intravenous fluid therapy.
Intravenous fluid therapy is recommended for C-sections. It is not unusual for the dam to be dehydrated at presentation and electrolyte, acid-base, calcium and glucose abnormalities should be corrected before surgical intervention whenever possible.5 Crystalloids are suitable although, if dehydration is severe, colloids may be required for rapid volume replacement.3
The potential for hypoxia in the dam, and therefore the foetuses, is greater than in the majority of healthy non-pregnant animals. This is due to decreased functional residual capacity and increased metabolic rate. In brachycephalic breeds the risk is exacerbated further. McNally et al (2009) reported that, compared to room air, oxygen supplied via a facemask (100 ml/kg/minute) for three minutes increased the time to desaturation from 69.6 ± 10.6 seconds to 297.8 ± 42.0 seconds. Although this study was performed in non-pregnant animals preoxygenation is still likely to be beneficial.3 Therefore, provided preoxygenation does not significantly stress the dam it should always be attempted.
Pre-clip and initial prep
The clipping and initial preparation of the of surgical site prior to induction of anaesthesia, with the aim of reducing anaesthetic and surgical time up to delivery of the neonates, is beneficial if the dam will allow the procedures to be performed.
Due to the changes in gastrointestinal physiology discussed earlier there is a greater risk of reflux, regurgitation and aspiration. Administration of a suitable antiemetic e.g. maropitant, a neurokinin receptor antagonist should be considered.3 The 30-60 minute time to onset of action following subcutaneous administration should be taken into account, and the product administered as soon as possible. Maropitant has an added benefit of potentially reducing inhalant anaesthetic requirements and providing a degree of visceral analgesia.15 Maropitant is not currently licenced in the UK for administration to the pregnant animal, therefore it’s use must be following a risk-benefit analysis by the attending Veterinary Surgeon.
SUMMARY OF THE PRE-ANAESTHETIC MANAGEMENT OF THE CAESAREAN SECTION PATIENT
- Communication with owner & informed consent
- Comprehensive clinical history and physical examination
- Experienced staff available: anaesthetists, surgeons, nurses, support personnel
- Checklists available: AVA & C-section specific
- Resuscitation protocols and emergency drugs available
- Considerate handling to minimise excitement
- Imaging if time permits – number and viability of neonates
- Simple laboratory analysis: PCV, BUN, creatinine. Other tests if time permits.
- IV cannulation. Permits IV fluid therapy, anaesthetic and other drugs can be titrated. Immediate IV access for emergency drugs
- IV fluid therapy
- Pre-clip and initial surgical prep if the dam is amenable
- Consider antiemetic administration
- Reduced anaesthetic and CNS depressant drug requirements
A downloadable bundle including the Summary of the Physiology of the Pregnant Bitch, C-Section Specific Checklist and the Summary of Pre-anaesthetic Management of the Caesarean Section Patient may be obtained here.
Key an eye open for part 2 of the Caesarean Section Survival Guide which will be published on 27th September 2018. In this second instalment we will examine premedication, induction, maintenance & monitoring, recovery and analgesia.
Originally published: Thursday, 13th September 2018
- Moon PF et al (1998). Perioperative management and mortality rates of dogs undergoing caesarean section in the United States and Canada. JAVMA 213, 365-369
- Evans KN & Adams VJ (2010). Proportion of litters of purebred dogs born by caesarean section. JSAP 51, 113-118
- Robertson S (2016). Anaesthetic management for caesarean sections in dogs and cats. In Practice 38, 327-339
- Moon PF et al (2000). Perioperative risk factors for puppies delivered by caesarean section in the United States and Canada. JAVMA 36, 359-368
- Ryan SD & Wagner AE (2006). Caesarean section in dogs: Physiology and perioperative considerations. Compendium. 34-43
- BSAVA Manual of Canine and Feline Anaesthesia and Analgesia. 3rd edition. 2016.
- Kushnir Y & Epstein A (2012). Anesthesia for the pregnant dog and cat. Israel J. Vet Med. 67, 19-23
- Tan EK & Tan EL (2013). Alterations in physiology and anatomy during pregnancy. Best Practice & Res: Clinical Obs & Gynae. 27, 791-802
- Probst CW & Webb AI (1983). Postural influence on systemic blood pressure, gas exchange and acid/base status in the term-pregnant bitch during general anesthesia. Am J Vet Res. 44, 1963-1965
- Probst CW et al (1987). Postural influence on systemic blood pressure in large full-term pregnant bitches during general anesthesia. Vet Surgery. 16, 471-473
- Marx CE (1979). Physiology of pregnancy: High risk implications. Am Soc Anesth. 1251-1254
- Schnider SM (1978). The physiology of pregnancy. Am Soc Anesth. 1251-1258
- BSAVA Manual of Advanced Veterinary Nursing. 1999
- McNally EM et al (2009). Comparison of time to desaturation between preoxygenated and non-preoxygenated dogs following sedation with acepromazine maleate and morphine and induction of anaesthesia with propofol. Am J Vet Res. 70, 1333-1338
- Boscan P et al (2011). Effect of maropitant, a neurokinin 1 receptor antagonist, on anesthetic requirements during noxious visceral stimulation of the ovary in dogs. Am J Vet Res. 72, 1576-1579
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