Obstetric Anaesthesia and Analgesia
Obstetric anaesthesia and analgesia involve caring for women during childbirth in three situations:
provision of analgesia for labour, usually by epidural or spinal analgesic techniques
anaesthesia for instrumental (e.g. forceps or Ventouse) or caesarean delivery
ANATOMY AND PHYSIOLOGY OF PREGNANCY
Progesterone
The hormone progesterone may be considered the most important physiological substance in pregnancy. It is secreted initially in increasing amounts during the second half of the menstrual cycle to prepare the woman for pregnancy. Following conception, the corpus luteum ensures adequate blood concentrations until placental secretion is adequate. The most important physiological role of progesterone is its ability to relax smooth muscle. All other physiological changes stem from this pivotal function (Fig. 35.1).
FIGURE 35.1 Summary of the main actions of progesterone – it establishes the maternal physiological adaptation to pregnancy. PaCO2, arterial carbon dioxide tension; ODC, oxyhaemoglobin dissociation curve; P50, partial pressure of oxygen when haemoglobin is 50% saturated at pH 7.4 and temperature 37°C; bicarbonate.
Haemodynamic Changes
Blood volume increases from 65–70 to 80–85 mL kg–1 mainly by expansion of plasma volume, which starts shortly after conception and implantation and is maximal at 30–32 weeks (Fig. 35.2). Red cell volume increases linearly but not as much as plasma volume (Table 35.1). Thus, the haematocrit decreases, causing the ‘physiological anaemia’ of pregnancy.
TABLE 35.1
Haematological Changes Associated with Pregnancy
Variable | Non-Pregnant | Pregnant |
Haemoglobin | 14 g dL–1 | 12 g dL–1 |
Haematocrit | 0.40–0.42 | 0.31–0.34 |
Red cell count | 4.2 × 1012 L–1 | 3.8 × 1012 L–1 |
White cell count | 6.0 × 109 L–1 | 9.0 × 109 L–1 |
Erythrocyte sedimentation rate | 10 | 58–68 |
Platelets | 150–400 × 109 L–1 | 120–400 × 109 L–1 |
The increase in blood volume is accompanied by an increase in cardiac output (Fig. 35.3) within the first 10–12 weeks by approximately 1.5 L min–1. By the third trimester, cardiac output has increased by about 40–50% as a result of significant increases in heart rate and stroke volume (Table 35.2). In labour, cardiac output may increase by a further 45%.
TABLE 35.2
Cardiovascular Changes in Pregnancy
Variable | Change | % change |
Heart rate | Increased | 20–30% |
Systolic blood pressure | Decreased | 10–15% 2nd trimester |
Diastolic blood pressure | Decreased | |
Stroke volume | Increased | 20–50% |
Cardiac output | Increased | 40–50% by 3rd trimester |
Systemic vascular resistance | Decreased | 20% |
Central venous pressure | Unchanged | |
Pulmonary vascular resistance | Decreased | 30% |
Pulmonary capillary wedge pressure | Unchanged |
FIGURE 35.3 Diagrammatic representation of changes in blood flow to various organs during pregnancy, together with percentage changes in cardiac output, and blood and plasma volumes.
Respiratory Changes
Respiratory function undergoes several important modifications (Table 35.3), also as a result of the actions of progesterone. The larger airways dilate and airway resistance decreases. There are increases in tidal volume (from 10 to 12 weeks’ gestation) and minute volume (by up to 50%). Progesterone exerts a stimulant action on the respiratory centre and carotid body receptors.
TABLE 35.3
Changes in Respiratory Function in Pregnancy
Variable | Non-Pregnant | Term Pregnancy |
Tidal volume ↑ | 450 mL | 650 mL |
Respiratory rate | 16 min–1 | 16 min–1 |
Vital capacity | 3200 mL | 3200 mL |
Inspiratory reserve volume | 2050 mL | 2050 mL |
Expiratory reserve volume ↓ | 700 mL | 500 mL |
Functional residual capacity ↓ | 1600 mL | 1300 mL |
Residual volume ↓ | 1000 mL | 800 mL |
PaO2 slight ↑ | 11.3 kPa | 12.3 kPa |
PaCO2 ↓ | 4.7–5.3 kPa | 4 kPa |
pH slightly ↑ | 7.40 | 7.44 |
PaO2, arterial oxygen tension; PaCO2, arterial carbon dioxide tension.
The functional residual capacity (FRC) and residual volume are reduced at term because of the enlarged uterus (Table 35.3). This substantial reduction, combined with the increase in tidal volume, results in large volumes of inspired air mixing with a smaller volume of air in the lungs. The composition of alveolar gas may be altered with unusual rapidity and alveolar and arterial hypoxia develop more quickly than normal during apnoea or airway obstruction. In normal pregnancy, closing volume does not intrude into tidal volume.
The incidence of failed intubation in term parturients is approximately 1 in 300 cases, compared with 1 in 2200 in the non-pregnant population. This is caused in part by changes in pregnancy which affect the airway (Table 35.4). These factors increase the difficulty in seeing the larynx and increase the rate at which hypoxaemia develops in an apnoeic patient.
TABLE 35.4
Physiological Changes of Pregnancy Which Increase the Risk of Hypoxaemia
Interstitial oedema of the upper airway, especially in pre-eclampsia
Enlarged tongue and epiglottis
Enlarged, heavy breasts which may impede laryngoscope introduction
Increased oxygen consumption
Restricted diaphragmatic movement, reducing FRC
Renal Changes
These changes are shown in Table 35.5. Renal blood flow is increased (Fig. 35.3). By 10–12 weeks, glomerular filtration rate (GFR) has increased by 50% and remains at that level until delivery. Glycosuria often occurs because of decreased tubular reabsorption and the increased load. The renal pelvis, calyces and ureters dilate as a result of the action of progesterone and intermittent obstruction from the uterus, especially on the right.
TABLE 35.5
Parameter | Non-Pregnant | Pregnant |
Urea (mmol L− 1) | 2.5–6.7 | 2.3–4.3 |
Creatinine (μmol L− 1) | 70–150 | 50–75 |
Urate (μmol L− 1) | 200–350 | 150–350 |
Bicarbonate (mmol L− 1) | 22–26 | 18–26 |
24 hour creatinine clearance | Increased |
Gastrointestinal Changes
These also stem from the effects of progesterone on smooth muscle.
Changes in liver function are summarized in Table 35.6.
TABLE 35.6
Liver Function Changes in Pregnancy
Parameter | Change in Pregnancy |
Albumin | Decreased |
Alkaline phosphatase | Increased (from placenta) |
ALT/AST | No change |
Plasma cholinesterase | Decreased |
Haematological Changes
Haemoglobin concentration decreases from 14 to 12 g dL–1 (Table 35.1). Cell-mediated immunity is depressed. There is an increase in platelet production but the platelet count falls because of increased activity and consumption. Platelet function remains normal. Haematological changes return to normal by the sixth day after delivery.
Coagulation
Pregnancy induces a hypercoagulable state. These changes are summarized in Table 35.7.
TABLE 35.7
Coagulation Changes in Late Pregnancy
Fibrinogen increased from 2.5 (non-pregnant value) to 4.6–6.0 g L–1
Factor II slightly increased
Factor V slightly increased
Factor VII increased 10-fold
Factor VIII increased – twice non-pregnant state
Factor IX increased
Factor X increased
Factor XI decreased 60–70%
Factor XII increased 30–40%
Factor XIII decreased 40–50%
Antithrombin IIIa decreased slightly
Plasminogen unchanged
Plasminogen activator reduced
Plasminogen inhibitor increased
Fibrinogen-stabilizing factor falls gradually to 50% of non-pregnant value
The Epidural And Subarachnoid Spaces
The epidural space is the space between the periosteal lining of the vertebral canal and the spinal dura mater. It contains spinal nerve roots, lymphatics, blood vessels and a variable amount of fat (Figs 35.4, 35.5). Its boundaries are as follows:
FIGURE 35.4 The vertebral column. Note that the spinal cord ends at the level of L1 or L2 and that the dural sac extends to the level of the S2 vertebra.
superiorly – the foramen magnum, where the dural layers fuse with the periosteum of the cranium; hence, local anaesthetic solution placed in the epidural space cannot extend higher than this
inferiorly – the sacrococcygeal membrane
anteriorly – the posterior longitudinal ligament
posteriorly – the ligamentum flavum and vertebral laminae
laterally – the pedicles of the vertebrae and the intervertebral foramina.
Spread of local anaesthetic in either the subarachnoid or epidural space is more extensive as a result of the reduced volume.
Progesterone-induced hyperventilation leads to a low PaCO2 and a reduced buffering capacity; thus, local anaesthetic drugs remain as free salts for longer periods.
Pregnancy itself produces antinociceptive effects. The onset of nerve block is more rapid, and human peripheral nerves have been shown to be more sensitive to lidocaine during pregnancy. Increased plasma and CSF progesterone concentrations may contribute towards the reduced excitability of the nervous system.
Increased pressure in the epidural space facilitates diffusion across the dura and produces higher concentrations of local anaesthetic in CSF.
Venous congestion of the lateral foramina decreases loss of local anaesthetic along the dural sleeves.
Pain Pathways in Labour and Caesarean Section
The afferent nerve supply of the uterus and cervix is via Aδ and C fibres which accompany the thoracolumbar and sacral sympathetic outflows. The pain of the first stage of labour is referred to the spinal cord segments associated with the uterus and the cervix, namely T10–T12 and L1. Pain of distension of the birth canal and perineum is conveyed via S2–S4 nerves (Fig. 35.6). When anaesthesia is required for caesarean section, all the layers between the skin and the uterus must be anaesthetized. It is important to remember that the most sensitive layer is the peritoneum, and therefore the block should extend up to at least T4 and also include the sacral roots (S1–S5) to cold and T5 to touch.
The Placenta
Functions of the Placenta
Transport of Respiratory Gases: This is the most important function of the placenta and is described above.
Hormone Production: Human chorionic gonadotrophin (hCG) is secreted by placental syncytiotrophoblasts and production commences very early in pregnancy and peaks at 8–10 weeks. Its role is to stimulate the corpus luteum to secrete progesterone. hCG levels increase again near term gestation but its role in late pregnancy is unclear.
Oestrogens are secreted by the placenta and have a role in breast and uterus development.
Alkaline phosphatase is secreted by the placenta but its role in pregnancy is uncertain.
Immunological: The placenta modifies the fetal and maternal immune system so that the fetus is not rejected.
Placental Transfer of Drugs: The barrier between maternal and fetal blood is a single layer of chorion united with fetal endothelium. The surface area of this is vastly increased by the presence of microvilli. Placental transfer of drugs occurs, therefore, by passive diffusion through cell membranes which are lipophilic. However, this membrane appears to be punctuated by channels which allow transfer of hydrophilic molecules at a rate that is around 100 000 times lower.
Factors Determining Placental Transfer
Materno-Fetal Concentration Gradient: Drug transfer occurs down a concentration gradient in either direction. The maternal drug concentration depends on the route of administration, dose, volume of distribution, drug clearance and metabolism. The highest concentration is achieved after intravenous administration, although epidural and intramuscular administration result in similar concentrations. Fetal drug concentration depends on the usual factors of redistribution, metabolism and excretion. The fetus eliminates drugs less effectively due to immature enzyme systems. The distribution differs because of the anatomical and physiological organization of the fetal circulation; for example, drugs accumulate in the liver because of the umbilical venous flow to the liver and are metabolized before distribution. The relatively high extracellular fluid volume explains the large volumes of distribution of local anaesthetics and muscle relaxants.
Molecular Weight and Lipid Solubility: The placental membrane is freely permeable to lipid-soluble substances, which undergo flow-dependent transfer. The majority of anaesthetic drugs are small (molecular weights of less than 500 Da) and lipid-soluble and so cross the placenta readily. The main exceptions are the neuromuscular blocking drugs.
Protein Binding: A dynamic equilibrium exists between bound (unavailable) and unbound (available) drug. Reduced albumin concentration increases the proportion of unbound drug. Many basic drugs are bound to α1-glycoprotein, which is present in much lower concentrations in the fetus than in the adult.
Degree of Ionization: The placental membrane carries an electrical charge; ionized molecules with the same charge are repelled, while those with the opposite charge are retained within the membrane. The rate of this permeability-dependent transfer is inversely proportional to molecular size. Size limitation for polar substances begins at molecular weights between 50 and 100 Da. Ions diffuse much more slowly. Factors affecting the degree of ionization alter the rate of transfer.
Maternal and Fetal pH: Changes in maternal or fetal pH alter the degree of ionization and protein binding of a drug, and thus its availability for transfer. This has most significance if the pKa is close to physiological pH (local anaesthetics), and is clinically relevant in the acidotic fetus. Fetal acidosis increases the ionization of the transferred drug, which is then unable to equilibrate with the maternal circulation, resulting in accumulation of the drug. This is known as ion trapping.
PHARMACOLOGY OF RELEVANT DRUGS
Tocolytic Drugs
BASIC OBSTETRICS
contractions occurring every 3 min and lasting 45 s
progressive dilatation of the cervix (approximately 1 cm h–1)
progressive descent of the presenting part
vertex presenting with the head flexed and the occiput anterior
labour not < 4 h (precipitate) or > 18 h (prolonged)
delivery of a live healthy baby
delivery of a complete placenta and membranes
The First Stage of Labour
Initially, the cervix effaces (i.e. becomes thin along its vertical axis and soft in consistency) and then cervical dilatation begins. The rate of cervical dilatation should be about 1 cm h–1 for a nulliparous woman and 2 cm h–1 for a multiparous woman. It is standard practice to examine the woman every 4 h, or more frequently if there is cause for concern. Routine observations are made as per National Institute of Clinical Excellence (NICE) guidelines and these are charted on the partogram (see Fig. 35.7):
fetal heart rate (FHR) every 15 min
blood pressure and temperature 4-hourly
frequency of emptying of the bladder
frequency of contractions half-hourly