Obstetric Anaesthesia and Analgesia Arun Kumar Paul
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Physiological ConsiderationsCHAPTER 1

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The internal female genital organs include the ovaries, the uterus, fallopian tubes and the vagina (Fig. 1.1). Following ovulation, the ovum is released from a ripened graafian follicle and gathered into the fimbriated end of the fallopian tube. Spermatozoa passes up through the vagina and cervical canal and meets the ovum in the tube and fertilisation takes place there. The fertilised ovum forms a blastocyst and it passes down the uterine cavity. Ultimately it becomes implanted in the endometrium of uterus. Various structural changes occur within the uterus, and it grows to maturity during gestation in about 40 weeks, if pregnancy continues effectively. But if fertilisation does not take place, the endometrial lining is shed cyclically under hormonal control out of the uterus accompanied by bleeding in the act of menstruation.
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Fig. 1.1: Female pelvis with genital organs
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Bisexual reproduction results from the fertilisation of a female egg cell, ovum by a male sperm cell, spermatozoa. Usually one ovum is produced and released at a time in about 28 days during child bearing age, but the spermatozoa are produced in large numbers by the two testes.
During pregnancy there is progressive anatomical and physiological changes not only of the genital organs but also to all systems of the body. This is mostly to adopt the increasing need of the growing foetus.
 
GENITAL ORGANS
  • Vulva: oedematous, hyperaemic, pigmented.
  • Vagina: hypertrophied, oedematous, increased vascularity, secretion increased, thin and white and acidic discharge.
  • Uterus: increased growth and weight, enlarged. Muscles hypertrophied, fibrous and elastic tissues increased, endometrium changes to decidua, structural and functional changes in isthmus. Hypertrophy and hyperplasia and increased vascularity of the cervix. Cervix becomes bulky.
    Fallopian tube: congested, length increased.
  • Ovary: growth of corpus luteum, bright orange later becomes yellow and finally pale.
  • Breasts: best evident in primigravida. Size increases, vascularity increases, nipples become larger, erectile and deeply pigmented. Secretion can be squeezed out at the breast at about 12th week. It becomes thick and yellowish by about 16th week.
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Cutaneous: Pigmentation on face, breasts, abdomen striae graviderum, linea nigra present.
Weight gain: Progressive, may be variable, total weight gain may be 11 kg on average. Periodic and regular weight checking is important. Rapid gain of weight of more than 0.5 kg a week or more than 2 kg a month may indicate toxaemia of pregnancy. Stationary or falling weight may suggest intrauterine growth retardation or intrauterine foetal death. Poor weight gain may be due to prematurity or dysmaturity. Obese patients are likely to gain more weight.
Body fluid and electrolytes: During pregnancy there is variable amount of retention of electrolytes — sodium, potassium and chloride. The retention is mostly due to increased oestrogen and progesterone, increased aldosterone, increased antidiuretic hormone. Water retention during pregnancy at term may be roughly 6 litres.
 
Haematological Changes
  1. The blood volume is markedly increased during pregnancy. It starts to rise from about 10th week and to maximum about 30% above the nonpregnant level at about 30 to 32 weeks and then remains stable till term.
  2. Plasma volume increases, the maximum to the extent of 45%.
  3. Red blood cell volume and haemoglobin increase to about 20% due to increased demand of oxygen transport during pregnancy. The disproportionate increase in plasma and RBC volume produces haemodilution and there are some manifestations of anaemia.
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  4. Neutrophilic leucocytosis may also occur.
  5. Total plasma protein increases. But due to haemodilution the plasma protein concentration falls. There is marked fall in albumin level and rise in globulin level and thus the normal albumin/globulin ratio is diminished.
 
Blood Coagulation Factors
  1. Fibrinogen level increases.
  2. Platelets usually not affected, but may decrease.
  3. Fibrinolytic activity depressed.
  4. Erythrocyte sedimentation rate markedly increases.
  5. Increased coagulability.
 
Heart and Circulation
  1. Heart is somewhat pushed upwards and outwards with slight rotation to left due to elevated diaphragm. Apex beat is shifted.
  2. Pulse rate is slightly raised by about 15%.
  3. Systolic blood pressure decreases by 0 to 15 mmHg. Diastolic blood pressure decreases by 10 to 20 mmHg.
  4. Cardiac output is elevated during first trimester by 30 to 40%. This is related to a 15% increase in heart rate and a 30% increase in stroke volume. The cardiac output further increases during labour with uterine contractions and bearing down efforts.
  5. Blood pressure usually decreases during pregnancy secondary to about 15% decrease in total peripheral resistance.
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  6. Central venous pressure usually does not alter, but compression on inferior vena cava by gravid uterus may raise femoral venous pressure by about 15%.
  7. Some mothers at near-term may show reduced blood pressure and this may be associated with diaphoresis, nausea/vomiting, paleness, etc. particularly when in supine position. It is termed as supine hypotensive syndrome which is due to compression of inferior vena cava and/or abdominal aorta by gravid uterus. It also results uteroplacental insufficiency and foetal hypoxia. Left uterine displacement may be helpful in such cases.
  8. Uterine blood flow is increased. Pulmonary blood flow increases. Renal blood flow also increases during pregnancy.
  9. Anaesthetic implications:
    1. Increased cardiac output may hasten the rate of IV induction and slow the rate of inhalational induction.
    2. Compression of inferior vena cava by gravid uterus causes dilatation of azygos and epidural veins. This epidural venous engorgement decreases the epidural space. Thus the dose of drugs during block should be reduced.
 
Metabolic Changes
  1. Total metabolism increases. Basal metabolic rate is increased by about 30%.
  2. Protein metabolism: Positive nitrogen balance is found during pregnancy. Blood urea level falls by 20%.
  3. Carbohydrate metabolism: There is increased carbo-hydrate metabolism. Glycosuria may occur.
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  4. Fat metabolism: Plasma lipids increase significantly during pregnancy. Total lipid, cholesterol and phospholipid increase by about 40%.
  5. Iron requirement during pregnancy increases.
  6. Calcium metabolism: During pregnancy there is increased demand of calcium by the growing foetus.
 
Gastrointestinal System
  1. The motility of GI tract is decreased by the middle of second trimester most probably due to progesterone effect.
  2. The position of the stomach becomes more cephalad and horizontal due to pushing by the enlarged uterus.
  3. Gastric emptying time is slowed.
  4. Function of gastroesophageal sphincter disturbed.
  5. Gastrin production is increased causing more acid production.
  6. Increased risk of regurgitation and acid aspiration syndrome.
 
Hepatic Changes
  1. Liver blood flow usually does not change.
  2. There may be increase in liver function tests.
  3. Transaminases increase.
  4. Alkaline phosphatase increases.
  5. Pseudocholinesterase activity decreases by about 20%.
  6. Prolonged action of succinylcholine can occur.
  7. Prolonged activity of local anaesthetics from pseudocholinesterase deficiency is of little clinical significance.
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Renal Changes
  1. Renal blood flow increases during pregnancy.
  2. Glomerular filtration rate increases.
  3. Serum creatinine and blood urea nitrogen decrease.
  4. Creatinine clearance increases.
  5. Glycosuria may occur.
  6. Proteinuria may occur.
  7. Compression of the aorta by enlarged uterus may reduce glomerular filtration rate and renal blood flow at the third trimester. Compression of ureter may also occur. All these may lead to urinary tract infection.
 
Nervous System
  1. Minimum alveolar concentration (MAC) for volatile anaesthetics may be decreased by about 40% mostly due to progesterone and endorphin effect.
  2. Dose of anaesthetic for spinal/epidural block may be decreased.
  3. Engorged epidural veins reduce the epidural space and subarachnoid extravascular space. It decreases the volume of cerebrospinal fluid in the subarachnoid space. The decreased volume of these spaces facilitates the spread of local anaesthetics and there is decrease in dose requirements of local anaesthetics needed for spinal/epidural block.
 
Pulmonary System
  1. Upper airway: Capillary engorgement of the upper respiratory tract mucosa usually occurs during pregnancy. Instrumentation particularly direct laryngoscopy 9and endotracheal intubation may cause trauma and bleeding. Oedema of vocal cords and arytenoids may also be present. Laryngoscopy may be difficult in short obese parturients due to short neck and large breasts.
  2. Minute ventilation may increase significantly by about 50% and tidal volume by about 40% and respiration rate is raised by about 10%.
  3. Maternal PaO2 may raise by 10% and PaCO2 may decrease by about 10%. There is no change in pH. Pain associated with labour and delivery may aggravate hyperventilation.
  4. Oxygen consumption increases by about 20%. Supplemental oxygenation is recommended during induction of general anaesthesia or even during regional anaesthesia.
  5. Lung volumes:
    1. Total lung capacity remains unaltered.
    2. Vital capacity shows no change.
    3. Functional residual capacity and expiratory reserve volume decrease by 20%.
    4. Residual volume decreases.
    5. Airway resistance is also diminished by about 35%.
  6. Anaesthetic implications:
    Induction of anaesthesia, recovery from anaesthesia and changes in-depth of anaesthesia are significantly faster in parturients.
 
PHYSIOLOGY OF UTEROPLACENTAL CIRCULATION
The placenta is the main route for exchange of nutrients and metabolites between mother and foetus. It is also the 10major site of production of oestrogen and progesterone during the last two trimesters of pregnancy and the only site for production of chorionic gonadotrophin. Placenta also produces some enzymes such as choline acetylase, amine oxidase, acetyl cholinesterase and some other oxidative enzymes.
Uteroplacental circulation concerns with the circulation of the maternal blood through the intervillous space. A mature placenta has the volume of about 500 ml blood, of which 150 ml remains in intervillous space and the rest in the villi system. Blood flow at term ranges from 500 to 600 ml per minute. Pressure in intervillous space during contraction ranges from 30 to 50 mmHg, but during relaxation it is 10 to 15 mmHg. It should also be noted that the pressure in the supplying uterine artery is about 70 to 80 mmHg and in the draining uterine vein it is about 8 mmHg.
Uterine blood flow is most important as it determines the adequacy of placental circulation and foetal well being. Placenta receives maternal blood through the uterine arteries and foetal blood through two umbilical arteries. The pure nutrient-rich blood is delivered to the foetus by a single umbilical vein. Placental function mostly depends on uterine blood flow and the placental area available for exchange of nutrients.
Uterine blood flow is directly proportional to blood pressure and inversely proportional to uterine vascular resistance. Uterine blood is not autoregulated and thus directly related to the mean perfusion pressure across the uterus. Thus any drug or factor that decreases perfusion 11pressure either by decreased systemic blood pressure or by increased venous pressure will decrease the uterine blood flow.
Hypotension is common in supine hypotensive syndrome (aortocaval compression), sympathetic block, following epidural/spinal block or due to administration of potent analgesic and anaesthetic drugs.
Uterine vascular resistance may be increased by alpha adrenergic stimulants like methoxamine and metaraminol. Maternal stress/pain can decrease uterine blood flow due to increased secretion of catecholamines. Uterine contractions decrease uterine blood flow secondary to increased uterine venous pressure.
 
Foetoplacental Circulation
From the foetus the impure blood is carried by the two umbilical arteries. The arteries divide into small branches to enter the stems of chorionic villi. The blood flows into the corresponding venous channels either through capillary network or through shunts. Maternal and foetal blood streams flow side by side, but in opposite direction. This type of flow facilitates material exchange between mother and foetus. The foetal blood flow through placenta is about 400 ml per minute. The pressure in the umbilical artery is about 60 mmHg. The pressure in the umbilical vein is about 10 mmHg. Maternal and foetal blood is separated by tissues known as placental membrane (placental barrier). It consists of some layers like syncytiotrophoblast, cytotrophoblast and connective tissue stroma of villus.
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PLACENTAL TRANSFER/EXCHANGE
The nutrients and the waste products are being exchanged between mother and foetus. This is dependent on various factors:
  1. Physical properties of the substances in the foetal and maternal blood.
  2. Extent and functional integrity of placental exchange membrane.
  3. Rate of maternal and foetal blood flow on either side of exchange membrane.
 
Mechanism of Placental Transfer
  1. Simple diffusion: Water, salts and the end products of carbohydrate and fat metabolism can diffuse freely across the placenta depending mainly on the local concentration gradient.
  2. Facilitated (accelerated) diffusion: It is the process by which the substances like glucose are accelerated to cross in the direction of the concentration gradient at a higher rate.
  3. Active transport: Some substances like aminoacids, certain metallic ions, etc. are carried from mother to foetus and as such the placental membrane cells expend energy in this mechanism.
 
Respiratory Gas Exchange
Although the foetal respiratory movements can occur in utero, there is no gaseous exchange. Intake of oxygen and excretion of carbon dioxide mainly take place by simple diffusion across the foetal membrane. The oxygen supply 13to the foetus is at the rate of 5 ml/kg/min and this occurs with cord blood flow of 165 to 330 ml/min.
As already said that placental exchange is mostly dependent on diffusion from maternal circulation to the foetus and vice versa. This diffusion across the placenta to foetus or the rate and extent of transfer is determined by several factors like maternal to foetal concentration gradients, maternal protein binding, lipoid solubility and the degree of ionisation of that substance.
 
Maternal Protein Binding
Low maternal protein binding of the substances or drugs causes rapid diffusion as the portion of the drug not bound to protein is available for diffusion across the placenta. Similarly, high protein binding causes slow diffusion.
 
Molecular Weight
The high molecular weight (> 1000) of a drug has limited ability to cross the placenta. Conversely placenter transfer is facilitated by the relatively low molecular weight of the substance.
 
Lipid Solubility
Poor lipid solubility of a substance causes slow diffusion across the placenta. The more highly soluble the substance in lipids, the greater will be the transfer through the biological membranes including placenta. The membrane is composed of mostly of lipoproteins and this acts as a barrier against the passage of water soluble substances by simple diffusion.
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Degree of Ionisation
The highly ionised drug shows slow diffusion and the substances with minimal ionisation produces rapid diffusion across the placenta.
 
Concentration at Placental Site
The level of concentration of drug at the placental site largely determines the extent of transfer. The amount of drug received by the foetus is thus, increased by increasing the dose and concentration. Drugs given at the time of uterine contraction will reach less to the foetus as at the time of contraction uterine blood flow decreases secondary to increased uterine venous pressure.
 
Foetal Uptake and Distribution
The foetal blood pH is not always identical to maternal blood pH. Foetal uptake of a drug that crosses the placental barrier is facilitated by the lower pH of foetal blood compared to maternal blood. Thus, the weak basic drugs that cross the placenta in non-ionised form become ionised in foetal circulation. Then the ionised drug cannot readily cross the placenta back to maternal circulation and thus, accumulates in foetal blood. This is known as ion trapping.
About three-fourth of the umbilical venous blood in foetal circulation passes through the liver. Drugs administered to mother is diluted significantly by blood returning from the lower extremities and pelvic viscera of the foetus. This may protect the brain of foetus from exposure of high concentrations of drugs particularly depressants.
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Placental Drug Transfer
Placental transfer of drugs occurs by diffusion. This is mostly governed by Fick's equation:
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Where,
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Fick's equation relates various factors to determine the amount of drug can cross the placenta. Of these factors, some can be controlled but other factors like the surface area and thickness of placenta are not within any control.
 
NORMAL LABOUR
Labour is a series of events that take place in genital organs to expel the viable products of conception out of the uterus through the vagina. If it occurs before 37 completed weeks, it is termed preterm labour. Delivery is the expulsion or extraction of a viable foetus out of the womb. This can take place without labour as in elective caesarean section. It may be vaginal either spontaneous or aid or it may be abdominal.
Labour is said to be normal when spontaneous in onset and at term, with vertex presentation, without undue prolongation and without having any complications.16
Events of the labour are divided into three stages:
  1. First stage: It begins from the onset of true labour pain and ends with full dilatation of the cervix. The average duration is 12 hours in primigravidae and 6 hours in multiparae.
  2. Second stage: It starts from the full dilatation of cervix and ends with expulsion of foetus from the birth canal. It is about two hours duration in primigravidae and about 30 minutes in multiparae.
  3. Third stage: It starts after the expulsion of foetus and ends with expulsion of placenta. The average duration is 15 minutes on average in both primigravidae and multiparae.
True labour pains are associated with painful uterine contractions with the onset of labour. There is profuse cervical secretion with slight oozing and expulsion of cervical mucus plug with blood (show). There is progressive effacement and dilatation of cervix and formation of ‘bag of waters’. The cause of pain is mostly due to stretching of adjacent structures including nearby ganglia or more probably due to ischaemia. The pain of uterine contractions is along the cutaneous nerve distribution of T10 to L1. Pain of cervical dilatation and stretching is through the sacral plexus.
In the second stage, with full dilatation of cervix the membranes rupture and liquor amni comes out. Uterine contraction and retraction become stronger. Delivery of the foetus occurs by the downward thrust by uterine contraction enhanced by voluntary contraction of abdominal muscles (bearing down efforts).17
The third stage of labour involves the phase of placental separation, its descent to the lower segment and at last its expulsion with the membranes. Following parturition the uterus and cervix regress to near prepregnancy state.
 
Anaesthetic Implications
  1. The progress of labour may depend on various factors like maternal pain, parity, foetal size and presentation, drugs administered during labour, etc. Excessive sedation, catecholamine release in response to pain, cephalopelvic disproportion, foetal malposition, etc. may also be the other factors.
  2. Regional anaesthesia (spinal or epidural block) can prolong the latent phase of labour particularly in cases of premature initiation of block.
  3. During the active phase of labour regional anaesthesia may remove the reflex urge to bear down and thus may prolong the labour.
  4. Potent volatile anaesthetics decrease the uterine activity in a dose dependent manner. Halothane in low concentions 0.5% or enflurane (1%) may not decrease the uterine activity, prolong labour and increase post-partum haemorrhage. But higher concentrations of volatile anaesthetics may produce uterine relaxation significantly.
  5. Hypotension should always be avoided during anaesthesia either general or regional.
 
DIAGNOSIS OF PREGNANCY
The reproductive period of a female starts at the age of about 12 years and ends at about 45 years. The duration 18of pregnancy is said to be 10 lunar months or 9 calender months and 7 days or 280 days or 40 weeks calculated from the first day of last menstruation. This is gestational age. As fertilisation usually occurs 14 days prior to expected missed period the true gestational or embryologic age is 280−14 = 266 days. Features include:
  1. At 6 to 8 weeks:
    1. Amenorrhoea.
    2. Morning sickness.
    3. Frequency of micturition.
    4. Breast discomfort; engorgement, nipple and areola pigmented.
    5. Uterus remains a pelvic organ until at 12th week. It is just felt per abdomen. It feels soft and elastic.
    6. Immunological tests positive.
    7. Sonography: gestational ring.
  2. At about 16th week:
    1. Amenorrhoea.
    2. Breast changes: Pigmentation of areola, colostrums.
    3. Uterus: Palpable per abdomen midway between pubis and umbilicus.
    4. Uterine souffle: Soft blowing and systolic murmur heard low down by the sides of uterus, synchronous with maternal pulse. It is due to increased blood flow through the dilated uterine vessels.
    5. Internal ballottement.
    6. X-ray: Foetal shadow.
    7. Sonography.
  3. At about 20th week:
    1. Amenorrhoea.
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    2. Quickening.
    3. Breast changes.
    4. Uterus at the level of umbilicus at 24th week.
    5. Contractions.
    6. External ballottement.
    7. Foetal parts.
    8. Foetal movement 20 weeks.
    9. Foetal heart sound 20 weeks.
    10. Internal ballottement.
    11. X-ray − Foetal shadow.
    12. Sonography.
 
Absolute Signs of Pregnancy
  1. Palpation of foetal parts.
  2. Foetal movements.
  3. Foetal heart sound.
  4. Radiology: Foetal skeleton at or after 16th week.
  5. Ultrasonic evidence as early as 6th week.
 
Assessment of Foetal Maturity
  1. Ultrasonography: Foetal age and weight can be ascertained. It can detect foetal defects. But it can not assess lung maturity. Foetal biparietal diameter parallels foetal age.
  2. Amniocentesis and analysis of amniotic fluid for its lecithin/sphingomyelin ratio is needed to evaluate foetal lung maturity. A ratio greater than 2 to 3 indicates lung maturity. Amniotic creatinine content is also a reliable method.
  3. Radiological assessment of foetal maturity.
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  4. Clinical assessment of uterine size with serial fundal estimations is the most commonly used clinical method.
 
Foetal Monitoring During Labour (Reed, 1995)
  1. Continuous foetal heart rate and uterine contraction monitoring may be used to assess the foetal well being.
    Both direct and indirect methods (foetal electrocardiography, phonocardiography and ultrasonography) are used.
    1. Indirect method − Ultrasound probe is placed on the abdomen of the mother.
    2. Indirect method − A foetal scalp electrode is placed.
    Foetal heart rate patterns may have baseline and periodic features. Baseline features include heart rate with variability. These are measured between uterine contractions. Whereas periodic measurements occur along with contractions.
    The normal foetal heart rate ranges from 120 to 160 beats per minute with a variability of 5 to 20 beats per minute.
    Beat to beat variability indicates intact neural pathways and seems to be the most sensitive indicator of foetal well being. Depressant drugs and local anaesthetics in epidural block can decrease the variability.
    Periodic measurements may show three changes in association of uterine contractions as early, late and variable decelerations. In early deceleration there may be slowing of heart rate with the onset of contractions. A decrease of heart rate is observed in late decelerations 21after the onset of uterine contraction and ends after the contraction. But variable decelerations are not at all related to uterine contractions and it indicates the severe conditions when lasts for more than 30 seconds with a significant decreased heart rate.
    Antenatal cardiotocography (simultaneous recording of foetal heart rate and uterine contractions pattern) is widely used to assess the condition of foetus.
  2. Abnormal foetal heart rate may indicate further studies such as foetal blood sampling and analysis. Normal foetal scalp blood pH varies from 7.28 to 7.30. When it is less than 7.20, it indicates bad sign and there is need of early intervention that is immediate delivery of foetus. A pH ranging 7.20 to 7.25 may indicate to watch further and repeat the test after 15 minutes to do the needful. Uterofoetal resuscitation may be needed in such cases.
  3. Amnioscopy to observe the colour and clarity of amniotic fluid.
  4. Detection of foetal abnormality: Amniocentesis and detection of alpha foetoprotein (AFP).
 
Uterofoetal Resuscitation
  1. If hypotension caused by supine hypotensive syndrome, intravenous medications to mother, epidural local anaesthetics, etc. treatment should be aimed at
    1. 100% oxygen.
    2. Intravenous fluid infusion.
    3. IV ephedrine.
    4. Left lateral uterine displacement in case of supine hypotensive syndrome.
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  2. If it is due to prolonged uterine contraction due to oxytocin
    1. Stop oxytocin
    2. Tocolytics.
  3. If it is due to umbilical cord prolapse
    1. Foetal head to be pushed off the pelvic floor and
    2. Immediate delivery of foetus.