Non obstetric surgery during pregnancy

Non obstetric surgery during pregnancy is relatively common.

The most common indications for surgery during pregnancy are either pregnancy related or pregnancy non related. Pregnancy related surgery include interventions for cervical incompetence and surgery for ovarian cyst problems.

The most common non pregnancy related indications are acute abdominal problems( most commonly appendicitis and cholecystitis), maternal trauma and surgery for malignancies.

Anaesthetists who care for pregnant patients undergoing non-obstetric surgery must provide safeanaesthesia for both the mother and the foetus.  To maintain maternal  safety the physiological and anatomical changes of pregnancy must be considered and anaesthetic techniques and drug administration modified accordingly.  Foetal wellbeing is related to avoidance of foetal asphyxia, teratogenic drugs and preterm labour.

Physiological Changes during Pregnancy

Central Nervous System Effects

The minimal alveolar concentration (MAC) progressively decreases during pregnancy—at term, by as much as 40%—for all general anesthetic agents; MAC returns to normal by the third day after delivery. Changes in maternal hormonal and endogenous opioid levels have been implicated. Progesterone, which is sedating when given in pharmacological doses, increases up to 20 times normal at term and is probably at least partly responsible for this observation. A surge in -endorphin levels during labor and delivery also likely plays a major role

At term, pregnant patients also display enhanced sensitivity to local anesthetics during regional anesthesia; dose requirements may be reduced as much as 30%. This phenomenon appears to be hormonally mediated but may also be related to engorgement of the epidural venous plexus.

Obstruction of the inferior vena cava by the enlarging uterus distends the epidural venous plexus and increases epidural blood volume. The latter has three major effects: (1) decreased spinal cerebrospinal fluid volume, (2) decreased potential volume of the epidural space, and (3) increased epidural (space) pressure. The first two effects enhance the cephalad spread of local anesthetic solutions during spinal and epidural anesthesia, respectively, whereas the last may predispose to a higher incidence of dural puncture with epidural anesthesia

Respiratory Effects

         Oxygen consumption and minute ventilation progressively increase during pregnancy. Both tidal volume and, to a lesser extent, respiratory rate increase.

PaCO2 decreases to 28–32 mm Hg; significant respiratory alkalosis is prevented by a compensatory decrease in plasma bicarbonate concentration.

PaCO2 decreases to 28–32 mm Hg; significant respiratory alkalosis is prevented by a compensatory decrease in plasma bicarbonate concentration.

The combination of decreased FRC and increased oxygen consumption promotes rapid oxygen desaturation during periods of apnea (see Chapter 22). Preoxygenation prior to induction of general anesthesia is therefore mandatory to avoid hypoxemia in pregnant patients.

Closing volume exceeds FRC in up to 50% of all pregnant women when they are supine at term. Under these conditions, atelectasis and hypoxemia readily occur.

The decrease in FRC coupled with the increase in minute ventilation accelerates the uptake of all inhalational anesthetics.

Capillary engorgement of the respiratory mucosa during pregnancy predisposes the upper airways to trauma, bleeding, and obstruction. Gentle laryngoscopy and the use of small endotracheal tubes (6–6.5 mm) should be employed during general anesthesia.

Cardiovascular Effects

         Cardiac output and blood volume increase to meet accelerated maternal and fetal metabolic demands.

An increase (45%) in plasma volume in excess of an increase in red cell mass produces dilutional anemia and reduces blood viscosity.

At term, blood volume has increased by 1000–1500 mL in most women, allowing them to easily tolerate the blood loss associated with delivery; total blood volume reaches 90 mL/kg. Average blood loss during vaginal delivery is 400–500 mL, compared with 800–1000 mL for a cesarean section. Blood volume does not return to normal until 1–2 weeks after delivery.

The increase in cardiac output (40% at term) is due to increases in both heart rate (15–20%) as well as stroke volume (30%). Cardiac chambers enlarge and myocardial hypertrophy is often noted on echocardiography. Pulmonary artery, central venous, and pulmonary artery wedge pressures, however, remain unchanged. Most of these effects are observed in the first and, to a lesser extent, the second trimester. In the third trimester, cardiac output does not appreciably rise, except during labor. The greatest increases in cardiac output are seen during labor and immediately after delivery (see the section on Effect of Labor on Maternal Physiology). Cardiac output often does not return to normal until 2 weeks after delivery.

A decrease in systemic vascular resistance by the second trimester decreases both diastolic and, to a lesser degree, systolic blood pressure. The response to adrenergic agents and vasoconstrictors is blunted.

Decreases in cardiac output can occur in the supine position after week 28 of pregnancy. Up to 20% of women at term develop the supine hypotension syndrome, which is characterized by hypotension associated with pallor, sweating, or nausea and vomiting. The cause of this syndrome appears to be complete or near-complete occlusion of the inferior vena cava by the gravid uterus.

The gravid uterus also compresses the aorta in most parturients when they are supine. This latter effect decreases blood flow to the lower extremities and, more importantly, to the uteroplacental circulation

Lastly, elevation of the diaphragm shifts the heart's position in the chest, resulting in the appearance of an enlarged heart on a plain chest film and in left axis deviation and T wave changes on the electrocardiogram (ECG). Physical examination often reveals a systolic ejection flow murmur (grade I or II) and exaggerated splitting of the first heart sound (S1); a third heart sound (S3) may be audible. A few patients develop small, asymptomatic pericardial effusion.

Renal Effects

         Renal vasodilatation increases renal blood flow early during pregnancy but autoregulation is preserved. The kidneys often enlarge. Increased renin and aldosterone levels promote sodium retention. Renal plasma flow and the glomerular filtration rate increase as much as 50% during the first trimester; glomerular filtration declines toward normal in the third trimester. Serum creatinine and blood urea nitrogen may decrease to 0.5–0.6 mg/dL and 8–9 mg/dL, respectively. A decreased renal tubular threshold for glucose and amino acids is common and often results in mild glycosuria (1–10 g/d) or proteinuria (< 300 mg/d). Plasma osmolality decreases by 8–10 mOsm/kg.

Hematological Effects

            Pregnancy is associated with a hypercoagulable state that may be beneficial in limiting blood loss at delivery. Fibrinogen and factors VII, VIII, IX, X, and XII concentrations all increase

In addition to the dilutional anemia (see the section on Cardiovascular Effects), leukocytosis (up to 21,000/L) and a 10% decrease in platelet count may be encountered during the third trimester. Because of fetal utilization, iron and folate deficiency anemias readily develop if supplements of these nutrients are not taken. Cell-mediated immunity is markedly depressed and may increase susceptibility to viral infections.

FOETAL SAFETY

Prevention of foetal asphyxia

One of the most serious risks to the foetus during maternal surgery is intrauterine asphyxia.  This must be avoided by maintaining maternal oxygenation and haemodynamic stability.  It is extremely important to avoid hypoxia, extreme hyper and hypocarbia, hypotension and uterine hypertonus.

Maternal hypoxaemia causes uteroplacental vasoconstriction and decreased perfusion, causing foetal hypoxia, acidosis and ultimately death. 

Uteroplacental circulation is not autoregulated and hence perfusion is entirely dependant on the maintenance of an adequate maternal blood pressure and cardiac output.  Hypotension can be caused by anaesthetic drugs, central neuraxial blockade, hypovolaemia or aortocaval compression.  Maternal hypotension needs to be treated aggressively by ensuring left lateral tilt and boluses of IV fluids.

Additional vasopressors may be required and currently it is felt alpha agonists such as phenylephrine and metaraminol produce a better foetal acid balance than indirect sympathomimetic agents such as ephedrine. Ephedrine also has a relatively slow onset and long duration of action and tachyphylaxis can occur making titration difficult. 

Drugs and teratogenicity

Teratogenicity is defined as the observation of any significant change in the function or form of a child secondary to prenatal treatment.

Inhaled anesthetic agents

Nitrous oxide

There is some controversy regarding the teratogenic effects of nitrous oxide.

Nitrous oxide inactivates vitamin B12, an essential cofactor for methionine

synthetase, and may interfere with DNA synthesis.

Another drug of concern is ketamine.  This causes increased uterine tone and foetal asphyxia and should not be used in the first two trimesters.  The effect is not seen in the third trimester.

Benzodiazepines have been associated with a cleft lip and palate in animal studies.  The association in humans is controversial.  A single dose has not been associated with teratogenicity.  Long term use should be avoided as neonatal withdrawal may occur.  Single doses may be useful to provide anxiolysis preoperatively.

Prevention of pre-term labour / foetal monitoring

Surgery during pregnancy increases the risk of spontaneous abortion, preterm labour and preterm delivery. This risk is increased with intra-abdominal procedures. Uterine manipulation should be kept to a minimum and drugs that increase uterine tone (e.g. ketamine) should be avoided.  Prophylactic tocolytic therapy is controversial as there are associated maternal side effects and efficacy during nonobstetric surgery has not been proven. perioperative foetal monitoring

Although perioperative foetal monitoring has not been shown to improve foetal outcome, a sensible approach would be to use cardiotocography (CTG) monitoring where possible and practical when the foetus is of a viable age.  The obstetricians and neonatologists should be informed, appropriately trained personnel available to interpret the CTG and a prior action plan in place for when there is evidence of foetal distress unresponsive to conservative measures. 

   

If the foetus is not of a viable age or perioperative CTG monitoring is not possible / practical, FHR monitoring should occur pre and post-operatively and staff should be alert to the signs of premature labour.

Practical Anesthetic Considerations

Timing of Surgery

As a general rule, elective surgery should not be performed during pregnancy. If surgery is necessary (cardiac, neurosurgery, abdominal emergencies, or malignancies), choice of timing is essentially a balance between maternal and fetal risks and the urgency of surgery. For the fetus, the second trimester is the optimal

time to perform surgery.

Anesthetic Management

Preoperative Care.

 Gastric aspiration prophylaxis (H2-receptor antagonist and 30 mL of a nonparticulate antacid before the induction) should be considered after 16 –20 weeks gestation. Premedication anxiolysis (e.g., midazolam 1 mg) may be necessary for the anxious parturient, as elevated catecholamines may decrease uterine blood flow.

Prevention of Aortocaval Compression. After 20 weeks gestation, the pregnant patient should be transported in the lateral position, and left uterine displacement instituted when positioned on the operating table. The effectiveness of left uterine displacement can be assessed by measuring the blood pressure on the right leg or observing the pulse oximeter waveform on the right foot. For surgery in the prone position, the abdomen should hang unobstructed and any external compression should be avoided.

Anesthetic Technique

No study has correlated improved fetal outcome with any anesthetic technique and the choice of anesthesia should be guided by maternal indications as well as the site and nature of the surgery. When possible, a local or regional anesthetic technique may be preferable. Regional techniques minimize fetal drug exposure and maternal perioperative complications. However, laparoscopy and most upper abdomen operations usually require general anesthesia. Regardless of the

technique used, avoidance of hypoxemia, hypotension, hypovolemia, acidosis, and hypercarbia/ hypocarbia are the most critical elements of the anesthetic management. Blood glucose levels should be checked, especially during prolonged surgery or

in patients with gestational diabetes or glucose intolerance.

General Anesthesia

Induction. General anesthesia mandates endotracheal intubation beginning at approximately 16 –20 weeks gestation. Preoxygenation with 100% oxygen

administration for 3– 4 min (or 4 vital capacity breaths if time is restricted) before a rapid sequence induction with cricoid pressure should be performed. Fasciculation after the administration of succinylcholine does not occur consistently. The use of a smaller endotracheal tube (6.0 –7.0 mm) is recommended because of respiratory tract mucosa edema and engorgement associated with pregnancy. Avoid nasal intubations that may precipitate bleeding because of increased mucosal vascularity. Use drugs with a history of safe use during pregnancy including thiopental, morphine, fentanyl, succinylcholine, and most nondepolarizing muscle relaxants. Although not similarly “time-tested,” many consider propofol safe during pregnancy.

Maintenance. A moderate concentration of a volatile agent ([1]1.5–2.0 MAC) with a high concentration of oxygen (Fio2

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 0.5) is recommended. Although scientific evidence does not support avoidance of nitrous oxide during pregnancy, using concentrations of [1]50% and limiting its use in the first trimester and during extremely long operations is suggested. If nitrous oxide is avoided, adequate analgesia should be administered to minimize the need for higher doses of a volatile agent that may cause maternal hypotension.

Opioids and induction agents decrease FHR variability to a greater extent than do the inhalation agents.

Opioid-induced fetal respiratory depression is relevant only if cesarean delivery is to be performed at the same time as the surgical procedure.

Positive-pressure ventilation may reduce uterine blood flow and decrease uteroplacental perfusion as a result of increased intrathoracic pressure and decreased venous return. Hyperventilation should be avoided and end-tidal CO2

should be maintained in the normal range for pregnancy.

The effects of reversal agents are unpredictable. Because of theoretical concerns of anticholinesterase agents increasing uterine tone and precipitating preterm labor, slow administration after a preceding dose of atropine is recommended. Glycopyrrolate is often recommended because it crosses the placenta less readily than does atropine.

Regional Anesthesia

Maternal hypotension associated with spinal or epidural anesthesia should be prevented or minimized by fluid preloading and leg compression devices. Appropriate vasopressors should be available to treat hypotension if it occurs. Phenylephrine has been shown to be the preferred vasopressor for the treatment of hypotension after neuraxial anesthesia for cesarean delivery, as the use of ephedrine causes greater fetal

acidosis.

 Pregnant patients may have reduced requirements for local anesthetics and

appropriate dose reduction is necessary to prevent a high block. Patients are at higher risk for systemic local anesthetic toxicity because decreased protein binding during pregnancy results in a greater fraction of unbound drug.

Postoperative Care

The FHR and uterine activity should be monitored during recovery from anesthesia.

 Regional anesthesia may be preferable because systemic opioids may reduce FHR

variability. The routine and prolonged use of nonsteroidal antiinflammatory drugs is best avoided because of potential fetal effects.

Special Situations

Laparoscopy

Benefits include:

• faster return to normal activities

• shorter hospitalization

• less pain

• decreased risk of complications

Laparoscopy may be performed during pregnancy for both diagnostic and therapeutic surgery (e.g., cholecystectomy, appendectomy, ovarian torsion).

There were previous concerns regarding foetal safety during laparoscopic surgery.  These included fears of direct uterine and foetal trauma, foetal acidosis due to absorbed carbon dioxide and decreased maternal cardiac output secondary to the increased intra-abdominal pressure and positioning with a subsequent decrease in uteroplacental perfusion.

Pneumatic stockings should be used to promote venous return and the lowest pressure pneumoperitoneum (<12mmHg) should be used where possible.

Aortocaval compression should be avoided and changes in position should be undertaken slowly.

PaCO2 should be closely monitored by the routine use of end tidal carbon dioxide monitoring and consideration of arterial blood gas analysis in selected cases.  FHR monitoring may be advisable to detect foetal compromise early allowing optimisation of maternal haemodynamics.  FHR changes may indicate the need for temporary deflation of the pneumoperitoneum.