Imaging that may be routine in the non-obstetric population may present increased risk in pregnancy; the benefits often outweigh the risks, but this warrants consideration.
The multisystem changes of pregnancy must be considered when interpreting examination and investigation findings.
An informed discussion about maternal and foetal risks from the anaesthetic, surgery and underlying pathological processes must occur.
Multidisciplinary liaison between the anaesthetic, surgical, obstetric and neonatal teams should occur, depending on the stage of pregnancy.
Changes to usual anaesthetic techniques may be necessary depending on the stage of pregnancy; the key to foetal well-being is maintaining utero-placental perfusion and maternal oxygenation.
The parturient presenting for non-obstetric surgery presents specific challenges to the medical team coordinating her care, so thorough preoperative assessment and planning are essential. Anaesthesia and surgery during pregnancy are subject to additional considerations posed by both the altered physiology of pregnancy and risks to the foetus, which include teratogenicity, preterm labour and miscarriage. Elective procedures should not occur in pregnancy, and the American College of Obstetricians and Gynecologists (ACOG, 2011) advises that elective surgery should be postponed until after delivery. Urgent surgery should be undertaken in the second trimester if possible, when the incidence of spontaneous abortion and preterm labour is at its lowest. However, by its very nature, emergency surgery may be necessary at any time throughout pregnancy.
Preoperative assessment is similar to that in a non-pregnant patient, but the history should cover the additional points that follow and the clinical examination must take into account the specific physiological changes of pregnancy, particularly when assessing an unwell patient. Marked cardiovascular and respiratory changes accompany the increased metabolic demands of pregnancy; the anaesthetist must be well informed of these and alter anaesthetic management accordingly.
Additional investigations are not indicated in an otherwise uncomplicated pregnancy. Investigations should be undertaken as indicated by the patient’s clinical condition and pathological process necessitating surgery. Imaging that may be routine in the non-obstetric population presents increased risk in pregnancy and warrants further consideration (see later in this chapter).
The presenting problem requiring surgery and any other health conditions must be explored. The focus of the history-taking then turns to the pregnancy. Gestational age affects physiological changes that influence anaesthetic conduct, e.g. aspiration risk and aortocaval compression. The specific trimester is also important as this affects discussion about risk and informed consent. Common disorders associated with pregnancy should be specifically sought, e.g. hypertensive disease, gestational diabetes and gastro-oesophageal reflux.
Airway and Respiratory System
The incidence of failed intubation is higher in the obstetric population (1:224 to 1:267) than in the elective non-obstetric setting (1:1000 to 1:2000), although this may be due in part to the additional non-technical challenges of intubation on the labour ward as well as physiological changes (Quinn et al., 2013; Rose and Cohen, 1996; Samsoon and Young, 1987). Weight gain and airway oedema can cause problems with mask ventilation and tracheal intubation. The potential consequences of a difficult or failed intubation are more serious in the parturient due to the higher oxygen consumption and reduced functional residual capacity.
A recent retrospective study of failed intubation in the obstetric population determined several risk factors: increasing age and BMI; recording a Mallampati score; and a Mallampati score >1. Increasing age was not an anticipated outcome; it was hypothesised that older parturients may be at increased risk of pregnancy complications associated with weight gain. A significant finding was that airway assessment was poor in this population. Simply documenting the Mallampati score was associated with a difficult airway, suggesting that airway assessment was undertaken more commonly if the anaesthetist suspected difficulty (Quinn et al., 2013).
The following investigations of the respiratory system are altered in pregnancy:
Spirometry (see Table 20.1).
Arterial blood gases: the increase in minute ventilation rate commonly leads to a respiratory alkalosis by mid-pregnancy (PaCO2 3.75–4.25 kPa) with a pH at the upper end of normal. A compensatory decrease in levels of bicarbonate and base is normal.
Imaging (see later in this chapter).
Legend: FEV 1 = Forced Expiratory Volume in the first second
Major cardiovascular changes occur from 6 weeks’ gestation. Cardiac output increases through increased stroke volume and heart rate. Preload increases due to increased blood volume and venous return; afterload is decreased through reduced systemic and pulmonary vascular resistance. Aortocaval compression may occur from 20 weeks’ gestation, necessitating 15° lateral tilt intraoperatively.
Blood pressure readings are lower in normal pregnancy, with a greater decrease in diastolic than systolic pressure. These are up to 20 per cent lower than baseline around the 28th week of gestation, before gradually returning towards non-pregnant levels (Gunderson et al., 2008; Wilson et al., 1980). Heart rate increases from 5 weeks’ gestation and peaks at 32 weeks to approximately 25 per cent above baseline (Hunter and Robson, 1992). Examination of a healthy pregnant woman may reveal distended neck veins with prominent a (atrial) and v (ventricular) waves and brisk x and y descents. The apex beat may be displaced laterally by upward movement of the diaphragm, and a prominent right ventricular impulse may be palpated. An ejection systolic murmur may be heard over the pulmonary or tricuspid area in more than 90 per cent of pregnant women at term. This is caused by increased turbulent flow and regurgitation across dilated cardiac chambers and valves. The first heart sound may be loud and the second heart sound split owing to increased right-sided flow. A third heart sound may or may not be normal. Peripheral oedema is seen in up to a third of pregnancies.
ECG: changes in cardiac position and size lead to ECG changes. In addition to increased heart rate, common findings include left axis deviation, sagging ST segments and inverwsion or flattening of the T wave in lead III.
Echocardiography: changes in cardiac position actually facilitate the parasternal and apical views. Echocardiographic results are altered in keeping with the increased cardiac output seen in pregnancy, as illustrated by a recent study of 24 non-pregnant women and 97 women at varying stages of pregnancy (Table 20.2).
|Stroke volume (mL)||63.7 ± 14||56.2 ± 11.7||<0.04|
|Heart rate (bpm)||91 ± 15||79.3 ± 18.1||<0.02|
|Cardiac index (L/min/m2)||3.0 ± 0.7||2.4 ± 0.4||<0.0001|
Legend: mL = millilitre, bpm = beat per minute, L/min/m2 = litre per minute per metre square
Reflux is common in pregnancy owing to hormonal and mechanical changes leading to impaired lower oesophageal competence and altered gastric anatomy. It is unclear exactly when this becomes clinically relevant; some reports state that an aspiration risk is present from the second trimester onwards, but most will treat as clinically relevant and alter anaesthetic management from 16 weeks’ gestation.
Biochemistry: alterations in pregnancy must be remembered when interpreting preoperative investigations (Cunningham, 2014). An increase in glomerular filtration rate (by about 50% of baseline) and renal plasma flow (by about 75% of baseline) lead to an increase in creatinine clearance, so serum creatinine levels typically measure about 25 per cent less than pre-pregnancy levels in the first and early second trimesters; these physiological changes have implications for drug metabolism and excretion.
Altered liver biochemistry may be seen. By term, serum levels of bilirubin, aminotransferases and lactate dehydrogenase can increase to the upper limits of the normal range. Placental production of alkaline phosphatase may lead to levels up to three times normal.
Urinalysis: increased glomerular filtration rate and permeability of the glomerular basement membrane increase protein excretion from about 100 mg/24 h to 200 mg/24 h in the third trimester; this is not considered abnormal until it exceeds 300 mg/24 h, which equates to 1+ protein on dipstick urinalysis. However, 1+ protein on dipstick urinalysis as a threshold for significant proteinuria leads to a high false positive rate due to alterations in urine concentration (Waugh et al., 2004).
The greater increase in plasma volume (45% by term) than red cell mass (25% by term) leads to a dilutional anaemia from early pregnancy onwards (Table 20.3a). While a normal finding, its presence may lead to a decreased ability to compensate for significant blood loss during the peripartum period, so may warrant a lower threshold for ordering a preoperative group and save sample. Leucocytosis is seen throughout pregnancy and especially during labour; though benign, it may complicate the diagnosis of peripartum infection. Platelet count may be widely variable; however, even thrombocytopenic women can remain hypercoagulable due to increases in fibrinogen and clotting factors (Szecsi et al., 2010) (Table 20.3b).
|Decreased||Within non-pregnant reference ranges||Increased|
|Protein S||Factors II, V, X, XI, XII||Factors VII, VIII, IX|
|Activated prothrombin time|
It is important to understand that clotting is altered in pregnancy and therefore minor derangements from non-pregnant values may indicate significant derangement in haemostatic function. Thromboelastography and rotational thromboelastometry are useful tools to measure functional haemostasis, although the normal references ranges used for the non-pregnant patient may not be appropriate for the parturient (Armstrong et al., 2011; Karlsson et al., 2012; Macafee et al., 2012).
Before surgical intervention, imaging is often required for diagnosis and planning of treatment or as part of the preoperative assessment. There are multiple modalities available, X-rays and CTs raising particular concern for patient and physician. Risks and benefits must be weighed, taking into consideration that delayed diagnosis and management may result in worse outcome. When possible an abdominal shield should be worn.
Potential risks associated with exposure to ionising radiation include teratogenicity and carcinogenesis. A review of the literature concluded that there was no evidence that teratogenicity was increased at levels below 50 mGy, and malignancy below 10 mGy (Lowe, 2004). It concluded that the only statistically proven adverse effect at dose levels associated with diagnostic radiation procedures was a small increase in malignancy, with an estimated increase of one cancer death per 1700 10 mGy exposures. There is no evidence that this effect is dependent on gestational age. Non-cancer health effects may be expected after foetal doses ≥100 mGy and appear to have the most significant effects between the gestational ages 8–25 weeks.
Most diagnostic procedures result in low levels of radiation exposure, but direct imaging of the abdomen and pelvis can be considerably higher (Valentin, 2000). The highest radiation doses are seen with CT of the abdomen (max 49 mGy) and pelvis (max 79 mGy), and barium meals (max 24 mGy). Radiation doses associated with chest imaging are < 0.01 mGy for a radiograph and < 0.96 mGy for CT. It must also be remembered that interventional radiology can result in prolonged and significant radiation exposure. If multiple exposures are anticipated, a specialist in dosimetry calculation should be consulted to calculate foetal exposure.
Ultrasound is the primary imaging tool used in pregnancy. Animal studies have shown that when ultrasound is used at higher intensities than those used clinically, repeated exposure has neurobehavioural effects (Hande and Devi, 1995). Concern has also been expressed regarding the ultrasound’s thermogenic properties, which may be teratogenic (Miller et al., 2002). However, there have been no documented adverse outcomes associated with the use of ultrasound in pregnancy and it remains the investigation of choice.
Magnetic Resonance Imaging
Certain animal studies have raised concerns regarding MRI and teratogenicity (Heinrichs et al., 1986; Tyndall and Sulik, 1991). While there have been no human studies suggesting adverse effects, research in this area remains limited so the International Commission on Non-ionising Radiation (2004) has advised caution using MRI in pregnancy, particularly in the first trimester, suggesting that while remaining preferable to ionising radiation, it should be used only after carefully considering whether it will provide critical information unavailable from ultrasound.
Risk Assessment and Informed Consent
Prospective, randomised studies with matched controls examining the risks posed by anaesthesia and surgery to the parturient do not exist. Most information in this area comes from animal studies or large observational studies, so extrapolation must be cautious. Potential adverse outcomes may be associated with anaesthesia, surgery or the pathological process necessitating the procedure. Concerns relate to maternal morbidity, foetal teratogenicity, foetal loss and premature labour.
Observational study seems to suggest that maternal morbidity and mortality are not increased by non-obstetric surgery during pregnancy (Erekson et al., 2012). An American study looking at cholecystectomy and appendectomy outcomes in the obstetric and non-obstetric population over a 5-year period found a similar composite 30-day major morbidity, despite an increased likelihood in the obstetric group to have preoperative systemic infections, suggesting a higher threshold for surgical intervention (Silvestri et al., 2011).
Studies do, however, suggest an increased risk of spontaneous miscarriage in pregnant women exposed to anaesthesia and surgery. At present, the most reliable information comes from a large systematic review of 54 papers and more than 12,000 patients, published in 2005 (Cohen-Kerem et al., 2005). Miscarriage rates in patients undergoing surgery during pregnancy were found to be 5.8 per cent, increased to 10.5 per cent if surgery took place in the first trimester. Premature delivery was seen in 8.2 per cent. The absence of matched controls made interpretation difficult. Sub-analysis indicated that appendectomy was a particular risk factor, accounting for 3.5 per cent of premature labours. Foetal loss associated with appendectomy was 2.6 per cent, increased to 10.9 per cent when peritonitis was present, suggesting that the underlying pathological process is a major contributor to morbidity.
Teratogenicity is also a potential risk of surgery in pregnancy, but again, formal investigation is lacking. Teratogenicity refers to a structural or functional postnatal change seen as a consequence of a prenatal exposure. Most patients will be concerned about drug and radiation exposure, but may not realise that factors such as pyrexia, hypoxia and acidosis can be more detrimental.
During the first 15 days post-conception, an all-or-nothing phenomenon is seen, in which the embryo is lost or preserved. Major congenital malformations are most likely to occur from exposure to teratogens from days 15–60. After this, functional changes may occur, but structural changes are unlikely.
Anaesthetic drugs affect cell-signalling, cell mitosis and DNA synthesis, so theoretically have the potential to cause teratogenicity. At clinically relevant doses, however, studies have not indicated a link between birth defects and the use of opioids, intravenous induction agents, inhalational agents, local anaesthetic or neuromuscular-blocking drugs. Earlier concerns over benzodiazepines in pregnancy have not been substantiated.
Concerns have been raised regarding nitrous oxide and congenital anomalies due to its effects on methionine synthetase; prolonged administration of high-concentration nitrous oxide has been shown to be teratogenic in rats, and the possibility of an association between occupational exposure and spontaneous abortion was reported in the 1980s (Cohen et al., 1980; Mazze et al., 1984). Further epidemiological studies have not supported these associations to date, but it would seem prudent to avoid its use, or limit its concentration to below 50 per cent (Mazze and Kallen, 1989).
Case-control studies have shown no increase in birth defects resulting from pregnancies exposed to surgery and anaesthesia, and women should be reassured of this (Reitman and Flood, 2011). However, subtle functional changes can be difficult to identify, and it must be reiterated that randomised controlled trials in this area do not exist. Medications should therefore be administered after consideration of necessity, benefit and risk. There is no clear anaesthetic technique of choice during pregnancy. The key to foetal well-being is maintaining utero-placental perfusion and maternal oxygenation.