The increase in the number of pregnant women with significant coexisting medical disease has led to a need for high-dependency facilities during labour, delivery and the puerperium. In addition, women with complications of pregnancy such as pre-eclampsia may require high-dependency care. In these situations, an understanding of the pathophysiological changes that are taking place may be improved by the use of invasive monitoring.
Successive reports of the Confidential Enquiries into Maternal Deaths (CEMD) have recommended the more frequent and earlier use of invasive monitoring in the management of obstetric haemorrhage.
Problems and special considerations
Midwives are not intensive care nurses, and invasive monitoring may only aid management if the data obtained are reliable and correctly interpreted. All invasive cardiovascular monitoring has significant morbidity associated with its use, such as line sepsis, accidental arterial puncture, pneumothorax and even death. Insertion of pulmonary artery catheters is associated with a particularly high morbidity, and their use is rarely indicated nowadays.
The relative risks and benefits of invasive monitoring need to be assessed carefully - the difficulties of conducting labour and delivery on an intensive care unit may, in some circumstances, outweigh the potential benefits of such monitoring. The implications of the CEMD recommendations are that all obstetric units should be able to care for women with central venous pressure (CVP) monitoring, and that if it is not possible to do so, these women should be transferred to an intensive care unit.
Management options
Direct arterial pressure monitoring
Intra-arterial monitoring provides valuable information in conditions where even brief periods of hypotension or hypertension might cause significant morbidity or mortality. Women with severe cardiac disease resulting in a fixed cardiac output require continuous blood pressure monitoring if epidural analgesia is used in labour, and before induction of either general or regional anaesthesia for operative delivery.
Direct arterial pressure monitoring is desirable in women with severe pre-eclampsia, or those receiving intravenous infusions of antihypertensive agents.
On rare occasions, an intra-arterial cannula may be inserted to facilitate frequent arterial blood gas analysis in women with severe respiratory pathology.
Most midwives are not used to managing arterial lines, and it is therefore vital to ensure that the line is clearly labelled to minimise the risk of its being confused with an intravenous line. The insertion site must be readily accessible and kept visible at all times. It is sensible to explain the purpose of the line to the mother and to involve her in responsibility for its care.
Central venous pressure monitoring
Insertion of a central venous catheter to measure right atrial pressure may give valuable information in women with coexisting cardiac disease. Most protocols for managing women with severe pre-eclampsia also suggest insertion of a CVP line to aid in fluid management, although it is important to realise that the information obtained (right atrial pressure) may not accurately reflect pulmonary artery or left atrial pressure (see Chapter 86, Hypertension, pre-eclampsia and eclampsia).
The least invasive technique is recommended for women who are undelivered, i.e. use of a long line inserted from a peripheral vein (usually in the antecubital fossa). It may be technically difficult to cannulate the subclavian or internal jugular vein in the neck of a pregnant woman. She will be intolerant of the head-down position, and the need to adopt lateral tilt or the supine wedged position may distort the usual anatomical landmarks. The apprehension caused by attempting insertion of a neck line may provoke hypertension or arrhythmias caused by increased circulating catecholamines. In addition, the pre-eclamptic woman may have considerable soft tissue oedema of the face and neck and may also be thrombocytopenic.
A clear right atrial pressure waveform on a directly transduced trace may provide confirmation of correct placement of the line without the need for radiological confirmation, although the decision about whether to perform radiography should follow consideration of the relative risks and benefits. Chest radiography should be performed regardless of whether the woman is delivered if there is any anxiety about correct placement or complications of insertion.
Midwives should be able to look after women with CVP lines, and there should be continued education led by the anaesthetic team to ensure that new and existing staff are familiar with this aspect of care of high-risk patients.
Pulmonary artery catheterisation
Though the traditional gold standard for measurement of cardiac output, pulmonary artery catheterisation is becoming less common generally, since its use has been linked to increased mortality in some studies (although patient selection may be a confounding factor). In obstetrics, many authorities would reserve its use for extreme cases of impaired global cardiac function such as cardiomyopathy, or for severe pre-eclampsia with impaired left ventricular function. The risks are those of CVP monitoring plus the potential for pulmonary artery rupture and infarction, as well as technical problems such as knotting of the catheter. Recent studies have suggested that non-invasive measurements of cardiac output such as transthoracic echocardiography may give comparable results without the risk of such complications (see below).
Other cardiac output monitoring techniques
Cardiac output correlates better with uteroplacental perfusion than heart rate and blood pressure, so cardiac output monitoring may be a useful adjunct in obstetric care. The choice of cardiac output monitor depends on whether accuracy (demonstrated by validation studies) or trend measurements are required. The degree of invasiveness and whether the woman is conscious must also be considered. The information obtained should be clinically useful within the correct time frame and impact positively on clinical care.
Non-invasive methods of cardiac output monitoring, such as transthoracic or trans- oesophageal echocardiography, suprasternal Doppler ultrasound and electrical bioimpedance, have been used in the obstetric setting. These techniques have inter-observer variation but may be useful in specific cases (e.g. with severe cardiac disease) or as research tools. Point-of-care echocardiography is gaining popularity, particularly within the obstetric critical care setting; basic echocardiography skills can help distinguish between causes of hypotension and help guide fluid therapy. Arterial waveform analysis techniques have been suggested as alternatives in obstetric practice; these include the LiDCOplus (LiDCO, Cambridge, United Kingdom) and PiCCOplus (Pulsion Medical Systems, Munich, Germany), and the uncalibrated Vigileo monitor (Edwards Lifesciences, Irvine, California). These devices provide beat-to-beat data and are generally precise and reliable in terms of cardiac output measurement, and can thus monitor the patient’s responsiveness to fluids and vasopressors. They are therefore thought to be more useful than the measurement of filling pressures.
Key points
• High-dependency care of women with coexisting medical disease or obstetric complications of pregnancy may require invasive monitoring.
• If appropriate monitoring cannot be provided in the maternity unit in which the woman is intending to deliver, arrangements should be made to transfer her care to another unit.
Further reading
Armstrong S, Fernando R, Columb M. Minimally- and non-invasive assessment of maternal cardiac output: go with the flow! Int J Obstet Anesth 2011; 20: 330-40.
Dennis AT, Dyer RA. Cardiac output monitoring in obstetric anaesthesia. Int J Obstet Anesth 2014; 23: 1-3.
Fujitani S, Baldisseri MR. Hemodynamic assessment in a pregnant and peripartum patient. Crit Care Med 2005; 33 (10 Suppl): S354-61.
Paech M, James M. Maternal hemodynamic monitoring in obstetric anesthesia. Anesthesiology 2008; 109: 765-7.