Physiological Changes to Fluid Status in Pregnancy

The maternal body accumulates water in pregnancy. Total body water can increase by up to 8 L. Oestrogens increase plasma renin activity with enhanced renal sodium absorption and water retention (renin–angiotensin–aldosterone system). Plasma osmolality decreases by about 10 milliosmol/kg below non-pregnant levels [1, 2]. These changes promote sufficient placental perfusion as the plasma volume rises steadily throughout the first two trimesters and plateaus at around 32 weeks. Reduced maternal plasma volume is associated with abnormal pregnancy outcome [3].

The normal human heart is able to tolerate recurrent episodes of pregnancy-related volume overload without compromising normal function [4]. Disturbances in the cardiovascular system and the regulation of fluid balance can lead to changes in blood pressure and to hypotension or hypertension. Imbalance in salt and fluid regulation is common and can cause oedema in later stages of pregnancy. While in the first and second trimester maternal fluid regulation appears normal, water load is not easily excreted in the last trimester and fluid regulation is more vulnerable with the risk of overload and underload. During labour and postpartum, demands on the regulatory systems further increase. In the postpartum period, mobilisation of fluids can lead to a drop in colloid osmotic pressure of up to 30%. The outlined physiological changes to maternal fluid balance through the stages of pregnancy emphasise the need for careful fluid management, especially in patients with underlying medical disease or acute illness.

Fluid Overload

Fluid overload can lead to pulmonary oedema, which is a significant cause of morbidity and mortality in pregnancy and early postpartum. The incidence in pregnancy is estimated between 0.08% and 0.5 % [5, 6]. In patients with preeclampsia, fluid overload can also cause cerebral oedema with high maternal mortality. Attention to volume status is important in any woman in labour, but strict attention must be paid to fluid balance in patients with any pre-existing medical disease. This includes careful monitoring of fluid input and output on an early-warning or high-dependency chart in a dedicated high-dependency unit. A senior clinician should have overall responsibility for the management of the patient. There should be early involvement of consultant obstetric anaesthetists and/or intensivists as part of a multidisciplinary approach. Always check for signs of fetal distress due to maternal compromise and consider early delivery to improve maternal outcome.

Risk Factors for Fluid Overload

Clinical Signs

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  Peripheral oedema, dyspnoea, tachypnoea, inappropriate weight gain

  
  
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  Paroxysmal nocturnal dyspnoea, inability to lie flat

  
  
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  End-expiratory crackles on auscultation, pink frothy sputum

  
  
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  Hypoxia and radiological signs of pulmonary oedema on chest radiograph (Figure 33.1)

Figure 33.1 Chest radiograph showing pulmonary oedema.

Management of Fluid Overload

If the patient is in respiratory distress with clinical signs of pulmonary oedema, start treatment before investigations.

Principles of Management

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  Look for causes of volume overload, for example, underlying cardiac disease, sepsis, tocolytic therapy or preeclampsia

  
  
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  Administer oxygen. Position the patient upright. Check airway, breathing and circulation. Remember laryngeal swelling can make mask ventilation and/or intubation significantly more difficult; therefore careful airway assessment is essential.

  
  
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  Establish venous access. Peripheral oedema may make this difficult. Consider central venous access and invasive heamodynamic monitoring [8, 9]. Non-invasive cardiac output monitoring and echocardiography may be useful to distinguish cardiac and non-cardiac causes of fluid overload in women with severe preeclampsia and/or underlying cardiac disease.

  
  
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  Urinary catheter with hourly fluid balance. Aim for urine output > 0.5 mL/kg per hour [8]. Women with preeclampsia frequently have a brief period of oliguria up to 6 hours after delivery. This should be anticipated and not overcorrected [10].

  
  
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  Fluid restriction. Pay attention to intravenous drug therapy and fluid maintenance. Overall fluid input should not exceed 85 mL/hour or 1 mL/kg per hour. It may often be necessary to use double/triple concentrations of drugs to avoid infusion of excessive amounts of fluid. The choice between colloid or crystalloid solutions remains controversial. Crystalloid may reduce plasma colloid pressure even further. Colloid solutions may contribute to volume overload during postpartum fluid mobilisation [11]. Consider colloid solutions if the colloid osmotic pressure is markedly decreased (<12 mm Hg).

  
  
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  Take blood for renal and liver function (including albumin) tests and full blood count. Consider blood gas analysis, especially in cases where the cause of fluid overload is unclear.

  
  
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  Arrange for a chest radiograph to identify pulmonary oedema but also look for other underlying causes (pneumonia, cardiac disease).

  
  
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  Use diuretics in cases of acute pulmonary oedema and respiratory compromise with underlying cardiac disease or iatrogenic fluid overload. The use of diuretics in women with preeclampsia remains controversial because of the intravascular hypovolaemic state of preeclamptic women, despite the presence of peripheral oedema [9].