Fluid overload after cardiac surgery

Fluid Overload After Cardiac Surgery – to Pee or to PD?

In this case-based article, we discuss the significance of fluid overload following paediatric cardiac surgery by evaluating the two main strategies – diuretics and peritoneal dialysis (PD).

A 3-month-old 4.5kg infant underwent elective repair of a significant ventricular septal defect because of worsening congestive cardiac failure symptoms and poor growth despite fluid restriction and diuretics.

The operation was performed via open sternotomy on cardiopulmonary bypass. It was uncomplicated, and they were admitted, intubated, to paediatric cardiac intensive care on a low-dose adrenaline infusion and had chest and peritoneal drains in situ.


Around 8 hours post-operatively, signs of low cardiac output syndrome (LCOS) developed. They were tachycardic (170), mildly hypotensive with a narrow pulse pressure (50/38, mean 42), weak pulses, and cool peripheries with a delayed capillary refill time (4s). In addition, they had mild lactataemia (3) and an increased oxygen extraction ratio (50%)¹⁰. The LCOS was treated with cautious intravenous crystalloid boluses (in 5ml/kg aliquots), escalating vasoactive infusions (adrenaline, noradrenaline and milrinone) and deep sedation.

By 20 hours post-operatively, the LCOS was improving, but urine output remained poor (0.6ml/kg/hr). They were up 130ml, and blood tests revealed a stage 2 AKI with mild hyperkalaemia. Oxygenation had worsened (PEEP 8, FiO2 0.45).

An echocardiogram found no residual structural lesion but mildly impaired biventricular systolic function. The PICU pharmacist reviewed the medication list given the renal impairment and adjusted doses accordingly.

The team diagnosed fluid overload which was hampering progress. They wondered whether they should commence a furosemide infusion or peritoneal dialysis. 

What are the consequences of fluid overload?

Fluid overload has consequences in critically ill children. Even after adjusting for disease severity, overload (defined by a positive measured fluid balance or increase in weight) is associated with increased mortality, prolonged PICU/hospital stay, prolonged mechanical ventilation and acute kidney injury (AKI).^1,2,3,4 One key mechanism is pulmonary oedema, but interstitial oedema can develop and cause dysfunction in any organ (e.g. heart, brain, kidneys, liver).⁵ Fluid may also accumulate in other body compartments (e.g. pleural effusions, ascites) is also problematic.

Around 50% of infants experience fluid overload (a gain of more than 10% of their body weight) after cardiac surgery.^{6 This may be due to acute kidney injury (AKI), syndrome of inappropriate antidiuretic hormone secretion (SIADH), post-cardiopulmonary bypass systemic inflammatory response syndrome (SIRS) with capillary leak, post-cardiopulmonary bypass low cardiac output syndrome (LCOS) coupled with iatrogenic fluid administration.}

Fluid overload in the post-congenital heart surgery population is associated with increased morbidity and mortality. It is also associated with increased mortality, prolonged PICU/hospital stay, increased duration of mechanical ventilation, increased vasoactive medication use, AKI and infection rate.⁷

How do we remove fluid post-cardiac surgery?

Achieving haemodynamic stability is the priority Immediately following paediatric cardiac surgery. This can be tricky because the consequences of being on cardiopulmonary bypass (SIRS, LCOS, AKI) usually take hours to become apparent.

Once haemodynamic stability has been achieved, however, the focus shifts towards combatting the near-inevitable accumulation of fluid. Minimising fluid input is often challenging. Our small patients often require multiple drug infusions. Other inputs to consider include resuscitation fluid/blood products, chest/peritoneal drain loss replacement, and parenteral nutrition. Modifying the other side of the equation is easier – fluid removal.

The two options in this patient group are diuretics or peritoneal dialysis.

Diuretics

Diuretic medications are the mainstay of treatment. The loop diuretic, furosemide, is often the first-line agent. Therapy aims to increase urine output, though it may have limited efficacy due to the prevalence of acute kidney injury. The primary complication of their use is electrolyte disturbances. 

Peritoneal dialysis

Peritoneal dialysis (PD) may be used alone or with diuretic therapy after cardiac surgery. Warmed dialysate is instilled into the peritoneal cavity and left to dwell. The peritoneum acts as a semi-permeable membrane, perfused by the peritoneal circulation. The fluid is then drained off, and the cycle repeated.

The dialysate is a buffered crystalloid solution. This is hyper-osmolar to serum, so osmosis draws fluid across the peritoneal membrane. In addition, small molecules (solutes) such as electrolytes equilibrate across the peritoneum by diffusion.

When prescribing PD in this setting, consider:

• Dialysate solution – particularly the glucose concentration (typically 1.5% or 2.5%). This determines the osmolarity and, therefore, the rate of fluid removal, including any additional electrolytes/other drugs.
• Cycle volume – Starting at 10ml/kg (can be increased for more fluid/solute removal)
• Cycle time – the sum of instillation, dwell and drainage times
• Instillation and drainage times – instillation and drainage are usually gravity-mediated processes and so we have little control here
• Dwell time – typically 20-60 minutes. Shorter times lead to more frequent cycles with more fluid/solute removed

The absolute indications for acute PD are the same as for other forms of renal replacement therapy – renal failure with intractable and symptomatic fluid overload/hyperkalaemia/metabolic acidosis/uraemia or certain intoxications.

AKI occurs in ~50% of infants/children following cardiac surgery.⁸ Because of this, there is increasing interest in using PD earlier in treating (or even preventing) fluid overload. Peritoneal dialysis has many advantages over the other method of renal replacement therapy used in the PICU – continuous venovenous haemodiafiltration (CVVHDF) – in the post-cardiac surgery population.

Peritoneal dialysis catheters are often placed routinely following major paediatric cardiac surgery. This allows easy access for PD and drains the peritoneal fluid that accumulates. In contrast, central venous access for CVVHDF is often extremely challenging in these usually tiny patients.

Fluid shifts are slower in peritoneal dialysis and may be better tolerated in patients at risk of haemodynamic instability. PD has no absolute contraindications, though it should be used with caution in recent abdominal surgery, peritonitis, peritoneal adhesions, diaphragmatic defect or severe cardiac/respiratory failure.

Complications of acute peritoneal dialysis include impaired ventilation, hypovolaemia, abdominal compartment syndrome, pain/discomfort, electrolyte disturbances, hyperglycaemia, protein loss and peritonitis. The risk is higher with more aggressive dialysis (larger cycle volumes and shorter cycle times).⁹ 

Peritoneal dialysis can remove larger fluid volumes and is better placed to balance electrolytes and other solutes, particularly in renal failure.

What is the evidence for one treatment over the other?

Only one randomised controlled trial has directly compared PD with diuretic therapy for oliguria following cardiac surgery.

This single-centre non-blinded study recruited infants (<6 months) undergoing cardiac surgery with peritoneal catheter placement. If they developed oliguria (4 hours of urine output <1ml/kg/hr in the first 24 postoperative hours), they were randomised to either furosemide (IV 1mg/kg 6-hourly) or a standardised PD regimen.

There was no significant difference between groups in the primary outcome – incidence of negative fluid balance on postoperative day 1. However, the furosemide group was three times more likely to have 10% fluid overload (OR 3.0, 95% CI 1.3-6.9) and prolonged ventilator use (OR 3.1, 95% CI 1.2-8.2).

Vasoactive medications were used longer, and more electrolyte abnormalities were found in the furosemide group. There was no statistically significant difference in mortality, although the study was underpowered. The length of stay was similar between groups. No serious complications were noted in either group, though PD was discontinued early in 22% of patients due to pleuro-peritoneal communications.¹¹

Two recent systematic reviews and meta-analyses have been conducted (including the above trial), and a small number of observational studies (with marked heterogeneity in the results).

The first compared PD with diuretics in children following cardiac surgery. It found that PD was associated with a shorter duration of mechanical ventilation (-1.25 days, p = 0.008) but led to increased mortality (OR 2.27; p = 0.02). There were no differences in PICU length of stay and degree of negative fluid balance by postoperative day one or peritonitis.¹²

The second meta-analysis looked explicitly at early PD (commenced within 24 hours of operation) versus late/not at all in infants following cardiac surgery. Early initiation of PD was associated with decreased mortality (OR 0.43, 95% CI 0.23-0.80), a shorter duration of mechanical ventilation (-1.09 days, 95% CI -1.09d to -0.33d) and decreased length of PICU stay (-2.46d, 95% CI -3.57d to -1.35d).¹³

Peritoneal dialysis and diuretics are usually used to treat fluid overload or oliguria, however one single-centre non-blinded prospective cohort (before-and-after) study found improved clinical outcomes in children following cardiopulmonary bypass when PD was used prophylactically.

It included 52 infants who were at high risk for post-operative fluid overload. Half received passive peritoneal drainage and diuretics, whilst the other half received prophylactic PD in the immediate postoperative period. The groups were similar in terms of demographics, diagnoses and intra-operative variables.

Patients receiving PD were more likely to have a negative fluid balance at 24 hours (-24ml/kg vs +18ml/kg, p = 0.003). Interestingly, this was more due to fluid input than output. Urine, peritoneal and chest drain outputs were similar in both groups at 24 hours.

Those receiving PD had lower mean vasoactive infusion scores at 24 hours, had earlier sternal closures (24h vs 63h, p <0.001) and lower inflammatory markers (IL-6 and IL-8) at 24 hours. There was a non-significant signal towards a shorter duration of mechanical ventilation in those receiving PD (71h vs 125h, p = 0.10).¹⁴

Take-homes

Fluid overload is extremely common following paediatric cardiac surgery and carries serious morbidity and mortality risks.

It can occur with or without AKI.

The two main methods of fluid removal are diuretic medications and peritoneal dialysis (PD).

Firm reasons for choosing dialysis over diuretics exist, but we need to prevent and treat fluid overload even in the absence of intractable renal failure.

There is some evidence to suggest improved outcomes with PD compared to diuretics when used to treat or prevent fluid overload following paediatric cardiac surgery.

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