Administration of fluid boluses is considered to be one of the cornerstones of sepsis resuscitation. The surviving sepsis guidelines continue to ardently recommend a fluid bolus of 30 ml/kg within 3 h of presentation to hospital in patients who are hypotensive and considered to have sepsis.1 Let us consider the physiological rationale behind fluid administration in patients with sepsis and hypotension.
The physiological rationale behind fluid resuscitation
Fluid resuscitation is based on the Starling curve that correlates preload with stroke volume. The early, ascending part of the curve represents fluid responsiveness; if the preload is increased along this part of the curve, the stroke volume increases. The increase in stroke volume plateaus off towards the latter part of the curve, with no further increase in stroke volume with an additional increase in the preload. Based on this physiological response, bolus fluids are administered, with an expectation that the stroke volume would increase and improve blood flow to the vital organs. However, there are two important considerations with this approach. First, echocardiography based studies have demonstrated that up to 50% of normal subjects may also be fluid responsive;2 second, the stroke volume is normal or high in most patients with sepsis and it may seem counter-intuitive to attempt to increase it further by increasing the preload. Indeed, studies aimed at increasing the cardiac output and oxygen delivery to “supranormal” targets have uniformly demonstrated a lack of benefit or possible harm using this strategy.3
How much fluid, for how long?
Among patients with sepsis, only 5% of a crystalloid solution may remain in the circulation after an hour of infusion.4 This corroborates with the transient improvement in hemodynamic parameters observed after bolus fluid therapy in septic patients. Hence, the question arises, considering the lack of sustained improvement, how often could fluid boluses be repeated? Inevitably, excessive fluid administration leads to the breakdown of the endothelial glycocalyceal barrier and tissue edema, with impaired organ function. Furthermore, fluid resuscitation may also lead to reduced systemic vascular resistance5 and reduce perfusion pressure to vital organs. There is provocative evidence from a human volunteer study suggesting that an increase in mean arterial pressure may be more related to the temperature of the fluid, compared to the volume administered; administration of intravenous fluid at room temperature (22°C) resulted in a higher mean arterial pressure compared to fluid that was warmed to 38°C.6
Fluid administration, increasing venous pressures, and effects on organ perfusion
The central venous pressure (CVP) has been recommended to guide intravenous fluid therapy in septic patients. Blood flow to the vital organs is determined by the difference between mean arterial pressure, the upstream pressure, and venous pressure, the downstream pressure. An inappropriate rise in CVP may thus compromise organ perfusion, besides reducing venous return. In fact, the venous pressure may be a more decisive factor that determines organ perfusion. A drop in MAP within the autoregulatory range may not compromise organ perfusion. However, capillary blood flow may be solely determined by venous pressures within the autoregulatory range of arterial pressures.7
Effect of fluid boluses on cardiac function
Sepsis is generally characterized by a preserved or higher than normal cardiac output. Left ventricular systolic function is usually well maintained; however, there is increasing evidence of significant diastolic dysfunction in septic patients.8 Aggressive fluid resuscitation in patients with impaired diastolic dysfunction leads to raised systemic venous and pulmonary artery pressures, with no significant increase in the stroke volume. Furthermore, excessive fluid resuscitation may exacerbate and perpetuate diastolic dysfunction.
What is the clinical evidence?
Measurement of the stroke volume by echocardiography demonstrated fluid responsiveness following 500 ml of a crystalloid solution in only 53% of patients with septic shock.9 A study of over 3000 children with severe sepsis revealed convincing evidence of significantly higher 48 h mortality with the use of bolus intravenous fluid compared to maintenance fluids alone as part of the initial resuscitation strategy.10 In a Zambian study among patients with sepsis and hypotension, early resuscitation with bolus intravenous fluid, vasopressors, and blood transfusion was compared with usual care based on clinician judgment. The intervention group received significantly more intravenous fluid in the first 6 h; however, in-hospital mortality was significantly lower among patients who received treatment based on clinician judgment.11 The CLASSIC study randomized patients with septic shock to receive fluid boluses until circulatory parameters continued to improve or a restrictive strategy with the administration of bolus fluid only if signs of severe hypoperfusion were present. In the fluid-restrictive group, the incidence of worsening of acute kidney injury was significantly less common.12
What may be a more optimal strategy?
Conventional measures, including CVP, inferior vena caval diameter variation, and central venous oxygen saturation have limited value in the assessment of fluid responsiveness. The passive leg raising test (PLR) coupled with stroke volume monitoring may be more efficacious and safer compared to the administration of bolus fluid as a “challenge dose”. When performed optimally, the PLR mobilizes approximately 300 ml of blood from the peripheral to the central compartment. A change in stroke volume by this intervention may be conveniently evaluated by measuring the velocity-time integral by bedside transthoracic echocardiography. Unlike administration of a fluid bolus, any effect on the hemodynamic status is completely reversed on reassuming the horizontal position of the legs. PLR-induced increase in cardiac output has been shown to be highly predictive of fluid responsiveness in a meta-analysis.13 The importance of reassessment of fluid responsiveness needs to be emphasized prior to the administration of repeated fluid boluses. If fluid responsiveness cannot be reliably assessed, a mini-fluid challenge with 200-500 ml of fluid over 30 min may be much safer compared to larger boluses of 30 ml/kg.14
Early use of vasopressors
Alpha-1 agonists including norepinephrine, at low doses, result in venoconstriction earlier than arterial constriction. Venoconstriction mobilizes unstressed blood volume from the skin and the splanchnic circulation, leading to increased preload and cardiac output. Besides, norepinephrine effectively produces arterial constriction, reversing the vasodilation-induced hypotension among patients with septic shock. Intuitively, a restricted fluid strategy combined with early vasopressor therapy, may be more appropriate in septic shock, characterized by intense vasodilation. Such an approach needs future studies to evaluate clinical outcomes, compared to a fluids-first strategy.
The bottom line:
- Empirical, large volume fluid boluses during resuscitation of septic patients may be ineffective and lead to harm.
- The hemodynamic effect of bolus fluid administration is usually transient; repeated boluses lead to tissue edema and impaired organ function.
- Excessive fluid administration has been shown to cause increased mortality compared to a fluid-restrictive strategy.
- The passive leg raising test combined with echocardiographic assessment of the stroke volume may be a safer, more effective method of evaluating fluid-responsiveness compared to repeated fluid challenges.
- Early use of vasopressors such as noradrenaline combined with restricted fluid resuscitation may be a more optimal strategy among septic patients.
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