Four Phases of Intravenous Fluid Therapy: A Conceptual Model
Four Phases of Intravenous Fluid Therapy: A Conceptual Model
Several trials in recent years have examined the effect of different compositions of i.v. fluids in varying scenarios. Identifying the stage of fluid resuscitation in which these trials were conducted may affect the way they are interpreted. The FIRST trial described a group of patients undergoing resuscitation after major trauma. These patients were severely injured with high injury severity scores and significantly elevated plasma lactate levels. The patients required in excess of 5 litre of i.v. fluid within the first 24 h demonstrating that this trial took part in the Rescue phase of resuscitation. Similarly, the CRISTAL trial enrolled severely hypotensive septic patients who required very large volumes of fluid—again demonstrating a 'Rescue Trial'.
In contrast, when we examine the baseline characteristics of the large SAFE and CHEST studies, which both included ~7000 ICU patients, most patients were not, at the point of entry into the trial, in the Rescue phase. These patients were more commonly (at the point of ICU admission) in either the Optimization phase of resuscitation as evidenced by the significantly lower volumes of fluid administered and longer time from presentation to enrolment. In a similar vein, the majority of trials in the perioperative fluid therapy have generally been conducted within the Optimization phase.
One particular subgroup of patients is those receiving fluids in the perioperative setting (typically in the Optimization phase). In this category, several clinical trials (and indeed meta-analyses and systematic reviews) have demonstrated the benefit of using minimally invasive monitors of fluid responsiveness to guide goal-directed fluid therapy in order to optimize tissue oxygen delivery. However, this evidence may need to be reconsidered, as the respiratory conditions used in these studies may have been suboptimal. A recent large study found that in patients at risk for pulmonary complications who underwent major abdominal surgery, a lung-protective (low tidal volume) ventilation strategy during the operation resulted in less pulmonary and extra-pulmonary complications within the first week after surgery compared with a non-protective mechanical ventilation. This change in ventilation management with lower tidal volumes will lead to a reduction in changes in intra-thoracic pressure during the respiratory cycle and a subsequent decrease in variation in venous return and resulting stroke volume/systolic pressure.
Fluid may also be administered to patients without significant, or even any, fluid losses. For example, fluid may be given for the prevention of organ damage, for example, before contrast administration, in cirrhotic patients with spontaneous bacterial peritonitis, or maintenance fluid administration in patients who cannot tolerate oral fluid intake. In these situations, fluid infusions are generally utilized; however, the amount and type of fluid may vary. Consensus guidelines for preventing contrast nephropathy recommend using crystalloids (saline or bicarbonate-based solutions) at rates of 1–1.5 ml kg h for 12 h before and 12 h after the contrast procedure. These recommendations are based on achieving urine volumes >150 ml h as these levels have been associated with decreased risk for AKI. For emergent cases, when a 12 h prehydration regimen is not possible, 3 ml kg h is recommended for 1 h before and continued for 6 h after the procedure. In contrast, in cirrhotic patients, albumin solutions are preferred to manage spontaneous bacterial peritonitis and to reduce the effects of large-volume paracentesis. It should be emphasized that fluid management (fluid infusion: see Box 2) in these cases is designed to optimize tissue perfusion and reduce risk for organ toxicity; however, it needs to be carefully titrated based on underlying co-morbidities, particularly decreased renal function and heart failure. Appropriate de-escalation of fluids and fluid mobilization are equally important in these situations to prevent the cumulative effect of fluid administration during the patient's hospital course. Maintenance fluids given for patients who cannot tolerate oral fluids or who are awaiting surgical or radiological procedures are similarly subject to wide variation. Underlying co-morbidities including diabetes and chronic kidney disease often dictate the amount and type of fluid used. We recommend that underlying co-morbidities should be considered in the same context of 'fit for purpose' to individualize maintenance fluid therapy with careful monitoring to prevent fluid accumulation.
Discussion
Stages of Fluid Therapy: Relevance to Clinical Trials
Several trials in recent years have examined the effect of different compositions of i.v. fluids in varying scenarios. Identifying the stage of fluid resuscitation in which these trials were conducted may affect the way they are interpreted. The FIRST trial described a group of patients undergoing resuscitation after major trauma. These patients were severely injured with high injury severity scores and significantly elevated plasma lactate levels. The patients required in excess of 5 litre of i.v. fluid within the first 24 h demonstrating that this trial took part in the Rescue phase of resuscitation. Similarly, the CRISTAL trial enrolled severely hypotensive septic patients who required very large volumes of fluid—again demonstrating a 'Rescue Trial'.
In contrast, when we examine the baseline characteristics of the large SAFE and CHEST studies, which both included ~7000 ICU patients, most patients were not, at the point of entry into the trial, in the Rescue phase. These patients were more commonly (at the point of ICU admission) in either the Optimization phase of resuscitation as evidenced by the significantly lower volumes of fluid administered and longer time from presentation to enrolment. In a similar vein, the majority of trials in the perioperative fluid therapy have generally been conducted within the Optimization phase.
Fluid Resuscitation in the Perioperative Period
One particular subgroup of patients is those receiving fluids in the perioperative setting (typically in the Optimization phase). In this category, several clinical trials (and indeed meta-analyses and systematic reviews) have demonstrated the benefit of using minimally invasive monitors of fluid responsiveness to guide goal-directed fluid therapy in order to optimize tissue oxygen delivery. However, this evidence may need to be reconsidered, as the respiratory conditions used in these studies may have been suboptimal. A recent large study found that in patients at risk for pulmonary complications who underwent major abdominal surgery, a lung-protective (low tidal volume) ventilation strategy during the operation resulted in less pulmonary and extra-pulmonary complications within the first week after surgery compared with a non-protective mechanical ventilation. This change in ventilation management with lower tidal volumes will lead to a reduction in changes in intra-thoracic pressure during the respiratory cycle and a subsequent decrease in variation in venous return and resulting stroke volume/systolic pressure.
Fluid Therapy for the Prevention of Organ Damage in Specific Cohorts
Fluid may also be administered to patients without significant, or even any, fluid losses. For example, fluid may be given for the prevention of organ damage, for example, before contrast administration, in cirrhotic patients with spontaneous bacterial peritonitis, or maintenance fluid administration in patients who cannot tolerate oral fluid intake. In these situations, fluid infusions are generally utilized; however, the amount and type of fluid may vary. Consensus guidelines for preventing contrast nephropathy recommend using crystalloids (saline or bicarbonate-based solutions) at rates of 1–1.5 ml kg h for 12 h before and 12 h after the contrast procedure. These recommendations are based on achieving urine volumes >150 ml h as these levels have been associated with decreased risk for AKI. For emergent cases, when a 12 h prehydration regimen is not possible, 3 ml kg h is recommended for 1 h before and continued for 6 h after the procedure. In contrast, in cirrhotic patients, albumin solutions are preferred to manage spontaneous bacterial peritonitis and to reduce the effects of large-volume paracentesis. It should be emphasized that fluid management (fluid infusion: see Box 2) in these cases is designed to optimize tissue perfusion and reduce risk for organ toxicity; however, it needs to be carefully titrated based on underlying co-morbidities, particularly decreased renal function and heart failure. Appropriate de-escalation of fluids and fluid mobilization are equally important in these situations to prevent the cumulative effect of fluid administration during the patient's hospital course. Maintenance fluids given for patients who cannot tolerate oral fluids or who are awaiting surgical or radiological procedures are similarly subject to wide variation. Underlying co-morbidities including diabetes and chronic kidney disease often dictate the amount and type of fluid used. We recommend that underlying co-morbidities should be considered in the same context of 'fit for purpose' to individualize maintenance fluid therapy with careful monitoring to prevent fluid accumulation.
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