Changes in circulating blood volume after infusion of hydroxyethyl starch 6% in critically ill patients

Colloids Yes or No? - a "Gretchen Question" Answered

Colloid solutions, both natural and synthetic, had been widely accepted as having superior volume expanding effects than crystalloids. Synthetic colloid solutions were previously considered at least as effective as natural colloids, as well as being cheaper and easily available. As a result, synthetic colloids (and HES in particular) were the preferred resuscitation fluid in many countries. In the past decade, several cascading events have called into question their efficacy and revealed their harmful effects. In 2013, the medicines authorities placed substantial restrictions on HES administration in people which has resulted in an overall decrease in their use. Whether natural colloids (such as albumin-containing solutions) should replace synthetic colloids remains inconclusive based on the current evidence. Albumin seems to be safer than synthetic colloids in people, but clear evidence of a positive effect on survival is still lacking. Furthermore, species-specific albumin is not widely available, while xenotransfusions with human serum albumin have known side effects. Veterinary data on the safety and efficacy of synthetic and natural colloids is limited to mostly retrospective evaluations or experimental studies with small numbers of patients (mainly dogs). Large, prospective, randomized, long-term outcome-oriented studies are lacking. This review focuses on advantages and disadvantages of synthetic and natural colloids in veterinary medicine. Adopting human guidelines is weighed against the particularities of our specific patient populations, including the risk-benefit ratio and lack of alternatives available in human medicine.

[Perioperative fluid management in major abdominal surgery]

Intravascular fluid administration belongs to the cornerstones of perioperative treatment with a substantial impact on surgical outcome especially with respect to major abdominal surgery. By avoidance of hypovolemia and hypervolemia, adequate perioperative fluid management significantly contributes to the reduction of insufficient tissue perfusion as a determinant of postoperative morbidity and mortality. The effective use of intravascular fluids requires detailed knowledge of the substances as well as measures to guide fluid therapy. Fluid management already starts preoperatively and should be continued in the postoperative setting (recovery room, peripheral ward) considering a patient-adjusted and surgery-adjusted hemodynamic monitoring. Communication between all team members participating in perioperative care is essential to optimize fluid management.

The half-life of infusion fluids: An educational review

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An understanding of the half-life (T_1/2) of infused fluids can help prevent iatrogenic problems such as volume overload and postoperative interstitial oedema. Simulations show that a prolongation of the T_1/2 for crystalloid fluid increases the plasma volume and promotes accumulation of fluid in the interstitial fluid space. The T_1/2 for crystalloids is usually 20 to 40 min in conscious humans but might extend to 80 min or longer in the presence of preoperative stress, dehydration, blood loss of <1 l or pregnancy.

The longest T_1/2 measured amounts to between 3 and 8 h and occurs during surgery and general anaesthesia with mechanical ventilation. This situation lasts as long as the anaesthesia. The mechanisms for the long T_1/2 are only partly understood, but involve adrenergic receptors and increased renin and aldosterone release. In contrast, the T_1/2 during the postoperative period is usually short, about 15 to 20 min, at least in response to new fluid.

The commonly used colloid fluids have an intravascular persistence T_1/2 of 2 to 3 h, which is shortened by inflammation. The fact that the elimination T_1/2 of the infused macromolecules is 2 to 6 times longer shows that they also reside outside the bloodstream. With a colloid, fluid volume is eliminated in line with its intravascular persistence, but there is insufficient data to know if this is the same in the clinical setting.

Automated pre-ejection period variation indexed to tidal volume predicts fluid responsiveness after cardiac surgery

BACKGROUND

Reliable continuous monitoring of fluid responsiveness is an unsolved issue in patients ventilated with low tidal volume. We hypothesised that variations in the pre-ejection period (PEP) defined as the time interval between electrocardiogram (ECG) R-wave and onset of systolic upstroke in arterial blood pressure could reliably predict fluid responsiveness in patients ventilated with moderately low tidal volume. Furthermore, we hypothesised that indexing dynamic parameters to tidal volume would improve their prediction. The aim was to refine and automate a previously suggested algorithm for PEP variation (DeltaPEP) and to test this new parameter indexed to tidal volume (PEPV), as a marker of fluid responsiveness along with central venous pressure (CVP), pulse pressure variation (PPV) and DeltaPEP. Additionally, the aim was to evaluate the concept of indexing dynamic parameters to tidal volume.

METHODS

Arterial pressure, CVP, ECG and cardiac index (CI) were acquired from 23 mechanically ventilated post-cardiac surgery patients scheduled for volume expansion. PEPV, PPV and DeltaPEP were extracted.

RESULTS

Using responder/non-responder classification (response=change in CI>+15%), sensitivity and specificity were 100% and 83%, respectively, for PEPV, 94% and 83% for DeltaPEP, and 94% and 83% for PPV. CVP offered no relevant information. Tidal volume indexing improved sensitivity for DeltaPEP to 100%.

CONCLUSION

In this study in post-cardiac surgery patients, a refined parameter, PEPV, predicted fluid responsiveness better than PPV and DeltaPEP. Our results suggest that dynamic parameters using variations in PEP should be indexed to tidal volume.

Therapeutic hypertension: principles and methods

The aspects of cardiovascular physiology important for the safe and effective implementation and titration of hypertensive therapy among neurosurgical patients with neurological or neurosurgical illness/injury are reviewed. Therapeutic hypertension may be an appropriate treatment for some neurological or neurosurgical conditions, e.g., vasospasm or support of cerebral perfusion pressure. Initiation and maintenance of hypertension should be done safely to avoid complications and/or undesired side effects. Accurate measurement of the arterial and central vascular pressures, the limitations of those methods, and alternative estimates of intravascular volume are reviewed. Hypertensive therapy is accomplished by modifying cardiac output and systemic vascular resistance, the principal physiological determinants of blood pressure. The goals of hypertensive therapy can be achieved by proper evaluation and manipulation of the four components of cardiac output, preload, afterload, heart rate and contractility. Measurement or calculation of estimates of these parameters is important in the selection of proper medications or supplemental fluid administration.

[Exact measurement of the volume effect of 6% hydoxyethyl starch 130/0.4 (Voluven) during acute preoperative normovolemic hemodilution]

BACKGROUND

What is the effect of preoperative acute normovolemic hemodilution (ANH) with 6% hydroxyethyl starch (HES) 130/0.4 (Voluven) on blood volume?

METHODS

In 10 patients undergoing radical hysterectomy, ANH was performed to a hematocrit of 21% using 6% HES 130/0.4 (Voluven) whereby a replacement of blood with 115% of colloid was planned. Plasma volume (indocyanine green dilution technique) and hematocrit were determined before, 30 and 60 min after ANH. Red cell volume (labelling erythrocytes with fluorescein) was determined before and 30 min after ANH.

RESULTS

After removal of 1,431+/-388 ml of blood and simultaneous replacement with 1,686+/-437 ml of colloid, blood volumes were 218+/-174 ml higher than before (at 105+/-4%). The volume effect was 98+/-12%, 30 min after ANH. Even 60 min after ANH, mean blood volumes were with 4,228+/-986 ml slightly higher than before ANH (102+/-5%). The hematocrit decreased disproportionally in relation to the residual intravascular volume. Consequently, estimating the volume effect from the changes in hematocrit led to an overestimation (about +30%).

CONCLUSION

Double label measurements of blood volume demonstrated that the volume effect of 6% HES 130/0.4 (Voluven) is about 100% in the course of ANH. The reason for the disproportionally large decrease in hematocrits could be the mobilization of a fraction of the plasma volume which was retained within the endothelial glycocalyx.