Vol. 21 •Issue 19 • Page 15
Identifying Optimal Hematocrit Levels
Hct Has a Direct Influence on Oxygen Transport
As respiratory therapists, our bread and butter is oxygenation. We are used to juggling FIO2 flow rates and PEEP levels to maintain a desired PaO2 But today there’s a renewed interest in looking at optimal hematocrit (Hct) levels.
The concept of an optimal Hct level has existed since the 1920s. While researchers of that era didn’t understand the physiologic mechanisms associated with oxygen transport, they did have an understanding of the necessity of maintaining adequate blood volume levels to improve outcomes.1 Today we have a better appreciation of how a patient’s Hct has a direct influence on oxygen transport.2 Researchers are now harnessing this understanding and investigating whether some patients may benefit from keeping Hct within a certain range.
In developing their data, they recognize that optimal Hct levels are directly related to shear rates and blood vessel radii within the circulatory system. Wall shear stress within the blood vessel is dependent on the shear rate and viscosity of the blood.
The blood vessel endothelia regulate this shear stress on their walls by altering their radius. This regulation of the vessel radius has led to the hypothesis that alterations in plasma volume will have a direct impact on the determination of the optimal
hematocrit.3,4Investigators are working to mimic or at least better understand the body’s capability to manipulate blood volume and subsequently Hct levels and translate this into clinical practice.5
First, let’s do a little house cleaning: The measurement of hemoglobin (Hb), rather than Hct, is the preferred method for assessing oxygen-carrying capacity.6The Hct, a calculated value, is, however, used in many studies, sometimes interchangeable with Hb; and this creates confusion.
A lot of the data related to identifying optimal Hct levels come from studies of renal patients undergoing dialysis. There’s an inverse relationship among Hb concentration, risk of hospitalization and mortality and cost in hemodialysis patients.7-9
To minimize these problems, the National Kidney Foundation Kidney Disease Outcomes Quality Initiative (K/DOQI) guidelines recommend a target Hb be in the range of 11 to 12 g/dL among both hemodialysis patients and chronic kidney disease patients not yet on dialysis.6Studies are under way to determine whether, in fact, this is the ideal target and what are the outcomes associated with patients falling either above or below this range. Both the CREATE and CHOIR studies attempted to determine whether higher Hb concentrations would be associated with improved outcomes in patients not yet requiring dialysis.10,11Neither study, however, demonstrated a benefit from the higher Hb concentrations. Some data suggest the longer a patient remains below the recommended K/DOQI Hb target, the greater the risk of additional hospitalizations and mortality.12
Fluctuation in Levels
It’s easy to see why the Hb/Hct levels of dialysis patients fluctuate. Their blood volume levels are routinely being manipulated via the dialysis process. Understanding why Hct levels fluctuate in the ICU patient is more complex.
Depending on the individual case, an acute drop in Hct may be related to acute blood loss post-trauma, gastrointestinal hemorrhage, surgery, treatment with chemotherapeutic agents or numerous chronic medical conditions.11From a pulmonary perspective, the optimum Hct in the ICU is one that permits
oxygenation and does not require the patient to undergo transfusion.13Several studies have shown that blood transfusions increase the potential for nosocomial infection as well as the occurrence of transfusion-related acute lung injury (TRALI).14,15It is believed TRALI—the third most common cause of transfusion-related death—occurs because of the activation of primed neutrophils. Researchers suggest that sub-clinical forms of TRALI may occur and are unrecognized and underreported.16,17Utilization of red blood cell transfusions also has been linked to the occurrence of ventilator-associated pneumonia.18At least from a pulmonary standpoint, the identification and maintenance of an optimal Hct level would be beneficial.
While complex etiologies in the ICU patient make research in optimal Hct daunting, researchers are making headway. Currently, there are trials under way investigating the role for both artificial Hb and exogenous erythropoietin.19,20Using Hb solutions as potential alternatives to blood transfusion to restore a patient’s Hct are among innovations being studied. Specifically, scientists are investigating polymerized bovine Hb solutions. Early data show that some of these compounds can restore higher arterial pressures and trigger vasoconstriction.5Further studies are needed to establish the correct dosage, efficacy, safety and effects on outcome in humans. In the near future, we could be presented with a patient who has started on these medications.
As therapists, our role in maintaining oxygenation is critical. To move oxygen around the body, we need the Hct to be stable and substantial enough to support the patient’s metabolic needs.
As our understanding of optimal Hct develops, we may soon find ourselves monitoring Hct ranges as part of our ventilator checks. And why not? How’s this different from any other parameter?
Margaret Clark is a Georgia practitioner.
For a list of references, visit www.advanceweb.com/rcp and click on the “magazine” tab.