Plasma Volume Study

Excerpt

Determination of a patient's blood volume status is an important and sometimes challenging area of clinical medicine. Fluid balance and circulation are essential in helping to maintain hemodynamic homeostasis. Physiologically, a balance exists between the intake of food and liquids versus output through respiration, urine, feces, and skin. In health, our bodies can efficiently regulate this balance to maintain hemodynamic stability. However, in illness, this stability is adversely affected. How is it affected? It varies significantly based on the pathophysiology of the disease, but fluid can easily shift high or low with vomiting, diarrhea, renal failure, intravenous fluids, blood transfusion, and fever, to name a few. Even within healthy individuals, the blood volume can vary based on body size, ideal body weight, body composition (lean body mass vs. fat), basal metabolic rate, and nutrition, among others.

Clinically, it is the physician's job to be able to accurately assess whether a patient is hypervolemic, euvolemic, or hypovolemic. To do this, physicians rely predominately on their physical exam skills. Findings, such as edema, jugular venous distension, moist mucus membranes, and crackles on lung auscultation, all suggest that a patient is volume overloaded. But, as exemplified by Acute Respiratory Distress Syndrome (ARDS), the status of the plasma volume correlated with mortality, ICU stay, and ventilator-free days. An estimated Plasma VOlume (ePV) can be useful in the setting of congestive heart failure to suggest morbidity and mortality. The Plamsam volume (ePV) has also served as a repository of inflammatory cytokines that increase the thrombotic risk in patients, particularly those with cardiovascular problems and patients having myeloproliferative disease. However, specific disease pathologies can lead to a shift of intravascular fluid into the extravascular space, leading to edema on clinical presentation, but in actuality, the patient is intravascularly depleted. The question then becomes, what can clinicians do to improve their diagnostic skills? One option is to use an ultrasound to look at the diameter and compressibility of the inferior vena cava (IVC). If the IVC is smaller in diameter and easily compressible, then this suggests that the patient is hypovolemic. Another more invasive option is to check central venous pressure (CVP). Unfortunately, the CVP is affected by multiple factors, which makes it less reliable.

Given the above difficulties with correctly assessing volume status, there is another test that has been around for decades to help physicians make their decisions. This test is a volume study, and it has traditionally taken place in the nuclear medicine departments of hospitals. The method by which the study works is through the use of radioactive tracers mixed into the patient's blood. Over a set period over time, the concentration of the tracer will dilute, and we can compare this concentration to the dilution of a concentration of a known volume over the same period. With this comparison, we can calculate the patient's blood volume. This method is known as the indicator dilution technique. This technique was first used for the measurement of plasma volume in 1915 using red dye(Vital Red).

Unfortunately, in the past, this test could, at best, yield results in four to six hours. This study is primarily needed to help clinicians determine volume status in critically ill patients. Therefore, the test was previously not very beneficial due to the time it took to obtain results. However, there has more recently been the development of a new semi-automated system for blood volume analysis that can yield results in around 90 minutes. Since results are more quickly obtainable now, there has been renewed interest in this study.

To understand how this study works, one must review blood volume physiology. Blood volume consists of two components, the red cell volume (RCV) and the plasma volume (PV). The RCV is composed of red blood cells (RBCs), which carry oxygen. The RCV represents slightly less than half of the total blood volume. The PV is predominately composed of water as well as plasma proteins, including albumin. The plasma proteins help to maintain the oncotic pressure that draws water from surrounding tissues into the vasculature. Patients with low albumin lose this oncotic pressure, which results in fluid shifting from the intravascular space into the extravascular space, as described above. The final topic to discuss is the hematocrit. Hematocrit represents the percentage of the total blood volume that is composed of RBCs. The range for hematocrit is 42 to 47% in men and 37 to 43% in women.

Another option for the assessment of plasma volume uses the optimized carbon monoxide-rebreathing method to determine the hemoglobin mass and from this the plasma volume.