Sandefur Cameron C., Anjum Fatima
St. Bernards Medical Center in Jonesboro, AR
Temple University, Lewis Katz School of Medicine
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.
确定患者的血容量状态是临床医学中一个重要且有时具有挑战性的领域。液体平衡和循环对于维持血流动力学稳态至关重要。生理上，食物和液体的摄入与通过呼吸、尿液、粪便及皮肤的排出之间存在平衡。在健康状态下，我们的身体能够有效调节这种平衡以维持血流动力学稳定。然而，在患病时，这种稳定性会受到不利影响。如何受到影响呢？这因疾病的病理生理学而有显著差异，但例如呕吐、腹泻、肾衰竭、静脉输液、输血及发热等情况都可轻易导致液体量的增减。即便在健康个体中，血容量也会因体型、理想体重、身体组成（瘦体重与脂肪）、基础代谢率及营养等因素而有所不同。临床上，医生的职责是能够准确评估患者是血容量过多、血容量正常还是血容量不足。为此，医生主要依靠体格检查技能。诸如水肿、颈静脉扩张、黏膜湿润及肺部听诊有啰音等表现均提示患者存在容量超负荷。但是，如急性呼吸窘迫综合征（ARDS）所示，血浆容量状态与死亡率、重症监护病房停留时间及无呼吸机天数相关。估计血浆容量（ePV）在充血性心力衰竭情况下有助于提示发病率和死亡率。血浆容量（ePV）还充当炎症细胞因子的储存库，这些因子会增加患者尤其是患有心血管疾病和骨髓增殖性疾病患者的血栓形成风险。然而，特定的疾病病理可导致血管内液体转移至血管外间隙，导致临床表现为水肿，但实际上患者血管内血容量减少。那么问题就变成了，临床医生能做些什么来提高他们的诊断技能呢？一种选择是使用超声查看下腔静脉（IVC）的直径和可压缩性。如果IVC直径较小且易于压缩，则提示患者血容量不足。另一种更具侵入性的选择是检查中心静脉压（CVP）。不幸的是，CVP受多种因素影响，这使其可靠性降低。鉴于上述正确评估容量状态的困难，还有一项已存在数十年的检查可帮助医生做出决策。这项检查是容量研究，传统上在医院的核医学科进行。该研究的工作方法是通过将放射性示踪剂混入患者血液中。在一段时间内，示踪剂的浓度会稀释，我们可以将该浓度与已知体积在同一时期的稀释浓度进行比较。通过这种比较，我们可以计算患者的血容量。这种方法称为指示剂稀释技术。该技术于1915年首次使用红色染料（活性红）测量血浆容量。不幸的是，过去这项检查最快也要四到六个小时才能得出结果。这项研究主要用于帮助临床医生确定重症患者的容量状态。因此，由于获取结果所需的时间，该检查以前不太有用。然而，最近开发了一种用于血容量分析的新型半自动系统，可在约90分钟内得出结果。由于现在能更快获得结果，人们对这项研究重新产生了兴趣。为了解这项研究的工作原理，必须回顾血容量生理学。血容量由两个部分组成，红细胞容量（RCV）和血浆容量（PV）。RCV由携带氧气的红细胞（RBC）组成。RCV占总血容量的略少于一半。PV主要由水以及血浆蛋白（包括白蛋白）组成。血浆蛋白有助于维持将水从周围组织吸引到血管系统的胶体渗透压。如上所述，白蛋白水平低的患者会失去这种胶体渗透压，导致液体从血管内空间转移到血管外空间。最后要讨论的主题是血细胞比容。血细胞比容表示总血容量中由RBC组成的百分比。男性血细胞比容范围为42%至47%，女性为37%至43%。评估血浆容量的另一种选择是使用优化的一氧化碳再呼吸法来确定血红蛋白量，并由此确定血浆容量。
