Monitoring, the gauges

Determining the presence and extent volume related conditions is crucial for diagnosing their underlying causes and to correct for them. Thus, one of the first steps in the treatment of critically and acutely ill patients is an accurate assessment of their intravascular volume.

From a clinical perspective, that is, from the perspective of someone trying to treat a patient, a more useful measure than volume status alone would be a patients volume responsiveness. Here a distinction must be made. It is not enough to merely know how full the tank is, clinicians must know how they can alter its contents (by either adding or removing fluids) and in what ways this alteration will affect the health of their patients. Therefore, in this context it is more pertinent to speak in terms of a patients potential response to a fluid than it would be to that patients overall volume. A simple enough test for responsiveness can be thought of easily: give patients some volume of a fluid and see how they respond. This is known as a fluid challenge and it gives clinicians the ability to gauge the preload reserve of patients [14], allowing them to identify those patients likely to benefit from further volume resuscitation. Such a technique is utilized in many cases, such as apparent hypovolemia, hypoperfusion, or in at-risk surgical patients [15, 16]. One must be careful in implementing and interpreting a fluid challenge, though, as research has shown that healthy volunteers can experience significant increases in stroke volume in response to virtual fluid challenges [17], suggesting that although one may respond to an initial fluid challenge, a continued fluid loading treatment may not be necessary.

Furthermore, a fluid challenge is limited to those patients whose ventricular wall could be distended from a small increase in volume. For those patients with stiffened hearts or those already hypervolemic, this type of test cannot yield any viable results of either volume status or usefully scale volume responsiveness. For those situations where a fluid challenge may not be possible or appropriate, many other techniques have been proposed and utilized throughout clinical practice.

However, several of the techniques used to predict this volume responsiveness only do so correctly about 50% of the time. Pair this with the fact that only about 50% of critically ill patients respond to volume expansion [18] and ones mind may begin to reel with the idea that clinicians need a better way of detecting volume status and volume responsiveness to treat their patients. The approach I have taken to assess volume responsiveness, one that I believe the evidence shows is better than each of those listed below, is outlined thoroughly elsewhere. For now, let us review the state of the field and see where each technique excels and where it may stumble.

Of the various ways clinicians have at their disposal to measure volume status and responsiveness, multiple different kinds of classification systems that can be made. One convenient classification system that many researchers on the topic have used is dividing the field into static and dynamic measures [19, 20, 21]. The distinction here between static and dynamic is whether a signal needs to change with respect to something (time, a respiratory maneuver, etc.) to reflect volume status/responsiveness. Those that require a change are said to be dynamic, those that do not require a change, static. A further useful subdivision exists between what each of these techniques measures. In the case of volume status assessment the primary parameters of interest are pressures, volumes, and variations there of. Pressures and volumes without variation lie within the static side of our categorization and those with variation constitute part of the dynamic side of our category.

  1. The physical exam
  2. Static measures of intravascular volume
    1. Pressures
      1. Central venous pressure
      2. Pulmonary artery occlusion pressure
      3. Other pressures
    2. Volumes
      1. Right ventricular end-diastolic volume
      2. Left ventricular end-diastolic area
      3. Inferior vena cava diameter
    3. Bioimpedance vector analysis
  3. Dynamic measures of intravascular volume
    1. Stroke volume variation and pulse pressure variation
    2. Changes in aortic flow velocity
    3. Respiratory induced changes in inferior vena cava diameter
    4. Passive leg raise induced variations
    5. Bioreactance and NICOM