Extending the metaphor of the heart as a pump, the vasculature as pipes, and the blood as the fluid affected by them, we will leverage a metaphor made popular by Pereras Rapid Ultrasound in Shock evaluation protocol [12, 13] and consider another key aspect of this overall system: the tank. The tank, representing the effective intravascular volume status (here and elsewhere often shortened to simply volume status), is the measure of how much blood a person has to work with in their circulatory system at any given time. Intravascular volume is a crucial conceptual framework employed by clinicians to guide the treatment of critically and acutely ill patients. Modulation of this circulating blood volume is the foundation of many types of fluid treatment, itself a pillar of current medical practice.
One of the most important things a clinician can do for their patient is insure optimal oxygen delivery to their tissues. If we were to peak beyond the borders of Figure 1.4, we would see that the function of cardiac output itself is to drive oxygenated blood to peripheral tissues for perfusion and exchange. To sustain this feedback-control loop, water and electrolytes are necessary. People obtain both through their diet and under normal circumstances will maintain a homeostatic condition of normovolemia (or euvolemia), sustaining a cardiac output sufficient to meet the needs of the whole body. This may, as many things can, go wrong in a number of ways.
Focusing on just those ways that in which volume status can impede proper tissue oxygenation, we can imagine that there are only two ways this can happen: a person can either have too much fluid in their blood hypervolemia or a person can have too little hypovolemia. Hypervolemia, also known as fluid overload, usually results from compromised regulatory mechanisms for electrolytes (primarily sodium) and water, as is the case in congestive heart failure, kidney failure , and liver failure (conditions responsible for millions of deaths around the globe annually ). Hypovolemia, also known as fluid depletion, results from large losses of volume such as vomiting, diarrhea, hemorrhage, etc. In either case, solutions exist to compensate for the fluid imbalance (sodium/fluid restriction or removal in the case of hypervolemia and infusions of replacement fluids in hypovolemia) in an at- tempt to optimize cardiovascular and tissue oxygenation parameters. Each solution type is predicating on altering the individual return and output curves constituting cardiac function (recall Figure 1.5).