When performed by an experienced clinician, physical examination of the patient will reveal a great deal about the adequacy of oxygen delivery and cardiac output. Many of the findings of the physical examination relate directly to regional or organ-specific blood flow (e.g., capillary refill time, pulse pressure, mentation, urine production). Although these physical parameters are invaluable in the repeated assessment of patients and require little more equipment than a wristwatch, some are subjective measures and correlate poorly with an individual patient’s actual cardiovascular status.3 However, it must be noted that although an individual value for capillary refill time, for example, may correlate poorly with more direct measures of cardiac output, the trends in serial physical examination findings in an individual patient provide the best and most reliable measure of alterations in that patient’s cardiovascular status. Unfortunately, the converse is not true: a patient whose physical examination findings are not changing may be experiencing a decline in cardiac performance that will not be detectable until compensatory mechanisms are exhausted or overcome.
The findings of a thorough physical examination, particularly when complemented with hemodynamic monitoring (see Chapter 203, Hemodynamic Monitoring), will be sufficient to guide the clinician in directing the care of most patients. However, there exists a subset of critically ill veterinary patients in whom more direct assessment of cardiac output (and its derived parameters) is essential to proper case management. Patients with sepsis, septic shock, systemic inflammatory response syndrome, and multiple organ dysfunction syndrome make up the bulk of veterinary patients that are likely to require more invasive measures of cardiac output. Patients with severe compromise of the pulmonary or cardiovascular system may also require cardiac output monitoring to optimize their care. It is in the care of these patients that clinicians may find themselves unable to make appropriate decisions regarding management without the additional information provided via cardiac output monitoring.
In patients with complex disease states such as those mentioned above, the individual’s cardiovascular and pulmonary systems may be compromised to such an extent that the typical measures of cardiovascular status and performance may give contradictory information and suggest therapies that have opposing mechanisms of action (e.g.,expanding or depleting extracellular fluid volume). An all- too-common example is a septic patient that has developed capillary leak syndrome (enhanced permeability of systemic capillaries and venules, promoting tissue edema). This patient typically has a low central venous or mean arterial pressure, or both (suggesting additional intravenous fluid therapy would be of benefit), while at the same time exhibiting marked peripheral edema (which might lead the clinician to want to be less aggressive with fluid administration). The treatment of such a patient would be enhanced by the knowledge of the cardiac output and oxygen delivery, which are always of primary importance and can mandate a course of action in the face of conflicting findings. Cardiac output can also be a much earlier indicator of deteriorating cardiovascular status, because compensatory mechanisms such as reflex vasoconstriction can maintain other indicators such as mean arterial pressure near normal levels in the face of worsening cardiac performance.
