Chapter 34

The purpose of hemodynamic monitoring is to identify cardiovascular insufficiency and to ensure optimal treatment of the unstable critically ill. Hemodynamic monitoring serves a diagnostic, therapeutic, and resuscitation target role. Although vital signs help assess the adequacy of tissue perfusion, they are a late indicator of tissue ischemia.1 Analysis of hemodynamic variables beyond traditional vital signs allows the clinician to differentiate various causes of hemodynamic instability and intervene appropriately.

Hemodynamic monitoring changes therapeutic decisions in up to 58% of patients and can unmask underlying cardiovascular compromise, allowing for early intervention.2–5 In this chapter, we discuss techniques applicable in the ED setting: blood pressure monitoring, central venous pressure (CVP) monitoring, cardiac output (CO) monitoring, and oxygenation and organ perfusion monitoring (Table 34-1).

Table 34-1 Hemodynamic Variables Obtainable in the ED

Direct measurement of blood pressure was first performed by Stephen Hales in 1714, when he inserted a brass pipe in the left carotid artery of a living horse tied down on its back. The blood rose 8 ft 3 in. in the pipe above the level of the heart.6 In modern medicine, arterial blood pressure has been the crucial element of the clinical vital signs. Blood pressure is the force exerted by the circulating blood through a blood vessel. The pressure difference (ΔP) between two points in the vessel is governed by Ohm’s law: ΔP = Q × R, where Q is blood flow and R is resistance to flow. The clinical application of Ohm’s law states that the arterial pressure is determined by the CO (or blood flow in liters per minute) and total peripheral resistance (TPR), which means mean arterial pressure (MAP) = CO × TPR. In evaluating blood pressure as a hemodynamic indicator of organ perfusion, the contribution by TPR (the impediment to flow) must be taken into account in relation to CO, which is the measure of effective blood flow to the tissue. Although the more common measure of resistance is systemic vascular resistance, TPR provides a better physiologic measure of impediment to flow. Systemic vascular resistance is calculated as the ratio of total flow (CO) to the pressure difference between MAP and right atrial pressure. Right atrial pressure is not the back pressure ...

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