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Basics of Cardiopulmonary Critical Care

II-A. Shock

1. DEFINITIONS AND PHYSIOLOGYa. What is shock?

Shock is defined as global hypoperfusion, and is routinely (but not always) associated with hypotension and

metabolic acidosis. Operationally, shocks is defined as systolic blood pressure (SBP) < 90 mm Hg or mean

arterial pressure (MAP) < 60 mm Hg for at least one hour despite adequate fluid resuscitation. Clinically, patients

in shock demonstrate signs of end-organ dysfunction, such as oliguria or confusion.

b. What physiologic principles help to characterize shock?To identify the physiologic derangements causing shock in a particular situation, it is necessary to understand the

determinants of mean arterial pressure. It is also necessary to understand the fundamental principles represented in

the Frank-Starling curve.

c. What are the determinants of mean arterial pressure (MAP)?Recall the pressure gradient in any circuit is determined by Ohms Law: P = Flow x Resistance. In the

circulatory system, P is the difference between the upstream pressure, or MAP, and the downstream pressure, the

central venous pressure (CVP). The flow is cardiac output (CO) and the resistance is the systemic vascular

resistance (SVR). Therefore the relationship is: MAP CVP = CO x SVR. Additionally, CO is the product of

heartrate (HR) and stroke volume (SV). Solving for MAP and accounting for units of measure, the physiologic

relationship ultimately is: (units: MAP, CVP (mmHg); CO (L/min); SVR (dynes/sec/cm-5)

; HR (bpm); SV (L))

d. Does low blood pressure always indicate shock?As mentioned above, shock refers to a state ofglobal tissue hypoperfusion and it is operationally defined as SBP 65, CVP >8-12, and urine output >0.5 ml/kg/h); and (4) ideally avoid

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higher preload than necessary for achieving optimal cardiac functionthis may require the use of dynamic metrics

of volume status and responsiveness.

Suggested Reading

Book Chapters & Review ArticlesMarino. The ICU Book. Williams & Wilkins 2007 Third Edition. Ch. 9. The pulmonary artery catheter. pp. 163-180.

Landry. The pathogenesis of vasodilatory shock. New Engl J Med. 2001;345(8);588-95.