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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.
