The end-systolic pressure-volume line:
If pressure-volume loops are constructed for each cardiac cycle, small changes in preload and/or afterload will result in shifts of the point defining the end of systole. These end-systolic points on the pressure versus volume diagram describe a straight line, known as the end-systolic pressure-volume line. A steeper slope of this line indicates greater contractility.
The thermodilution technique for determining cardiac output:
The relationship used by the thermodilution technique for calculating QT is called the Stewart-Hamilton equation:
QT = [V X (TB - T1) X K1 X K2] /fTB(t)dt
where V is the volume of the indicator injected, TB is the temperature of blood, T1 is the temperature of the indicator, K, is a constant that is the function of the specific heats of blood and the indicator, K2 is an empirically derived constant, and fTB(t)dt is the area under the time-temperature curve. Determination of cardiac output by the thermodilution method is generally quite accurate, although it tends to systematically overestimate QT at low values. Changes in blood temperature and QT during the respiratory cycle can influence the measurement. Therefore, results generally should be recorded as the mean of two or three determinations obtained at random points in the respiratory cycle. Using cold injectate widens the difference between TB and T1 and thereby increases signal-to-noise ratio. Nevertheless, most authorities recommend using room temperature injectate (normal saline or 5% dextrose in water) to minimize errors resulting from warming of the fluid as it is transferred from its reservoir to a syringe for injection.
All of the following are true regarding the fractional saturation of hemoglobin in mixed venous blood (SVO2) EXCEPT:
The Fick equation for cardiac output can be rearranged as follows:
CVO2 = Cao2 - VO2/QT
If the small contribution of dissolved oxygen to CVO2 and Cao2 is ignored, the rearranged equation can be rewritten as:
SVO2 = Sao2 - VO2/(QT x Hgb x 1.36)
where SVO2 is the fractional saturation of hemoglobin in mixed venous blood, Sao2 is the fractional saturation of hemoglobin in arterial blood, and Hgb is the concentration of hemoglobin in blood. Thus it can be seen that SVO2 is a function of V02(ie, metabolic rate), QT,Sao2, and Hgb. Accordingly, subnormal values of SVO2 can be caused by a decrease in QT (eg, due to heart failure or hypovolemia), a decrease in Sao2 (eg, due to intrinsic pulmonary disease), a decrease in Hgb (ie, anemia), or an increase in metabolic rate (eg, due to seizures or fever).
The Surviving Sepsis Campaign guidelines recommend which of the following regarding the initial resuscitation of sepsis-induced hypoperfusion?
The Surviving Sepsis Campaign guidelines for the management of severe sepsis and septic shock recommends that during the first 6 hours of resuscitation, the goals of initial resuscitation of sepsis-induced hypoperfusion should include all of the following: CVP 8 to 12 mm Hg, MAP ≥65 mm Hg, urine output ≥0.5 mL!kg/h. ScVO2 of 70% or SVO2 of 65%.
Noninvasive methods of measuring cardiac output:
Noninvasive methods of monitoring cardiac output include impedance cardiography and pulse contour analysis among others. Impedance cardiography is attractive because it is noninvasive, provides a continuous readout of QT, and does not require extensive training. However, measurements of QT obtained by impedance cardiography are not sufficiently reliable to be used for clinical decision-making and have poor correlation with thermodilution. Measurements of QT based on pulse contour monitoring are comparable in accuracy to standard PAC thermodilution methods, but are less invasive since transcardiac catheterization is not needed. The use of pulse contour analysis has been applied using noninvasive photoplethysmographic measurements of arterial pressure. However, the accuracy of this technique has been questioned and its clinical utility remains to be determined.
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