Cardiac Output Flashcards
Cardiac output equation
CO = SV x HR
Normal CO and CI
CO: 4-8 L/min
CI: 2.5-4.0 L/min/m2
What are the primary and secondary determinants of CO?
Primary:
- HR
- SV
Secondary:
- preload (venous capacitance, return)
- afterload (arterial compliance, vasoconstriction)
- contractility
- rhythm (presence/absence of atrial kick)
Absolute contraindications to PAC placement
- TS
- PS
- friable mass in RA or RV
- RVAD
Relative contraindications include arrhythmias, coagulopathy, new pacemaker leads. Use caution in patients with conduction blocks.
What is the Fick equation?
Cardiac output = oxygen consumption / arteriovenous oxygen difference
CO = VO2/(CaO2-CvO2)
VO2 = 125ccO2/min * BSA (formula of Dehmer)
CaO2 = (SaO2 x Hgb x 1.34) + (PaO2 x 0.003) from arterial blood gas
CvO2 = (SaO2 x Hgb x 1.34) + (PaO2 x 0.003) from PA blood gas (note: this is an INDIRECT Fick b/c not directly calculating O2 extraction at the tissue level)
VO2 = total oxygen uptake at peripheral tissues
CaO2 = oxygen delivered from the lungs
CvO2 = oxygen returned to the lungs
Normal SvO2 value
60-80%
What factors can affect SvO2?
- Cardiac output
- Hemoglobin (anemia, bleeding, etc.)
- Arterial oxygenation (FiO2, gas exchange, etc.)
- Tissue consumption (shivering, sepsis, etc.)
What is the equation for estimating CO from SvO2?
CO = VO2 / [Hgb x 1.34 x (SaO2-SvO2)]
As SvO2 increases (all else being equal), CO increases
As SvO2 decreases (all else being equal) CO decreases
What is the general principle for CO by bolus thermodilution?
- Thermodilution method measures concentration of an indicator over time
- Rate at which the indicator disappears is related to flow
Cardiac output is the inverse of the area under the curve (high CO = fast washout, fast return of temp to baseline; low CO = slow washout, slow return of temp to baseline)
Note: at extremes of CO (<3 or >15L/min) Td will be inaccurate since thermistor has limited sensitivity to very slow/very rapid temp changes.
