Comparison with thermodilution [Linton NW, Linton RA. Estimation of changes in cardiac output from the arterial blood pressure waveform in the upper limb. Br J Anaesth. 2001;86:486-496.]
Ten patients were studied during cardiac surgery. If possible, a group of 3 (or more) TDCO measurements was made whenever the state of the patient changed or at least every 10 minutes. The blood pressure, recorded during each measurement period, was used to calculate PCO. The results from each group of measurements were then averaged to give a determination. In one patient the ejection duration could not be determined from the arterial pressure waveform and so this patient was excluded from the analysis. In the remaining 9 patients 152 TDCO determinations were compared with PCO.
PCO values were compared with TDCO values after being divided by the initial cardiac output value in each patient (see figure). This gives an indication of the ability of PCO to track changes in cardiac output following calibration at the start of an operation.
PCO can track cardiac output accurately despite large variations in the arterial pressure waveform within individual patients (see figure). In the 9 patients from whom results were obtained in this study, on average the maximum SVR was 2.5 times greater than the minimum SVR.
Radial artery pressure waveforms from two patients at different stages of cardiac surgery are shown - see patient 1 and patient 2. These examples show the large changes in arterial pressure waveform which can occur.
Response to phenylephrine [Linton NW, Linton RA. Estimation of changes in cardiac output from the arterial blood pressure waveform in the upper limb. Br J Anaesth. 2001;86:486-496.]
One of the problems with existing pulse contour analysis methods of measuring cardiac output is their inability to track cardiac output accurately in the face of large changes in SVR. Rödig et al infused phenylephrine in 10 patients to produce a 62% increase in SVR. Cardiac output measured by thermodilution showed no significant change, but the pulse contour analysis estimates showed an increase of more than 3 (± 1.5 SD) l min-1. Therefore, in another group of 5 cardiac surgical patients, we investigated the response to phenylephrine (0.5 mg or 0.25 mg bolus) using PCO.
A typical response to phenylephrine is shown in the figure.
For each patient, the haemodynamic variables at the first peak in mean arterial pressure were compared with those before injection. On average, mean arterial pressure increased from 71 (SD 9) mm Hg to 95 (SD 14) mm Hg. PCO indicated an associated increase in SVR of 59 (SD 35)% with a small fall in cardiac output (4.4 (SD 1.4) l min-1 to 3.8 (SD 1.5) l min-1) - see figure. This response is consistent with the known vasoconstrictor properties of phenylephrine.
Response to adrenaline [submitted for publication]
This study has been submitted for publication. Further details will be posted shortly. For an example of the response to 5mcg adrenaline click here.
We have described the display and formal studies comparing PCO with thermodilution and investigating the response to vasoactive drugs. Click here for other examples collected during cardiac surgery.