HEMODYNAMIC MONITORING: ACCURACY AND PRECISION MATTER

Published on Friday, October 23rd, 2020 by .

What is accuracy?

The degree to which the results of a measurement conform to the correct value or standard.

What is precision?

How close two or more measurements are to each other.

How does this apply to hemodynamic monitoring?

Hemodynamic monitoring is often used in two different ways to assess patient’s circulation. Cardiac Output (CO), Stroke Volume (SV) and Systemic Vascular Resistance (SVR) are used to assess preload and afterload to help determine the current state of a patients hemodynamics and if any interventions are needed to optimize the delivery of oxygen to the tissues. Hemodynamic monitoring is also used to monitor the response to therapies such as vasopressors, inotropes and fluids. (1)  

If a measurement of a patient’s cardiac output is accurate, that is saying that it is a statistically acceptable measurement of the actual amount of blood being pumped from that patient’s heart with each heartbeat. 

If a measurement of a patient’s cardiac output is precise, that is saying that when measuring the same cardiac output ten times you will get the same number with each of those ten measurements.

blood flow

What to expect when monitoring with LiDCO?

First let’s look at how LiDCO measures with pulse power analysis. The PulseCO™ algorithm uses the entire BP waveform, not just the area under the curve or systolic portion. The arterial pressure waveform is transformed from pressure to volume for analysis since the desired value is stroke volume. The use of autocorrelation then finds the proportional power in the volume waveform independent of the wave shape. LiDCO’s pressure waveform algorithm has remained completely unchanged since the launch of the first PulseCO™ monitor in 2001 and has over 100 papers and abstracts on the performance.

Acceptable limits of precision have been defined by Critchley&Critchley (1999), who state that if a new cardiac output method is to replace an older, established method, the new method should have errors not greater that the older reference method. The current gold standard in hemodynamic monitoring, although not used often these days because less invasive options are available, is the pulmonary artery catheter (PAC). LiDCO’s PulseCO™ algorithm has been validated against the PAC demonstrating a good agreement between the two methods (2,3). In addition, over the last 19 years the precision of LiDCO’s core PulseCO™ algorithm to trend changes in stroke volume has been evaluated in a wide number of challenging clinical situations – these include: general surgical patients (Heller et al. 2002), high cardiac output (Hallowell&Corley et al. 2005), hyperdynamic liver transplant (Costa et al. 2007), post-operative care (Pitman et al. 2005 and Hamilton, Huber and Jessen 2002), congestive heart failure (Kemps et al. 2008 and Mora et al. 2011), pre-eclampsia (Dyer et al. 2011) and intensive care (Jonas et al. 2010, Brass et al. 2011, Cecconi et al. 2008).

LiDCO recognizes that different patient groups, in different clinical settings, have different monitoring needs. Trend monitoring allows you to understand how your patient’s hemodynamics are changing and respond accordingly to it. If the monitor is showing a decrease in stroke volume or cardiac output, you should act on the drop independently of the number displayed. Because of LiDCO’s validated precision you can be confident that when you are seeing a change in CO, SV and SVR that is a real change of these values in your patient.

Within the different LiDCO platforms there are tools available to help you easily assess the changes or trending of your patient’s hemodynamics such as event response, guided protocols and SVV/PPV.

Trend Screen - FC night FC Prtcl

We also recognize that in some clinical situations accuracy is of utmost importance and LiDCO offers the only hemodynamic monitor that allows you many different options for calibration. As a comparison think of your decision tree when selecting how to monitor your patients blood pressure: standard cuff, arterial line, central line, pulmonary artery catheter. All will give you blood pressure and your decision is based on what information you need to best assess your patient and what’s possible with how they are presenting. All LiDCO trends start with autocalibration based on the patient demographic information you enter of age, weight and height. There is also the option to calibrate by entering another absolute cardiac output measurement value obtained by TEE or TTE. This is easily done by entering the obtained cardiac output value in the CO calibration box within the menu, pressing the equal sign and then the device is calibrated from that cardiac output. In addition, The LiDCOunity offers the option to calibrate using the lithium dilution process.

Screenshot (14)

Conclusion

  • Accuracy and precision in relation to hemodynamic monitoring is an important concept to understand so that cardiac output, stroke volume and systemic vascular resistance can be used optimally to create the best outcomes for our patients
  • When this concept is mastered clinicians are able to easily decide when and if to calibrate beyond autocalibration and trending
  • LiDCO and the PulseCO™ algorithm have been proven with a strong body of evidence to be both accurate and precise
  • LiDCO is the only hemodynamic monitor that allows for calibration options when accuracy is of utmost importance

If you’d like to learn more about how to use ECHO to calibrate LiDCO, I’ve included a link to our latest blog written by Dr Alexander Scott, a critical care physician from the James Cook University Hospital in Middlesbrough, United Kingdom.  In it Dr. Scott tells us how he and his colleagues have adapted their hemodynamic evaluation of COVID-19 patients to confidently obtain continuous hemodynamic values in challenging clinical situations:

Read our Latest Blog: Calibrate the LiDCO System with an ECHO in COVID-19

References

  1. Pinsky, M. Hemodynamic Evaluation and Monitoring in the ICU, Chest 2007; 132;2020-2029 DOI 10.1378/chest.07-0073
  2. Pittman, J., Bar Yosef S, SumPing J, Sherwood M, Mark J. Continues Cardiac Output Monitoring with Pulse Contour Analysis: A Comparison with Lithium Indicator Dilution Cardiac Output Measurement. Crit Care Med. 2005;33(9):2015-2021
  3. Costa, MG., Della Rocca, G., Chiarandini, P., Mattelig, S., Pompei, L., Barriga, MS., Reynolds, T., Cecconi, M., Pietropaoli, P. Continous and Intermittent Cardiac Output Measurments in Hyperdynamic Conditions: Pulmonary Artery Catheter vs Lithium Dilution Technique. Intensive Care Med. 2007. DOI 10.1007/s00134-007-0878-6.
  4. Linton R., Band D., O’Brien T, Jonas MM., & Leach R. Lithium dilution cardiac output measurement: A comparison with thermodilution. Critical Care Medicine, 1997; 25: 1796-1800 L1
  5. Hamilton TT., Huber LM., Jessen ME. PulseCO: a less-invasive method to monitor cardiac output from arterial pressure after cardiac surgery. Ann Thorac Surg. 2002;74:S1408–S1412. doi: 10.1016/S0003-4975(02)04059-6
  6. Critchley, L., Critchley, J. A. Meta-analysis of studies using bias and precision statistics to compare cardiac output measurement techniques Journal Clinical Monitoring and computing 1999 15 (2) 85-91
  7.  Mora, B., Ince, I., Birkenberg, B., Skhirtladze, K., Pernicka, E., Ankersmit, HJ., et al. Validation of cardiac output measurement with the LiDCOTM pulse contour system in patients with impaired left ventricular function after cardiac surgery. Anaesthesia. 2011;66:675–681. doi: 10.1111/j.1365-2044.2011.06754.x
  8. Dyer, RA., Piercy, JL., Reed, AR., Lombard, CJ., Schoeman, LK., James, MF: Hemodynamic changes associated with spinal anaesthesia for cesarean section in severe preeclampsia. Anesthesiology 2008; 108:802–11
  9. Kemps, H., Thijssen, E., Schep, G., Sleutjes B,De Vries, W,. Hoogeveen ,A., Wijn, P., Doevendans, P. Evaluation of two methods for continuous cardiac output assessment during exercise in chronic heart failure patients. J Appl Physiol. 2008;105:1822-1829
  10. Jonas, M., Mills, E., Wolff, C. et al. Effect of cardiac arrhythmias on PulseCO calibration and performance. Crit Care 14P101 (2010). doi: 10.1186/cc8333
  11. Brass, P., Mills. E., Latza, J., Peters, J., Berendes. E. Comparison of cardiac index: LiDCOrapid and PICCOplus in the ICU. 2011 doi:10.1186/cc9482
  12. Cecconi M., Dawson D., Grounds R., Rhodes A. Lithium dilution cardiac output measurement in the critically ill patient: determination of precision of the technique. Intensive Care Medicine, 2008
  13. Hallowell G., Corley K. Use of lithium dilution and pulse contour analysis cardiac output determination in anestheized horses: a clinical evaluation. 2005. Vet Anaes & Analgesia 32, 201-211

Written by Jennifer Monier

Jennifer is our West Coast Clinical Specialist residing in sunny San Diego. She has been specializing in hemodynamics for the last 10 years and is passionate about changing practice within the OR, ICU and ED to allow clinicians to be proactive in their care with the understanding of what’s going on underneath their patient’s blood pressure. When not geeking out on micro and macro circulation Jennifer enjoys everything the outdoors has to offer such as backpacking, surfing and gardening.

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