Jos R.C. Jansen
Department of Intensive Care H4Q, Leiden University Medical Center, PO Box 9600,
2300 RC Leiden, The Netherlands. Tel: +31 71 526 4536, Fax: +31 71 526 6966, E-mail:
Accurate clinical assessment of the circulatory status is particular desirable in critically ill patients in the ICU and patients undergoing cardiac, thoracic, or vascular interventions. As the patient’s haemodynamic status may change rapidly, continuous monitoring of cardiac output will provide information allowing rapid adjustment of therapy.
For more than three decades the pulmonary artery catheter (PAC) thermodilution method has been generally accepted and is still the clinical standard to which all other methods are compared. The long history of use has led to much experience with its technology, clinical application and inadequacies. Over the years, many new methods attempted to replace the thermodilution technique, but none of these methods have gained acceptance.
There are eight desirable characteristics for cardiac output monitoring techniques; accuracy, reproducibility or precision, fast response time, operator independency, ease of use, continuous use, cost effectiveness, and no increased mortality and morbidity.
Novel techniques to monitor cardiac output.
Indicator dilution techniques. Recent advances in indicator dilution techniques involve: the re-introduction of transpulmonary thermodilution (PiCCO, Pulsion), the transpulmonary lithium dilution method (LiDCO), the PAC based continuous thermodilution methods (Vigilance, Baxter; Opti-Q, Abott; and TruCCOMS, AorTech). The transpulmonary indicator dilution methods with bolus injections are modalities of the conventional bolus thermodilution method. CO is calculated with use of the Steward-Hamilton equation. Application of this equation assumes three major conditions; complete mixing of blood and indicator, no loss of indicator between place of injection and place of detection and constant blood flow. The errors made are primarily related to the violation of these conditions. Of the mentioned methods the transpulmonary indicator dilution methods as well as the so-called ‘continuous cardiac output’ thermodilution methods [1] have been reasonably accepted in clinical practice. Especially, the clinical feasibility of the TruCCOMS system has been impossible to assess until now, as not enough clinical studies have been available.
Fick principle. The NICO (Novametrix) system is a non-invasive device that applies Fick’s principle on CO2 and relies solely on airway gas measurement. The method actually calculates effective lung perfusion, i.e. that part of the pulmonary capillary blood flow that has passed through the ventilated parts of the lung. The effects of unknown ventilation/perfusion inequality in patients may explain why the performance of this method shows a lack of agreement between thermodilution and CO2-rebreathing cardiac output [2].
Bio-Impedance and conduction techniques. The bio-impedance method was introduced, five decades ago, as a simple, low-cost method that gives information about the cardiovascular system and/or (de)-hydration status of the body in a non-invasive way. To improve the related thoracic impedance method, over the years, a diversity of thoracic impedance measurement systems appeared. These systems determine CO on a beat-to-beat time base. More than 150 validation studies have been reported, mostly with poor and exceptionally with good correlations compared to a reference method. In order to explain these controversial results, many of these studies refer to the poor physical principles of the thoracic impedance method [3]. The accuracy of this technique is dramatically increased (along with its invasiveness) when the electrodes are placed directly in the left ventricle, rather than on the chest.
Echo-Doppler ultrasound. New developments of ultrasound crystals have lead to new miniature echo-Doppler probes positioned inside the esophagus with their echo window on the thoracic aorta for measuring aortic flow velocity. Aortic cross sectional area is assumed (CardioQ, Deltex) or measured simultaneous with a 2d–echo (HemoSonic, ARROW). With these minimally invasive techniques aortic blood flow is measured, not cardiac output. However, a fixed relationship between aortic blood flow and cardiac output is assumed. Thus, CO can be calculated using this relationship. Abrupt changes in cardiac output are much better followed with Doppler systems than with the PAC based continuous cardiac output systems [4].
Arterial pulse contour analysis. The estimation of cardiac output base on pulse contour analysis is an indirect method, since cardiac output is not measured directly, as with an electromagnetic flow probe, but is computed from a pressure pulsation on basis of a criterion or model. The origin of the pulse contour method for estimation of beat-to-beat stroke volume goes back to the classic Windkessel model described by Otto Frank in 1899. Most pulse contour methods are, explicitly or implicitly based on this model [5-7]. They relate an arterial pressure or pressure difference to a flow or volume change. Nowadays, three pulse contour methods are available; PiCCO (Pulsion), PulseCO (LiDCO) and Modelflow (TNO/BMI). All these three pulse contour methods use an invasively measured arterial blood pressure and they need to be calibrated. PiCCO is calibrated by transpulmonary thermodilution, LiDCO by transpulmonary lithium dilution and Modelflow by the mean of 3 or 4 conventional thermodilution measurements equally spread over the ventilatory cycle. Output of these pulse contour systems is calculated on a beat-to-beat base, but presentation of the data is typically with a 30 seconds window. A no invasive pulse contour development is the combination of non-invasively measured arterial finger blood pressure with Modelflow [8] (TNO/BMI).
1. Rödig et al. Continuous cardiac output measurement: pulse contour versus thermodilution technique in cardic surgical patients. Br J Anaesth 1999; 50: 525
2. Nielsson et. al Lack of agreement between thermodilution and CO2-rebreathing cardiac output. Acta Anaesthesiol Scand 2001; 45:680
3. Patterson. Fundamentals of impedance cardiography. IEEE Engineering in Medicine and Biology 1989; 35
4. Roeck et al. Change in stroke volume in response to fluid challenge: assessment using esophageal Doppler. Intensive Care Med 2003; 29:1729
5. Rauch et al. Pulse contour analysis versus thermodilution in cardiac surgery. Acta Anaesthesiol Scand 2002; 46:424
6. Linton et al. Estimation of changes in cardiac output from arterial blood pressure waveform in the upper limb. Br J Anaesth 2001; 86:486
7. Jansen et al. A comparison of cardiac output derived from the arterial pressure wave against thermodilution in cardiac surgery patients. Br J Anaesth 2001; 87:212
8. Hirschl et al. Noninvasive assessment of cardiac output in critically ill patients by analysis of finger blood pressure wavevorm. Crit Care Med 1997; 25:1909
9. Jansen et al. An adequate strategy for the thermodilution technique in patients during mechanical ventilation. Intensive Care Med 1990; 16:422