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Over recent decades, hemodynamic monitoring has evolved from basic cardiac output monitoring techniques to a broad variety of sophisticated monitoring devices with extra parameters. In order to reduce morbidity and mortality and optimize therapeutic strategies, different monitoring techniques can be used to guide fluid resuscitation and other medical management. Generally, they can be divided in calibrated and non-calibrated techniques. In the first part of this review, the available calibrated techniques, ranging from invasive to non-invasive, will be discussed. We performed a review of the literature in order to give an overview of the current hemodynamic monitoring devices. For each monitoring system, a short overview of the physical principles, the advantages and disadvantages and the available literature with regard to validation is given. Currently, many promising hemodynamic monitoring devices are readily available in order to optimize therapeutic management in both perioperative and ICU settings. Although several of these calibrated techniques have been validated in the literature, not all techniques have been shown to reduce morbidity and mortality. Many new techniques, especially some non-calibrated devices, lack good validation data in different clinical settings (sepsis, trauma, burns, etc.). The cardiac output values obtained with these techniques need therefore to be interpreted with caution as will be discussed in the second part of this concise review. Transthoracic echocardiography forms a good initial choice to assess hemodynamics in critically ill patients after initial stabilisation. However in complex situations or in patients not responding to fluid resuscitation alone, advanced hemodynamic monitoring is recommended with the use of calibrated techniques like transpulmonary thermodilution. Calibrated techniques are preferred in patients with severe shock and changing conditions of preload, afterload and contractility. The use of the pulmonary artery catheter should be reserved for patients with right ventricular failure in order to assess the effect of medical treatment.

There is much evidence that fluid overload leads to adverse outcomes in perioperative and critically ill patients. Cardiac output monitoring can help us guiding initial and ongoing fluid resuscitation and can help us to assess whether a patient will be responsive to fluids when hypotensive. In recent years, many sophisticated devices that measure a variety of hemodynamic parameters have evolved on the market. We wanted to provide an overview of the different techniques available today, including their validation in different patient populations. In this second part of the review, we focus on non-calibrated techniques, both invasive and non-invasive. For each technique a short overview of the working principle, together with the advantages, disadvantages and the available validation literature is listed. Many promising minimal invasive monitoring devices can help us to further optimize our hemodynamic treatment in both the perioperative and critical care setting. However, the validation data are scarce for many of these techniques, especially in complex circumstances with changing hemodynamics (preload, afterload and contractility), as with the use of fluids and vasoactive medication. The measurements made by these devices, therefore, need to be interpreted with caution. Further improvements and more validation data are needed before these techniques can be implemented in common day practice. Moreover, in severely shocked hemodynamic unstable patients, calibrated techniques are to be preferred over those which are uncalibrated. Hence, the new techniques not only need to be accurate, but also need to be precise in order to keep track of changes.

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