Monitoring with the PiCCO technology (Pulse Contour Cardiac Output) has become a widely adopted method for hemodynamic assessment and fluid management in critically ill patients. The technique combines transpulmonary thermodilution and pulse contour analysis to provide continuous or intermittent measurements of cardiac output, global end-diastolic volume index (GEDVI), extravascular lung water index (EVLWI), and systemic vascular resistance index (SVRI). Despite its technological sophistication and broad clinical use, the validity, reproducibility, and universal applicability of PiCCO-derived data remain subject to ongoing debate. Particular concern arises in clinical scenarios involving altered physicochemical properties of blood, vascular dysregulation, vasoplegia, or hypoproteinemia, where thermodilution-based calculations may become inaccurate. Furthermore, neurohumoral and reflex circulatory mechanisms - such as the Schwik-Larin reflex - are not accounted for in the PiCCO model, yet may significantly impact hemodynamic dynamics and confound interpretation. This review provides a critical analysis of the methodological, physiological, and clinical limitations of PiCCO monitoring. Special emphasis is placed on the influence of blood rheology, temperature, microcirculatory changes, and endothelial dysfunction on the reliability of computed hemodynamic variables. The necessity of an integrative approach to data interpretation is emphasized, involving the correlation of PiCCO-derived parameters with the clinical picture, laboratory findings, therapeutic response, and the patient's pathophysiological status. In conclusion, PiCCO remains a potentially valuable tool in critical care; however, its effective use requires clinical vigilance, awareness of physiological constraints, and individualized therapeutic decision-making, particularly in fluid management strategies.

Modern intensive care is impossible without accurate and timely hemodynamic monitoring. Adequate infusion therapy is the cornerstone of stabilizing critically ill patients, particularly in cases of septic shock, ARDS (acute respiratory distress syndrome), trauma, severe infections, and multiple organ failure. However, traditional parameters such as arterial pressure, central venous pressure (CVP), urine output, and lactate levels often fail to provide a comprehensive picture of intravascular volume, preload, and tissue perfusion efficiency. This creates the risk of both hypovolemia and fluid overload, which may worsen the prognosis.

In the search for more reliable and informative tools to assess volume status, the PiCCO (Pulse Contour Cardiac Output) method was developed, combining transpulmonary thermodilution with arterial pressure waveform analysis. Unlike invasive pulmonary artery catheterization (Swan–Ganz method), PiCCO provides information on parameters such as cardiac output (CO), global end-diastolic volume index (GEDVI), extravascular lung water index (EVLWI), systemic vascular resistance index (SVRI), myocardial contractility (dPmax), and others [1–4]. This makes the technology particularly attractive for use in intensive care units, where rapid and precise hemodynamic assessment is required in unstable patients.

Moreover, the method allows for the evaluation of so-called “volume responsiveness” and enables tailoring of infusion strategies to individual patient needs, which is especially important in goal-directed therapy. In many guidelines and clinical protocols, PiCCO is recommended as a reference tool for determining the required volume of fluid resuscitation, preventing pulmonary edema, and ensuring the rational use of vasoactive agents [5–7].

Nevertheless, despite its attractiveness and technological sophistication, the PiCCO method is not without limitations. Its accuracy and reproducibility may be significantly affected by physiological, biochemical, and rheological factors such as blood properties, vascular wall condition, concomitant metabolic disturbances, as well as the specifics of the measurement procedure itself. In addition, certain theoretical assumptions underlying the interpretation of PiCCO-derived parameters remain controversial and require reconsideration in light of clinical practice.

The aim of this review is to critically examine the limitations, methodological challenges, and risks associated with the use of PiCCO technology in intensive care. Particular attention is given to physiological and clinical-laboratory factors influencing data interpretation, as well as to the rationale for adopting an integrative and balanced approach to the analysis of obtained parameters, which is especially important in the context of high clinical relevance of therapeutic decision-making.

Limitations and Methodological Challenges of the PiCCO Technology

1. Criticism of the Excessive Emphasis on Central Venous Pressure (CVP)

In recent years, the clinical significance of central venous pressure (CVP) as a predictor of volume responsiveness has been increasingly questioned [8]. A particularly influential position was presented in a meta-analysis [9], which concluded that CVP has low predictive value for assessing the response to fluid loading. However, such a viewpoint is one-sided and methodologically vulnerable.

First, the absolute value of CVP indeed cannot serve as a universal predictor of fluid responsiveness, as it depends on right ventricular compliance, intrathoracic pressure, and numerous other variables. Nevertheless, CVP dynamics over time—particularly in serial measurements before and after fluid administration—can provide valuable insights into changes in preload and hemodynamic adaptation. This is supported by clinical observations where an increase in CVP following a fluid challenge, without improvement in cardiac output, may indicate fluid overload [10].

Second, the cited meta-analyses lacked strict randomization, suffered from heterogeneous populations, and included studies with different methodologies for hemodynamic assessment. As rightly noted by Teboul JL and colleagues (2016), “meta-analyses are quantitative summaries, but not always qualitatively reliable recommendations for clinical practice” [11].

Thus, CVP should not be entirely dismissed as a hemodynamic parameter. Rather, it should be used in conjunction with other indicators, including dynamic tests, ultrasound findings, PiCCO-derived parameters, and laboratory markers of hypovolemia.

2. Influence of the Physicochemical Properties of Blood on the Accuracy of Transpulmonary Thermodilution in PiCCO Technology

The PiCCO (Pulse Contour Cardiac Output) technology is based on the method of transpulmonary thermodilution, in which changes in blood temperature are recorded after intravenous bolus administration of a cold indicator solution. This method enables the calculation of key hemodynamic parameters, including the global end-diastolic volume index (GEDVI) and the extravascular lung water index (EVLWI). The basis of these calculations is the thermodilution curve, which reflects standard physical interactions of the indicator with blood.

However, in clinical practice, the accuracy of these calculations directly depends on the physicochemical properties of blood. Unlike a homogeneous fluid, blood is a complex colloidal–cellular system composed of formed elements, plasma proteins, lipids, ions, buffering components, and biologically active molecules. Blood viscosity and thermal conductivity are dynamic parameters that can change under the influence of temperature, pH, osmolarity, albumin concentration, fibrinogen levels, and hemostatic activity [12,13]. It is also important to note that most PiCCO calculations are based on models of linear bolus distribution, which do not adequately reflect the true physiological heterogeneity of blood flow and vascular architecture in critically ill patients [14].

Therefore, changes in viscosity, hematocrit, erythrocyte and platelet aggregation, and vascular compliance may substantially distort the shape of the thermodilution curve and, consequently, lead to inaccurate values of GEDVI and EVLWI [15–18]. For example, in hypoproteinemia, reduced plasma viscosity accelerates indicator dispersion, resulting in overestimation of cardiac output and underestimation of volumes. Leukocytosis and thrombocytosis affect microcirculation and phase distribution, while hemolysis, the presence of microthrombi, and endothelial dysfunction (e.g., in sepsis) disrupt uniform bolus distribution within the vascular bed [19–23].

Thus, despite the high sensitivity of the method, PiCCO monitoring results must be interpreted with consideration of the physicochemical properties of blood, especially in patients with acute disturbances of homeostasis. This requires clinicians to recognize the limitations of the method and the necessity of periodic recalibration when significant changes in blood composition and properties occur.

3. Physiological Limitations of Thermodilution Monitoring: The Role of the Shwiegk–Larin Reflex

The hemodynamics of the pulmonary and systemic circulations are closely interconnected through mechanisms of neurohumoral and reflex regulation. One such underexplored yet important mechanism is the Shwiegk–Larin reflex, according to which an increase in pulmonary vascular pressure induces a reflex decrease in systemic arterial pressure, bradycardia, redistribution of blood to the reticuloendothelial system, and vasodilation in skeletal muscles [24–28]. This protective mechanism is aimed at unloading the pulmonary capillaries and preventing pulmonary edema [29–32].

However, during PiCCO monitoring, such adaptive responses are not taken into account, which may lead to underestimation of pulmonary circulation perfusion and overestimation of systemic vascular resistance. In conditions of hypoproteinemia, increased capillary permeability, and vasoplegia (e.g., in septic shock), the predictive accuracy of parameters such as GEDVI and SVRI is significantly reduced.

Thus, interpreting PiCCO-derived data without considering neurohumoral vascular regulation may result in misleading clinical conclusions and potentially irrational infusion strategies.

4. The Importance of Infusion Rate in the Interpretation of Preload Parameters

One of the key principles of infusion therapy is the assessment of so-called volume responsiveness, or the ability of cardiac output to increase in response to fluid loading. However, not only the infused volume but also the rate of administration is of critical importance. When infusion is performed slowly, the effect of rapid venous return to the heart and activation of the Frank–Starling mechanism may not be realized.

In this context, PiCCO-derived indicators such as SVV (stroke volume variation) and GEDVI (global end-diastolic volume index, reflecting changes in preload and circulating blood volume) are calculated without accounting for the kinetics of volume loading. As demonstrated by Monnet X et al. (2015) [33], the passive leg raising (PLR) test is reliable only when there is a rapid redistribution of venous blood into the thoracic cavity. If the response to infusion is too prolonged, the test results lose their validity [34].

Furthermore, PiCCO algorithms do not account for the pharmacological effects of vasoactive agents, which alter vascular tone and compromise the predictability of volume responsiveness. Therefore, the interpretation of SVV or GEDVI outside the context of infusion rate and concomitant drug therapy is methodologically vulnerable.

Thus, despite the considerable potential of the PiCCO technology in clinical practice, its application requires a balanced and critical approach. The reliability of data interpretation is achievable only when accompanied by a comprehensive analysis that includes assessment of blood rheological properties, the rate of fluid administration, vascular reflexes, and relevant clinical and laboratory parameters. Ignoring these factors substantially reduces the diagnostic value of the method and increases the risk of tactical errors in the management of critically ill patients. Viewing PiCCO as a universal tool for hemodynamic assessment in a generalized or simplified manner may lead to incorrect clinical decisions and, consequently, poorer therapeutic outcomes.