Association between elevated central venous pressure and outcomes in critically ill patients

Elevated right atrial pressure or central venous pressure (CVP) occurs frequently in critical care settings [1–4] and may be caused by several conditions, such as congestive heart failure syndrome, constrictive pericardial disease, tension pneumothorax, and resuscitation/evacuation phases of septic shock. Although CVP has been utilized as a surrogate of intravascular volume in critically ill patients, recent studies have challenged the validity of elevated CVP (ECVP) in critical care settings [5–9]. Based on rationale provided by the Starling curves and Guyton model on cardiac function, CVP is determined by the interaction of cardiac function and the return of blood to the heart. An ECVP might indicate an impediment to the venous return and microcirculatory blood flow as well as accompanying lung edema and splanchnic congestion [3, 10], which may further worsen the potential organ failure in critical patients.

Some prior studies have shown that increased mean CVP in certain patient populations and disease processes may lead to poor prognosis. In septic patients, increased mean CVP was associated with worse outcome, including increased 28-day mortality as well as the development and progression of acute kidney injury [6, 7, 11]. For patients with cardiovascular diseases, ECVP might be associated with impaired hepatic and renal function and may be independently related to all-cause mortality [8, 9]. These studies, however, could not generalize the concept of elevated central venous pressure (ECVP) load to all patients in critical care settings because of the limited cases and exclusive cohorts. Besides, the importance of ECVP duration was underestimated in estimating the ECVP load and analyzing its relationship with outcome, which is only accessible with the advantage of “big data” technology [12].

By using the large, public, de-identified clinical database, Multi-parameter Intelligent Monitoring in Intensive Care (MIMIC-III) [13], we evaluated the effect of ECVP on all patients in critical care settings, for whom CVP data were carefully recorded. We aimed to further characterize the association between mean CVP level and outcome as well as treatment received in the intensive care unit (ICU). Specifically, we investigated the potential effect of ECVP duration on outcome when mean CVP were stratified at different levels.

This study investigated the association between ECVP load and outcomes of patients in critical care setting. Based on comparison among patients with different levels of mean CVP, which is a measurement of ECVP load, we found that the higher mean CVP level was associated with increased 28-day mortality. Further investigation on length of hospitalization, duration of vasopressor treatment and mechanical ventilation, and laboratory results related to organ dysfunction also demonstrated that higher mean CVP level was associated with poor ICU outcome for patients in critical care settings.

In contrast to the common misleading statement of increasing CVP to a higher level to increase cardiac output, Starling curves, with the contribution of Guyton’s work, did not suggest any causal relationship between CVP and cardiac output, but emphasized the comprehensive role in the interaction between cardiac function and venous return of CVP. Recent studies have showed that not only the CVP failed as a useful measure for the assessment of preload and fluid responsiveness [20], but that the ECVP is independently associated with a higher mortality and increased risk of AKI in patients with sepsis and heart failure [7, 9, 11]. These results indicated that the CVP should be considered part of the evaluation in patients with hemodynamic instability. Furthermore, an ECVP may signify an impediment to venous return [3].

Based on the study for the consensus of congestive heart failure treatment, the pathophysiology of chronic systemic venous congestion should include decreased cardiac output from increased resistance to venous return, with the subsequent splanchnic damage and tissue perfusion insufficiency [1]. Integrated with the Starling–Guyton cardiac curve, the direct cause of ECVP may be defined as “the impediment to venous return by a relatively fatigued heart” [3]. Therefore, the potential causes of ECVP may include elevated venous return, cardiac dysfunction, increased vascular resistance, increased pericardial or intrapleural pressure, among other conditions. Our study showed that a higher ECVP₁₀ load was associated with prolonged duration of treatment and length of ICU hospitalization. In sensitivity analysis, the result showed that impact of ECVP on mortality was limited in some subgroup patients, especially in the sepsis group, which was inconsistent with previous result [11]. Considering the difference in enrolling criteria, our result indicated that for the sepsis patients who survived the resuscitation phase, the benefit from successful resuscitation, which was always presented as higher CVP level, may attenuate the impact of ECVP on outcome and did not lead to statistically significant result. The fluid balance, mean CVP level and duration of ECVP10 result in different subgroups, and outcome (survival and non-survival) is shown in Additional file 1: Supplement Table 3.

In contrast to the long-term influence of systemic venous hypertension caused by cardiac dysfunction such as right heart failure, pericardial effusion and other conditions, the pathophysiological changes that occur with ECVP, as manifestation of systemic venous congestion, generally have acute consequences in patients being treated in critical care settings. Based upon the significance of association between ECVP load and ICU outcome, we also showed that the ECVP load should be considered and corrected in two independent aspects, level and duration, because both played important pathophysiological roles. According to the present results, we suggest that ECVP, with its association with worse outcome in critical care settings, should be considered seriously and further actions should be undertaken to discover potential causes and treatment. Our study was based on data extracted from electronic medical records in the MIMIC-III v1.3 [13], a large, open clinical database, which allowed precise research on ECVP load. To our knowledge, this study is the first to evaluate ECVP on a large population of patients managed in critical settings. More than 500,000 CVP records and 9000 patients were involved in our study. Furthermore, the use of database technologies and statistics played a critical role in reaching a meaningful conclusion in the present study. Given the extreme complexity and diversity of critically ill patients and their treatment, the traditional approach of knowledge discovery may not satisfy the need to address clinical concerns, and further efforts should be made to organize, share and analyze the “big data” in critical care [21].

This study has several limitations. First, our study is limited by its retrospective nature and the source of data used. For this reason, no causal relationships could be established. Additionally, CVP records could only be assessed in critical care patients with central venous catheter and we only extracted data from the first 3 days of ICU admission. Thus, our conclusions cannot be generalized to patients treated in a setting other than a critical care setting or without opportunities of continuous treatment during the first 3 days in ICU, for the latter of which extremely low or high CVP values may occur during the rescue/emergency phase of certain disease or pathophysiology process. Second, unlike the other studies on elevated mean CVP in certain patient populations and disease processes [5–9], we focused on a relatively long phase (72 h from ICU admission) during the general management of patients in critical care settings. Besides, it should be noted that we do not oppose to the significance of CVP targeting in some emergency events of critical care, such as the initial resuscitation phase of septic shock [2]. Third, although some predictors of illness severity were included in our study and adjusted analysis confirmed the association between ECVP₁₀ load and poor outcome, our results may be affected by other confounding variables associated with organ damage or mortality. Additional prospective studies should be performed to investigate these parameters and the potential causes of ECVP load.

In conclusion, this study showed that for patients in critical care settings, higher ECVP load was associated with poor outcome as well as prolonged treatment in ICU. Level and duration of ECVP should be evaluated, and more effort should be made to establish the cause and appropriate treatment of ECVP.