Renal failure combined with acute heart failure involves a complex pathophysiological process. Metabolites and excessive fluid volume in the body cannot be discharged by the damaged kidneys; therefore. the accumulation of metabolites and fluid increases the heart load and can even result in acute heart failure¹¹.

Renal failure and heart failure interact and aggravate each other, forming a vicious circle, eventually leading to sharp deterioration in heart and kidney function, which is a serious threat to patient safety ¹². In clinical practice, CRRT is often used to replace the ability of the kidneys to remove excess fluid volume and metabolic products from the body to improve the patient’s condition¹³. However, the optimal method to achieve rapid and safe relief of heart failure symptoms remains unclear. Therefore, it is critical to correctly evaluate the patient’s blood volume status to facilitate CRRT dehydration adjustments.

Currently, the mainstream methods for assessing blood volume include the use of pulmonary artery catheters, pulse assessment (indicating continuous cardiac output), transoesophageal echocardiography, and bioimpedance^14,15,16,17. These techniques have advantages, as well as many limitations. Many clinicians still prefer to assess the blood volume of patients using general empirical methods, such as estimating the patient’s dry weight, assessing for presence of pulmonary moist rales or lower extremity and facial oedema, and evaluating changes in vital signs. Although these approaches are simple and easy, they have poor reliability and cannot meet the requirements for rapid, dynamic, accurate, and non-invasive clinical evaluations.

In this study, we used ultrasonic and empirical methods to determine the blood volume of patients in the ultrasound and experience groups and compared the results with those of the control group. We found that serum creatinine, potassium, and NT-proBNP levels among all three groups decreased within 24 h of CRRT, and there were no significant differences in pairwise comparisons among the three groups, indicating that different blood volume assessment methods do not affect the serum creatinine and potassium clearance efficiency during the initial treatment, and no significant effect was observed on the NT-proBNP level.

We also found that the time to improved heart failure, CRRT time, and ICU length of stay in the ultrasound and experience groups were significantly shorter than those in the control group. Compared with the control group, the ultrasound group had significantly shorter ventilator use duration, and the difference was statistically significant. The above results show that, compared with the control group without fluid volume assessment, the ultrasound and experience groups had faster improvement in heart failure symptoms and shorter CRRT times and ICU hospitalisation durations.

Our study demonstrates that timely dynamic fluid volume assessment during CRRT has better clinical value in guiding dehydration adjustments in patients with renal failure combined with acute heart failure.

In the comparisons of vasopressor use and incidence of adverse events (i.e., hypotension, arrhythmia, delirium), we found that the duration of vasopressor use in the ultrasound and control groups was significantly lower than that in the experience group, and the incidence of adverse events (hypotension, arrhythmia, delirium) in the ultrasound group was significantly lower than that in the experience and control groups.

We considered several reasons for these results. First, the experiential method has a certain value in the assessment of patients with high-volume status, such as rapid improvement of heart failure symptoms, reduced CRRT time and length of ICU stay, whereas in patients with insufficient volume status, its accuracy remains uncertain. Heart rate and blood pressure will increase reflectively in patients with insufficient fluid volume, which may present as a pseudo-high-volume status leading to rapid dehydration during CRRT, thereby increasing the incidence of hypotension and duration of vasopressor use. Second, the patients in the control group had slow and uniform dehydration. Although the duration of vasopressor use was shorter, the remission of heart failure symptoms was slower, CRRT time significantly increased, length of ICU stay was prolonged, and incidence of adverse events such as arrhythmia and delirium increased. Third, the duration of ventilator use was significantly longer than the time to improved heart failure symptoms in all three groups, possibly because the patients’ oxygen content improved after mechanical ventilation. Moreover the heart failure symptoms improved significantly, although the patients’ blood volume remained in an overloaded state. Heart failure symptoms recurred if the ventilator was discontinued. Therefore, the ventilator duration time should be extended to ensure that the patient’s heart failure symptoms do not recur.

In contrast, the heart failure symptoms improved rapidly, and CRRT time, length of ICU hospitalisation, and ventilator use duration were significantly shortened in the ultrasound group. More importantly, the incidence of CRRT-related hypotension, vasopressor use duration, and incidence of adverse events was significantly reduced.

The primary limitation of our study is that it was a single-centre study with a small sample size. Hence, multicentre prospective studies with large sample sizes are warranted to validate our conclusions and provide a better basis for clinicians.

In conclusion, owing to the rapid progression of renal failure combined with acute heart failure, blood volume evaluation needs to be more intuitive and accurate. Ultrasonic dynamic monitoring of IVCD and IVCCI can provide accurate guidance for CRRT dehydration adjustment in patients with renal failure combined with acute heart failure. It can quickly relieve heart failure symptoms, reduce the incidence of adverse events and ICU treatment costs, and improve the patients’ quality of life. Therefore ultrasonic dynamic monitoring of the IVCD and IVCCI has good social and economic benefits.