PRAM is a method for monitoring continuous cardiac output based on changes in arterial pressure, which is based on radial expansion caused by changes in volume of a given blood vessel. Many studies have demonstrated a good correlation of PRAM with other classical methods such as cardiac catheterization and Doppler ultrasound.(11, 12) SVV is a dynamic hemodynamic parameter that reflects changes in stroke volume.(13–15) SVV in patients with mechanical ventilation is less than 10ཞ15%. According to cardiopulmonary interaction under mechanical ventilation and Frank-Starling principle, a change in stroke volume caused by mechanical ventilation is more significant when the blood volume is insufficient, showing a negative correlation between SVV and the blood volume.(16, 17) Theoretically, SVV can therefore be used to estimate the intravascular volume state and predict the responsiveness of the circulatory system to the infusion treatment.
SVV has been used in predicting volume response in children receiving cardiac surgery. Some studies have shown that its accuracy is higher than CVP.(18, 19) To the best of our knowledge, this is the first study that demonstrated the good predictability of SVV for assessing fluid responsiveness after volume challenge in pediatric Fontan patients, proving that SVV is a simple, fast, direct and noninvasive method with good reproducibility. SVV can be measured with high-quality values in any individual patients. In this study, we implemented aggressive volume therapy by infusing 10ml·kg− 1 5% albumin instead of crystalloid solution within 10 minutes. (20, 21) The result of our experiment demonstrated that the hemodynamic parameters including the mean arterial pressure and cardiac index were significantly improved in the Fontan patients after volume challenge, as represented by a stable hemodynamic state, a balanced internal environment, an increase in urine volume, and the absence of significant adverse events. SVV < 16% indicated an insufficient blood volume of the patient and continuous volume expansion treatment had little effect in further improving the cardiac function in such a condition. Therefore, we believe that an appropriate volume and fluid type are primarily important for improving the circulation capacity and cardiac function of Fontan patients after operation. This may be related to the increase in TPG and pulmonary forward blood flow, though it needs to be verified in future research.
The lung and blood vessel wall compliance in children is better than that in adults. However, the lung of children with a Fontan procedure is pathologically different from that of people with a biventricular structure.(22) The higher positive airway pressure caused by mechanical ventilation will significantly reduce pulmonary blood flow and LVED volume. As Fontan patients have no normal right atrial and right ventricular structures in whom pulmonary vascular resistance (PVR) is relatively high, SVV produced by cardiopulmonary interaction may be significantly different between Fontan and normal children.
Currently, CVP is commonly used as a static hemodynamic parameter in clinical practice. However, some studies have demonstrated that CVP could not be used as a single reference in patients with volume therapy.(23) As the linear relationship between cardiac pressure and volume is weakened in Fontan patients, the pressure parameter such as CVP cannot really reflect the volume state. In addition, the superior vena cava of Fontan patients is connected to the pulmonary artery, and therefore CVP is significantly affected by pulmonary artery pressure. As a result, the value of CVP to predict the volume reactivity of Fontan patients could be influenced by many factors and the result is unreliable.
There is few studies reporting the application of SVV in children with single-ventricle Fontan circulation. In this study, we found that the cutoff of SVV was 16%, the sensitivity was 50%, and the specificity was 91.7%, suggesting that SVV is more specific than CVP in reflecting the volume state of children undergoing Fontan operation after CPB. Statistically, children with less SVV variation are more likely to be in a non-response state. Yoshitake et al(24) used noninvasive hemodynamic monitoring to evaluate the cardiac output after Fontan operation by measuring the parameters of cardiac function in the patients with single left ventricular (SLV) and single right ventricular (SRV). Their results showed that the mean SVV (SLV : SRV) was 13.9% : 15.5%, which is similar to our study. However, the sensitivity and specificity of SVV in our study suggest that SVV may have a high false-positive rate as a predictor of capacity reactivity in Fontan patients. This may be due to the more significant reduction of pulmonary blood flow in patients with single ventricle caused by positive pressure mechanical ventilation. In addition, the inhibition of myocardial contractility and the use of positive inotropic agents in early postoperative patients may affect the results. We believe that SVV derived from the expansion of the PPV index is a relatively safe volume reactivity parameter for patients who are not suitable to use volume dilation treatment, and therefore more suitable for patients whose current volume state is on the reaction section (steep section) of Frank-Starling curve rather than the smooth section. As each inspiratory and expiratory during mechanical ventilation can cause change in stroke volume, SVV can be used to indicate volume responsiveness and the current volume state of the patient.
This study has some limitations. First, the response of Fontan patients to volume infusion is affected by many factors. But as we only observed the effect of volume challenge alone, the influence of mechanical ventilation on the research results could not be excluded. In addition, we only observed the result of volume response immediately after volume challenge treatment without tracking changes in dynamic parameters in the cardiac intensive care unit. Finally, we did not use PiCCO as the control parameter for cost consideration.