EL and KE are new indices and can be measured using VFM technology in the perioperative period. However, there are some clinical concerns about interpreting these indices. In this case, we used the EPI to overcome the limitation and interpret EL, KE, and the efficacy of the surgery appropriately.
VFM technology uses both color Doppler images and speckle tracking images [1]. Intracardiac EL can be calculated using the following equation:
\( \mathrm{Energy}\ \mathrm{loss}\ \left(\mathrm{EL}\right)=\int \mu \left(2{\left(\frac{\partial u}{\partial x}\right)}^2+{\left(\frac{\partial v}{\partial y}\right)}^2+{\left(\frac{\partial u}{\partial y}+\frac{\partial v}{\partial x}\right)}^2\right) dA \), where μ is the viscosity of the blood, u and v are the velocity components along the Cartesian axes (x and y, respectively), and A is the area of the unit of the grid. EL has also been estimated by using computer flow simulation studies to assess hemodynamics following congenital heart disease [6,7,8]. Echocardiography VFM enables us to evaluate the cardiac workload. Evaluation of pulmonary valve stenosis and association between right ventricular function deterioration and EL using VFM technology have been reported [3, 9]. One of the well-known concept of EL is the “energy loss concept,” which is used to evaluate aortic stenosis [10], and the energy loss index is one of the predictors of prognosis in asymptomatic aortic stenosis [11].
However, there are some important points to be considered in interpreting the EL value during anesthesia. During CPB, the EL is zero because the heart undergoes asystole and does not produce energy. In a patient with severe aortic regurgitation [12, 13], or when systolic anterior motion exists [5], the patient’s left ventricular EL increases, which is caused by vortex and turbulent flow. In a hyper-dynamic state [14], the EL rises even if the patient does not need any therapeutic intervention. Therefore, when using VFM analysis during the early perioperative period, we should consider not only whether is the EL rising or falling, but also the clinical background in which the condition occurs. EL evaluation is difficult, which is why there are few reports about the relationship between EL and early perioperative prognosis.
To solve this problem, Akiyama et al. reported the utility of KE and EPI [14].
\( \mathrm{Kinetic}\ \mathrm{energy}\ \left(\mathrm{KE}\right)=\int \frac{1}{2}\rho {v}^2\times vdL \), where ρ is the density of the blood (1060 kg/m3), v is the velocity vector of the blood flow, and dL is an increment of the cross-sectional line. In this case, we calculated left ventricular output KE, because we could acquire optimal images from TEE. In our case, the EPI is defined as follows:
$$ \mathrm{Energetic}\ \mathrm{performance}\ \mathrm{index}\ \left(\mathrm{EPI}\right)=\frac{\mathrm{KEcycle}}{\mathrm{ELcycle}} $$
The EPI is useful for assessing the cardiac condition, effectiveness of treatment, and outcome of surgery [14, 15]. Nakashima et al. reported the energy efficiency ratio, which is the left ventricular EL divided by KE of the trans-mitral flow, to analyze the postoperative cardiac condition [16]. By considering energy efficiency as a ratio, it is easier to evaluate the cardiac condition.
Theoretically, after the Glenn procedure, blood flows from the superior vena cava directly into the pulmonary artery, and the patient’s left ventricular preload decreases [17, 18]. The mean EL depends on the left ventricular preload; therefore, after the procedure, the EL was decreased.
The mean kinetic energy depends on the ejection of blood from the left ventricle into the left ventricular outflow tract. After the DKS procedure, the KE would increase owing to the integrated systemic outflow; however, in this case, KE decreased, which was potentially explained by the fact that KE also depends on the left ventricular preload [14]. Interestingly, EL and KE were reported to decrease in an 11-month-old patient who underwent the Glenn procedure, which may be consistent with our findings [4]. Together, evaluation of EL and KE is difficult because the left ventricular preload changes dramatically during pediatric congenital heart surgery.
Thus, we tried to calculate the EPI. After the Glenn and DKS procedures, we detected an improvement in EPI, indicating that the cardiac workload improved as well. Using VFM technology, the efficiency of the congenital heart disease surgery can be assessed in the early postoperative period.
The systolic and diastolic EL were positively correlated with the heart rate, and the E wave peak velocity and negatively correlated with age [19]. In this case, EL/BSA is still higher than the reference value. If this patient undergoes Fontan surgery, cardiac preload will decrease and the EL will decrease appropriately. The EL measurement was two-dimensional in this case. Three-dimensional measurements using magnetic resonance imaging might allow a more accurate assessment of the ventricular workload [19]. Further research should be performed to analyze the blood flow changes between the Glenn and Fontan surgeries.
The VFM blood flow analysis has some advantages and limitations. We believe that intraoperative blood flow analysis can contribute to the understanding of the pediatric congenital heart disease physiology, thus helping in improving these patients’ prognosis. This report would be helpful and useful for anesthesiologists and surgeons to interpret perioperative blood flow analysis appropriately during congenital heart surgery.
In conclusion, we revealed the efficiency of the Glenn and DKS procedures using VFM. After Glenn and DKS procedures, we detected an improvement in EPI, with an appropriate improvement in the mean cardiac workload. The EPI might be a helpful index of the hemodynamic status in patients with a single ventricle.
