A mathematical model of the fetal cardiovascular system based on genetic algorithms as identification technique

The development of fetal cardiac surgery, considered the ultimate goal in the treatment of congenital cardiac malformations, needs to be supported by detailed knowledge of the blood circulation in the fetal cardiovascular system. The hemodynamic behavior in distal territories is usually inferred from vessel resistance indices, which give limited physiological information. This study presents a mathematical model of the human fetal global cardiovascular system, developed to clarify the relationships and differences existing between upper and lower body circulation. We modelled the heart with two time-varying capacitances, each representing the respective ventricle's pressure-volume relationship. The fetal vascular system was represented using two six-element Windkessel models, for the upper and lower body respectively. We obtained the identification of the set of circuital and elastance function parameters of the model using Genetic Algorithms (GAs), which follow the laws of evolutionary theory.

We compared the results of our numerical study on the model identified with data collected from measurements and literature, to validate the proposed global cardiovascular system model of the human fetus. This model is intended as an instrument to investigate the differences in blood distribution between the different vascular districts in the upper and lower fetal body and the role of the aortic isthmus, the small tract of vessel connecting upper and lower fetal vascular beds; it may also represent a useful tool in the assessment of dynamic balance during mechanical assistance of circulation.

[Aventilatory mass flow during apnea : investigations on quantification]

BACKGROUND

Aventilatory mass flow (AVMF) is routinely used for apneic oxygenation in various clinical procedures but no data exist to quantitatively describe the gas flow. This study was designed to determine the amount of AVMF during the clinical situation of apnea to force spontaneous respiration at the end of anaesthesia with controlled ventilation.

MATERIALS AND METHODS

A total of 200 patients undergoing anesthesia for routine surgery were examined. AVMF was analyzed with a high resolution, low gas stream, thermal mass flow analyzer. The intended recording time was 3 min.

RESULTS

Measurement was reliably successful and suitable for evaluation in only 23 patients. AVMF-induced gas flow started on average 17.9 + or - 9.4 s after onset of apnea. Maximum flow was reached within 158 + or - 20 s and determined to be 135 + or - 32 ml/min. The slope of increase of gas flow showed a rapid oscillation corresponding to the heart rate in all patients and in 14 out of 23 patients a slow oscillation with a frequency of 8.9 + or - 1.8/min.

CONCLUSIONS

During apnea AVMF develops in a non-linear fashion. The maximum flow observed is closely related to the estimated oxygen consumption. Heart rate synchronous flow variations are probably caused by intrathoracic volume variations due to heart action. The low frequency oscillations correspond to the frequency of Traube-Hering-Mayer waves.

Optimized veno-venous bypass with the affinity pump

Veno-venous bypass (VVBP) is increasingly used to avoid acute venous hypertension and low cardiac output after clamping the vena cava. Air embolism upon accidental decannulation of the inflow line and endothelial damage due to suction of the blood collecting cannula to the vessel wall are known complications specific to the currently used roller and centrifugal pumps, because they generate negative pressure at the inflow site of the pump. The Affinity pump has a unique chamber design with an occlusive segment, that collapses in low filling states preventing negative pressure at the inflow site of the pump chamber. This device was tested for VVBP in three pigs (each weighing 52.3 +/- 5.1 kg) with hepatic vascular exclusion. Blood was pumped from the femoral and portal veins to the external jugular vein and perfusion was maintained for 6 hours. The hemodynamic state of the animals was assessed by recording heart rate; systolic, mean arterial, and diastolic pressure; as well as central venous pressure. Mean pump flow during the experiment was 1,629.3 +/- 372.2 ml/min. After clamping, the inflow line of the pump mean arterial pressure significantly decreased (from 69.5 +/- 4.4 to 43.1 +/- 3.5 mm Hg), and mean pressure in the femoral vein increased significantly (from 16.1 +/- 2.6 to 26.8 +/- 5.9 mm Hg), whereas the mean pressure in the internal jugular vein did not significantly change (from 6.0 +/- 1.7 to 5.0 +/- 2.1 mm Hg). There was no suction by the blood collecting cannula on the vessel wall, and neither bubbles nor air emboli were detected and no operator intervention was needed. In conclusion, the Affinity pump eliminates device related complications due to negative pressure generated at the inlet, and guarantees stable hemodynamics. Its application is simple and safe and minimal operator intervention is needed, making the Affinity pump particularly suited for veno-venous bypass.

A centrifugal pump driven tidal flow extracorporeal membrane oxygenation system tested with neonatal mock circulation

In 1993, Chevalier published his experiences with tidal flow venovenous extracorporeal membrane oxygenation (ECMO) featuring a single lumen cannula, non-occlusive roller pump, and alternating clamps. Using a neonatal mock circulation (NMC), which enables different hemodynamic states for neonatal ECMO research, the tested hypothesis was that it is possible to create a centrifugal pump driven tidal flow neonatal venovenous ECMO system. Additionally, the resulting hemodynamic effects in a condition of circulatory impairment were investigated. The ECMO circuit tested was assembled using a pediatric centrifugal pump head, a distensible reservoir, and a rotary clamp separating drainage from the injection phase. Using the NMC, end tidal volumes, mock circulation flow, and arterial and venous pressures were measured at different pump speeds after the drainage and injection phases. Effective venovenous ECMO flow (evvEF) was calculated. Mock circulation baseline values (ECMO clamped) were compared to values during tidal flow ECMO. At 3,000 rpm, a centrifugal pump speed of 75 ml/kg/min evvEF was reached, and it increased with higher pump speeds. At this point, the end tidal mock circulation flow (representing cardiac output) after drainage differed significantly from that during the injection phase (p < 0.01) but not from the baseline value. The end tidal arterial and venous pressures after the drainage phase were found to be significantly decreased compared to the baselines (p < 0.01). In conclusion, a centrifugal pump driven tidal flow venovenous ECMO system can be created enabling sufficient tidal volumes. Tested in the described NMC simulating posthypoxic circulatory impairment, significant hemodynamic effects could be demonstrated. Animal experiments for confirmation are necessary.