The pulmonary vein Doppler flow velocity waveform: feature analysis by comparison of in vivo pressures and flows with those in a computerized fetal physiological model

Fetal Pulmonary Venous Return: From Basic Research to the Clinical Value of Doppler Assessment

The fetal pulmonary circulation represents less than 25% of the fetal cardiac output. In comparison with the pulmonary arteries, studies on pulmonary veins are few and limited, and many questions remain to be answered. The literature reports that pulmonary veins play an important role in regulating vascular flow, forming an active segment of the pulmonary circulation. The development of more sophisticated ultrasonography technology has allowed the investigation of the extraparenchymal pulmonary veins and their waveform. The recognition of the pulmonary vein anatomy in echocardiography is important for the diagnosis of anomalous pulmonary venous connections, with a significant impact on prognosis. On the other hand, the identification of the normal pulmonary vein waveform seems to be a reliable way to study left heart function, with potential applicability in fetal and maternal pathology. Thus, the goal of this narrative review was to provide a clinically oriented perspective of the available literature on this topic.

Added value of the pulmonary vein pulsatility index and its correlation to neonatal umbilical artery pH in fetal growth restrictions: a Vietnamese matched cohort study

BACKGROUND

In clinical obstetrics, many guidelines recommended the use of Doppler fetal ductus venosus blood flow to monitor and to manage fetal growth restriction (FGR). The ductus venosus and the pulmonary venous flow pattern of fetuses are similar. Umbilical artery pH (UA pH) is essential in identifying adverse pregnancy outcomes, particularly in fetal growth restriction cases. Nevertheless, the literature indicates that the relationship between pulmonary vein pulsatility index (PVPI) and UA pH in FGR cases has not been well investigated. This study aimed to identify the alteration in PVPI in FGR cases and evaluate the correlation between PVPI and UA pH in FGR newborns.

METHODS

This matched cohort study of singleton pregnancies from 28+ 0 to 40+ 0 weeks of gestation without congenital abnormalities included 135 cases of FGR (disease group) and 135 cases of normal growth (control group). The PVPI was measured at the proximal segment of the right or left pulmonary vein, approximately 5 mm from the left atrium wall. The umbilical artery pulsatility index (UAPI) was measured on the free umbilical cord. An elective cesarean section or labor induction are both options for ending the pregnancy, depending on the condition of the mother or fetus. Umbilical artery blood samples were collected within 5 min of delivery for UA pH measurement. SPSS version 20 and Medcalc version 20.1 were used for data analysis.

RESULTS

FGR cases had a significantly higher mean fetal PVPI than the control group (1.16 ± 0.26 vs. 0.84 ± 0.16; p < 0.01), and PVPI and UAPI were positively correlated (r = 0.63; p < 0.001). PVPI and UA pH were negatively correlated in FGR patients, with r = -0.68; p < 0.001. The PVPI value on the 95th percentile had a prognostic value of UA pH < 7.20 with a sensitivity of 88.2%, specificity of 66.3%, positive predictive value of 46.9%, and negative predictive value of 94.3%.

CONCLUSIONS

There was a statistically significant difference in PVPI values in FGR cases compared to the normal growth group, a positive correlation between PVPI and UAPI, and a negative correlation between PVPI and UA pH. PVPI might have a prognostic meaning in predicting UA pH at birth.

Evaluation of the selected parameters of the fetal diastolic functions in normally grown or macrosomic fetuses of gestational diabetic mothers with poor glycaemic control

OBJECTIVE

This study aimed to investigate whether the third trimester fetal cardiac diastolic function measured by selected conventional Doppler indices is affected in appropriate-for-gestational-age or macrosomic fetuses of gestational diabetic mothers with poor glycaemic control.

METHODS

This cross-sectional study included 93 pregnant women divided into two groups. Group 1 included 45 appropriate-for-gestational-age or macrosomic fetuses from gestational diabetic mothers with poor glycaemic control (study group). Group 2 included 48 appropriate-for-gestational-age fetuses from gestational age-matched healthy mothers (control group). Functional fetal cardiac parameters and fetoplacental Doppler parameters were measured. Data were compared between the two groups.

RESULTS

Maternal characteristics did not differ significantly between the study and the control group. There were no significant differences in the early and late velocity, early/late velocity ratio of both mitral and tricuspid valves, the fetal pulmonary vein pulsatility index, and the ductus venosus pulsatility index between the study and the control group. Moreover, the rate of abnormal Doppler findings in pulmonary vein (pulmonary vein pulsatility index >95th centile), ductus venosus (ductus venosus pulsatility index >95th centile), and peripheral vessels (umbilical artery pulsatility index >95th centile, middle cerebral artery pulsatility index <5th centile, cerebra-placental index >95th centile) were comparable in both groups.

CONCLUSIONS

The third trimester fetal diastolic functions measured by selected conventional Doppler techniques do not seem to be altered in appropriate-for-gestational-age or macrosomic fetuses of gestational diabetic mothers who have poor glycaemic control.

Fetal Pulmonary Vein Pulsatility Index in the Third Trimester of Pregnancy as a Predictor of Small for Gestational Age

OBJECTIVE

This study aimed to establish whether the increased fetal pulmonary venous pulsatility index (PVPI) in late pregnancy can independently predict small for gestational age (SGA) and to verify its cut point value and efficacy.

METHOD

The PVPI was measured in women with singleton pregnancies between 25 and 39 gestational weeks. Maternal hypertension and diabetes, estimated fetal weight (EFW) and percentile of the corresponding weeks of pregnancy (USG_PER), gestational weeks at delivery, and birth weight and percentile of the corresponding weeks of pregnancy (BABY_PER) were reviewed. To assess whether PVPI was independently correlated with BABY_PER, Pearson's correlation analysis was performed. The cut point value of PVPI for the prediction of SGA was established using a receiver operating characteristic (ROC) curve.

RESULTS

A total of 129 mothers were included in this study. Both USG_PER and PVPI were significantly related to SGA, independently (P <.001 and P = .004, respectively). The cut point value of PVPI was found to be 1.13. The AUCs of PVPI and USG_PER were not significantly different (P = .624). The sensitivity of PVPI was 70.27%, and the specificity was 92.39%.

CONCLUSION

PVPI could predict SGA independently, and the efficacy was comparable to EFW during pregnancy.

Z-Score Reference Ranges for Fetal Heart Functional Measurements in a Large Brazilian Pregnant Women Sample

The objective of this study was to determine the reference values for fetal heart functional measurements at 24 and 34 weeks of gestation and to develop Z-score equations for all measurements. A single-center, prospective, cross-sectional study with normal fetuses between 24 and 34 weeks of gestation was performed. All pregnant women underwent a comprehensive fetal Doppler echocardiogram with anatomical and functional analysis. Measurements of left and right cardiac output, combined cardiac output, mitral and tricuspid valve flow, inferior vena cava flow, and pulmonary vein flow were performed. The Shapiro-Wilk test and histogram evaluation were performed on all variables. Linear regression was used to assess the relationships between measurements and gestational age. A total of 612 pregnant women with singleton and normal fetuses were included. We assessed the reference values and percentiles of cardiac function as a function of gestational age. The variables that were not normally distributed were subjected to logarithmic or square root transformation. Eleven Z-score equations were developed, with equations for left and right ventricle output and combined cardiac output that were dependent on gestational age and with other equations that were independent of gestational age. The present study produced a large database, allowing the demonstration of reference values and percentiles as well as the development of Z-score equations to facilitate the echocardiographic evaluation of fetal heart function.

Functional cardiac measurements performed by two-dimensional Doppler echocardiography in normal fetuses: Determination of Z-scores and future prospects

Two-dimensional (2D) echocardiogram with the aid of color Doppler and pulsed Doppler allows one to record blood flow waveforms in several structures of the heart. The determination of normal values of these flows in the fetus can help understand cardiac hemodynamics. Given this importance, numerous surveys have been conducted with various existing echocardiographic techniques in order to improve the functional evaluation and consequently, planning of delivery. The aim of this review was to discuss the findings of the reference values of blood flows obtained by 2D echocardiography with Doppler, the current trend of the determination of Z-scores in the functional measurements, and their future prospects.

Comparison of global and regional right and left ventricular longitudinal peak systolic strain, strain rate and velocity in healthy fetuses using a novel feature tracking technique

AIMS

To compare the feature tracking derived measurements of the right (RV) and left ventricular (LV) myocardium in healthy fetuses and evaluate the correlation with advancing pregnancy.

METHODS

Global and segmental longitudinal peak systolic strain, strain rate and velocity of the RV and LV myocardium were assessed with feature tracking technique in 150 healthy fetuses (13-39, median 22 weeks).

RESULTS

RV global and free wall strain and strain rate values were significantly higher than those in the LV segments (P<0.001 and P<0.001). In all segments on both ventricles longitudinal peak systolic velocity measurements exhibited a significant base to apex gradient (P<0.001) and increased with advancing pregnancy (P<0.001). LV global strain and strain rate values remained constant throughout gestation, whereas RV measurements decreased (P<0.05). With advancing pregnancy the difference between RV and LV global longitudinal velocity increased significantly, whereas the difference of strain and strain rate decreased significantly.

CONCLUSION

RV deformation parameters and velocity values are significantly higher compared to LV values. LV deformation parameters are stable throughout gestation, whereas RV measurements decrease significantly. The difference between global RV and LV myocardial values were significant throughout gestation, velocity values increased, whereas deformation parameters decreased.

Paroxysmal coughing, subdural and retinal bleeding: a computer modelling approach

Unexplained subdural and retinal haemorrhages in an infant are commonly attributed to 'shaking', the mechanism of which is believed to be traumatic venous rupture. However, the haemorrhagic retinopathy reported as a result of Valsalva manoeuvres and the subdural bleeding that is a rare complication of pertussis together demonstrate that if a sustained rise in intrathoracic pressure is transmitted to cerebral and retinal vessels, it may result in bleeding, similar to that reported in inflicted injury. Such haemorrhages would be expected to occur whenever severe paroxysmal coughing were induced, whatever the cause. This study used a computer modelling approach to investigate feeding accidents as the trigger for bleeding. A dynamic circulatory model of a 3-month-old infant was induced to 'cough', and the response to changes in physiological variables monitored. It showed that coughing causes intracranial pressures to build up exponentially to approach a maximum, proportional to the amount of pressure the musculature of the thorax can produce, as venous return is impeded. They do not have time to become dangerous during individual coughs, as blood quickly returns after the cough is over, reestablishing normal pressures. Paroxysmal coughing, however, does not allow blood to return between coughs, with the result that very high luminal pressures may be generated, sufficient to damage veins. A history of coughing, vomiting or choking is not uncommon in otherwise normal infants with retinal and subdural bleeding. Our findings suggest that paroxysmal coughing could account for such bleeding in some cases.

The ‘Sutured Skull’ and intracranial bleeding in infants

It is known that retinal haemorrhages can result in adults when elevated intrathoracic pressures due to coughing, cardiopulmonary resuscitation, etc., force blood into the head. In infants under one year of age retinal and intracranial haemorrhage commonly occur together, but the same is not true for the older child and adult. The role of the elasticity of the infant skull (resulting from suture and fontanelle stretching) compared to the rigid mature skull, was investigated in a computer aided method. This showed that although in the event of Valsalva-like situations very high lumen pressures may be present in both groups, in the rigid adult skull an immediate corresponding increase in intracranial pressure is produced which surrounds and supports vascular walls leaving transmural pressures little changed. No such support is provided in the eye, and retinal vessels may rupture. Within the skull there may be drastic effects on brain circulation, but since changes in vascular transmural pressure are minimal vessel distension is not induced. In the infant skull the sutures stretch as pressure rises. Since vascular volume is only about 5% of intracranial volume each 1% increase in skull volume permits a 20% increase in vascular volume. Quite small skull expansions will allow dangerous vascular distension and risk of wall damage. Until skull bones fuse, intra-cranial bleeding will be expected in the soft infant skull in any situation where retinal haemorrhage alone is known to occur in the adult or child.

Kaiser Wilhelm syndrome: obstetric trauma or placental insult in a historical case mimicking Erb's palsy

Kaiser Wilhelm II of Germany (1859) developed a weak and noticeably short left arm during childhood, commonly attributed to nerve damage caused by the use of excessive force during his difficult breech delivery, Erb's palsy. However, Wilhelm's mother had a severe fall when about four months pregnant and the child was reported to be very thin at birth, suggesting intrauterine growth restriction (IUGR). Wilhelm blamed the British doctor for his deformity, and formed an enmity, which ultimately led to the 1914-1918 world war. We propose an alternative theory, considering the possibility of placental damage and consequential flow redistribution caused by the fall. In severe IUGR, the Doppler pulsatility index (PI) of the brachial arteries differs, that of the right arm being lower than the left. We used a computer model of the fetoplacental unit and reduced its functional placental area until such resistance asymmetry was produced. This would occur in extreme hypoxia when flow in the aortic isthmus is reversed, bringing right ventricular blood of lower oxygen content to the left subclavian artery. The reduced PI in the right arm is a normal vasodilatory hypoxic response, but the apparently normal PI in the left arm results from decreased demand due to metabolic failure. We suggest that the nerve damage affecting the Kaiser's left arm was due to placental insufficiency during pregnancy, and not mechanical brachial plexus injury during delivery. We further suggest that such a mechanism be called Kaiser Wilhelm syndrome to distinguish it from Erb's palsy originating from obstetric trauma.

Spiral artery associated restricted growth (SPAARG): a computer model of pathophysiology resulting from low intervillous pressure having fetal programming implications

Failure of adequate trophoblastic conversion of maternal spiral arteries is associated with intrauterine growth restriction (IUGR). In addition to poor oxygen delivery, raised spiral artery resistance reduces placental intervillous pressure. An iterative type computer model was formed by linking an existing model of the fetus and a new nine cotyledon placental model. Simulation of compression cuffing of the spiral arteries to progressively restrict uteroplacental flow was performed, while observing various fetal and placental variables. Water moved to the fetus in the cotyledonary core villi, and to the mother in the outer villous layers. While the fetus could match villous capillary pressure to changes in intervillous pressure, net transplacental water movement was minimal, but when spiral artery resistance was increased sufficiently to cause mean intervillous pressure to fall below that which the fetus could match, a net flow to the mother appeared. That continued until the resulting fetal blood hemoconcentration produced a sufficient increase in colloid osmotic pressure to restrict further loss. All cells within the fetal-placental unit are then required to operate in an abnormal ionic environment, which may significantly affect systems such as the renin-angiotensin set-point, with implications for post-natal homeostasis such as control of adult blood pressure. Furthermore, in vivo, cells of the feto-placental unit respond to the increased intravascular osmotic pressure by production of intracellular osmolytes in order to match intracellular and vascular/interstitial osmotic pressures. This may explain the observed effects on postnatal water balance in growth restricted infants and could also provide a possible mechanism for the association of the systemic maternal complications associated with impaired placentation and reduced intervillus flow.

The dynamic placenta: II. Hypothetical model of a fetus driven transplacental water balance mechanism producing low apparent permeability in a highly permeable placenta

In vitro and isotopic studies in vivo have reported the paradox that the human placenta is highly permeable, water exchanging at 3.6 litres per hour at 35 weeks of gestation, but clinical measurements in vivo show net transfer is minimal, around 2 ml/day. Current theories are based on osmotic pressure balances, but changes in maternofetal hydrostatic pressure change much faster than osmotic factors could respond. An alternative explanation might be that net transfer is not in fact the result of passive mechanisms, but is actively controlled by the fetus itself. The fetus is well equipped to monitor changes in blood volume, such as via sensors in venous and atrial stretch receptors to control ANF and hence urine production. Transplacental water regulation requires modification of transvillus pressures. Placental sub-chorial arteries and veins (of extra-embryonic origin) have different sensitivities from fetal body tissues to some vasoactive substances, and stem villous veins have unusually well developed vascular smooth muscle. It is thus theoretically possible for the fetus to modify subchorial venous resistances, and hence villous capillary pressure with a suitable circulating placental venous constrictive agent. A computer modelling study was undertaken using a fictitious placental venous constricting agent "fictensin", considered to be released by the fetus in proportion to disturbance of vascular volume. The effective placental permeability fell in proportion to the tightness of this fetal control mechanism, suggesting that the apparent placental permeability measured in vivo is a measure of fetal control, not true permeability. However, the range of compensatory pressures that the fetus can produce by this means is limited and failure of such a mechanism could allow flooding or dehydration of the feto-placental unit. This may shed new light on disorders such as polyhydramnios and fetal hydrops.

The dynamic placenta: I. Hypothetical model of a placental mechanism matching local fetal blood flow to local intervillus oxygen delivery

The placenta can be severely infarcted and yet return well oxygenated blood in spite of the potential shunt paths produced. Optimisation of oxygen transport by some form of local flow matching has been suggested, either via a direct action of hypoxia on subchorial vessels, or indirectly by syncytiotrophoblastic metabolic products. Using casts of cotyledonal vessels and software modelling, a mechanism of hypoxic fetoplacental vasoconstriction could be demonstrated. A simple previously described passive placental model was extended to include hypoxic sensitive arteries and dependence of syncytio-trophoblastic metabolism on intervillus (maternal) blood oxygen content. Such a mechanism of placental flow matching could maintain fetal pO2 by reducing flow through inadequately oxygenated cotyledons, therefore optimising pO2 at the expense of flow. A further modification stabilising fetal water transfer was required to avoid changes in intervillus oxygen delivery producing changes in fetal water content via placental capillary pressure alterations. Intervillus/villus flow matching is likely in the human placenta and this study suggests probable biologically plausible mechanisms for such a phenomenon.

Dynamics of the Pulmonary Venous Flow in the Fetus and Its Association With Vascular Diameter

Background— The usual positioning of the Doppler sample volume to assess fetal pulmonary vein flow is in the distal portion of the vein, where the vessel diameter is maximal. This study was performed to test the association of the pulmonary vein pulsatility index (PVPI) with the vessel diameter.

Methods and Results— Twenty-three normal fetuses (mean gestational age, 28.6±5.3 weeks) were studied by Doppler echocardiography. Pulmonary right upper vein flow was assessed adjacent to the venoatrial junction (“distal” position) and in the middle of the vein (“proximal” position). The vessel diameter was measured by 2D echocardiography with power Doppler, and the PVPI was obtained by the ratio (maximal velocity [systolic or diastolic peak]−minimal velocity [presystolic peak])/mean velocity. The statistical analysis used t test and exponential correlation studies. Mean distal diameter was 0.33±0.10 cm (0.11 to 0.57 cm), and mean proximal diameter was 0.16±0.08 cm (0.11 to 0.25 cm) ( P <0.0001). Mean distal PVPI was 0.84±0.21 (0.59 to 1.38), and mean proximal PVPI was 2.09±0.59 (1.23 to 3.11) ( P <0.0001). Exponential inverse correlation between pulmonary vein diameter and pulsatility index was highly significant ( P <0.0001), with a determination coefficient of 0.439.

Conclusions— In the normal fetus, the pulmonary venous flow pulsatility decreases from the lung to the heart, and this parameter is inversely correlated to the diameter of the pulmonary vein, which increases from its proximal to its distal portion. This study emphasizes the importance of the correct positioning of the Doppler sample volume, adjacent to the venoatrial junction, to assess pulmonary venous flow dynamics.