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Doppler studies are done using 2 modes of doppler:
- Color wave doppler: simply generates color over a moving object (blue if moving away from the transducer and red if moving towards the transducer or mix if turbulent flow) it is mainly used to "map" vessels and determine direction of flow.
- Pulsed wave doppler: generates pulses of doppler in a small gate chosen in the color doppler mode to furthermore examine the velocity of flow throughtout the cardiac cycle.
How to describe doppler study?
•Comment on Doppler studies of flow can be described by waveform pattern e.g.
preserved/ absent/ or reversed end diastolic flow, diastolic notch..etc.
•Doppler studies of flow can be quantified and described by the use of indices:
–Pulsatility index (PI)
–Resistance index (RI)
–Systolic/Diastolic ratio (S/D)
Why we use ratio (indices) rather than absolute figures of velocity (distance/time)?
The advantage of using indices rather than absolute velocities is that indices are angle independent (the beam can be at any angle with the vessel)
What components of fetal circulation can be examined?
1- Fetal Heart:
2- Arterial side of the fetal circulation
3- Venous side of the fetal circulation
2- Arterial Side of Fetal Circulation
- Flow velocity waveforms from the umbilical cord have a characteristic saw-tooth appearance of arterial flow in one direction and continuous umbilical venous blood flow in the other.
- Color wave doppler is used to demonstrate presence of 2 arteries and one vein in the umbilical cord.
- Pulsed wave doppler is used to demonstrate that blood is flowing in the umbilical artery both during systole and diastole.
- Indices are used to demonstrate normal quantified values of resistance to flow (in the placental bed) which is the resistance index (more important in umbilical arteries than PI or S/D).
In cases of placental insufficiency the blood will face increased resistance (RI increases) then will stop flowing during diastole (absent end diastolic flow) and in severe cases with impending fetal demise the flow during diastole will even be reversed (Systolic flow above the centerline and diastolic below the centerline)
- Color wave doppler is used to demonstrate the direction of flow in the aotra out of the left ventricle. This is important in cases of major cardiac anomalies when it is really difficult to identify the cardiac chambers, in such conditions the vessels are traced back to the relevant chambers (mainly aorta to the LV and IVC to the RA)
- Pulsed wave doppler and indices from the thoracic aorta are used to demonstrate quantified values of PI and RI to flow from abdominal organs (Placental bed resistance affects the abdominal aorta).
- Color wave doppler is used to identify the the middle cerebral artery can be seen as a major lateral branch of the circle of Willis, running anterolaterally at the borderline between the anterior and the middle cerebral fossae in transverse view of the fetal brain at the level of the biparietal diameter.
-Pulsed wave Doppler is used to obtain flow velocity waveforms. (care should be taken to apply minimal pressure to the maternal abdomen with the transducer, as fetal head compression is associated with alterations of intracranial arterial waveforms).
3- Venous side of fetal Circulation
The normal flow in the umbilical vein is continuous as seen in the figures above, in cases of insreased resistance to flow the flow in the UV becomes pulsatile.
The ductus venosus is a small connection between the umbilical vein and the Inferior vena cava (IVC)
- Color wave doppler is used to identify the ductus venosus; In an oblique transverse section through the upper abdomen, its origin from the umbilical vein can be found where color Doppler indicates high velocities compared to the umbilical vein, and sometimes this produces an aliasing effect (turbulent flow color with mix of red and blue).
- Pulsed wave doppler is used to demonstrate the typical waveform for blood flow in venous vessels consists of three phases (Figure 20). The highest pressure gradient between the venous vessels and the right atrium occurs during ventricular systole (S), which results in the highest blood flow velocities towards the fetal heart during that part of the cardiac cycle. Early diastole (D), with the opening of the atrioventricular valves and passive early filling of the ventricles (E-wave of the biphasic atrioventricular flow waveform), is associated with a second peak of forward flow. The nadir of flow velocities coincides with atrial contraction (a) during late diastole (A-wave of the atrioventricular flow waveform). During atrial contraction, the foramen ovale flap and the crista dividens meet, thereby preventing direct blood flow from the ductus venosus to the left atrium during that short period of closure of the foramen ovale
Fetal distress is fetus suffering from any or all of the following:
- Hypoxia: Reduced oxygen tension
- Asphyxia: + Rise of CO2
- Acidosis: + accumulation of acid products of anaerobic oxidation
- Ischemia: Reduced blood flow
As an obstetrician in need of a clear cut decision, you need to know:
-Nature of fetal distress
-Severity of fetal distress
-Duration of fetal distress
Doppler studies aim to identify these changes in fetal circulation.
So what is the sequence of events in cases of fetal distress?
An early stage in fetal adaptation to hypoxemia:
• There will be central redistribution of blood flow ( brain-sparing) reflects as increased blood flow to the brain, heart, and adrenals with reduced flow to the peripheral and placental circulations
This reflects in changes of doppler wave form with:
- Increased end-diastolic flow in the middle cerebral artery (lower MCA pulsatility index or resistance index)
- Decreased end-diastolic flow in the umbilical artery (higher umbilical artery RI or systole-to-diastole [S/D] ratio
With more Hypoxia:
Decrease cardiac output secondary to myocardial dysfunction will result in:
•Rise in CVP
•Increase in reverse flow in atrial systole which is transmitted down venous system - the further from the heart the greater degree of cardiac dysfunction
•Ductus venosus shows a reduced ‘a’ wave and later reverse ‘a’ wave.
•Reverse End-Diastolic-Flow in Umbilical Artery
•Pulsations seen in Umbilical vein
Then at the end:
•Constriction of cerebral circulation
•Death within 96 hours