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Re: Periodic variation of UA (and UV) waveformsFrom: James S Smeltzer MD (gaperina@mindspring.com)Sat Feb 16 09:57:00 2002
Hi, These are almost always a good sign, as they usually represent fetal breathing. The cyclic variation in the intrathoracic pressure (at an average period of about 0.8 per sec, varying by double or half, according to some work Dr. Michael Divon did a long time ago) shows up as alteration in both arterial and venous doppler wave forms for three reasons. All fluids flow only down a pressure gradient. Inspiration increases the gradient between the UV and atria, increasing cardiac filling and UV velocity measured close to the baby. This results in greater preload and the stroke volume increases by the frank starling mechanism. This causes fluctuation in the baroreceptor-perceived blood pressure, and feeds back to the brainstem. As the brainstem cares a lot about blood pressure regulation and nothing about heart rate except as it feeds back on that, decelerates the fetal heart rate by vagally mediated rapid response system (active over a second to few seconds). Slowing of the heart rate produces a change in the doppler indices without any change in the hemodynamics by increasing run-off time, raising the index. What you observe int he umbilical (and middle cerebral indices then if you are doing complete studies) is the result of these simultaneous complex processes. This all assumes that the chest is moving. A frequent rhythm of the fetus, which is usually insignificant but may be part of a pathological anatomy or physiology, is atrial bigeminy - and occasionally ventricular bigeminy, which can also produce this picture without chest movement. A serious problem associated with this picture is AV dissociation from antibody mediated lupus myocarditis (associated with a positive Ro or SSA antibody). Other arrhythmias, frequent PVCs or PACs may also produce this picture. Important: What is the clinical setting? Is the chest moving? Is the heart rhythm normal? Hope this helps! Jim S
At 08:50 PM 2/15/2002 -0600, you wrote:
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