Using Fetal Heart Rate Tracings to Assess Acidosis
In general, the amount of acid accumulation with late decelerations is dependent on the frequency and severity of the decelerations; the accumulation of acidosis may range from a base deficit increase of 1 mmol/L every 5 minutes to an increase of just 1 mmol/L every 15 minutes.
One of the weak links in our understanding of fetal acidosis today is our inability to recognize preexisting acidosis or preexisting injury. We have little experience in identifying fetal heart rate patterns associated with preexisting hypoxic injuries.
Adding to the challenge is the knowledge that a post-term fetus or one with intrauterine growth restriction may begin labor with a slightly greater level of acidosis. Furthermore, fetuses with true sepsis or severe anemia may accumulate acid at an increased rate compared with normal fetuses.
Where We Stand
Practically, attempting to avoid injury by recognizing mild, moderate, and potentially severe levels of fetal acidosis means that one must carefully examine fetal heart rate tracings, not only for the time we are present in the room, but at least back to the time of our previous assessment. As much as is possible, we should understand what the entirety of the monitoring has shown.
We should attempt to factor in the known changes in fetal acidosis associated with normal stages of labor together with estimated changes in acidosis related to superimposed fetal heart rate decelerations. With an understanding of the progress and stage of labor, the current fetal heart rate pattern, and the approximate level of fetal metabolic acidosis, we will be best prepared to manage the pregnancy for an optimal outcome.
ELSEVIER GLOBAL MEDICAL NEWS
ELSEVIER GLOBAL MEDICAL NEWS
Fetal Heart Rate Monitoring
Over the years, we have endeavored to assess fetal well-being by a number of electronic and nonelectronic means with varying degrees of success. Of all these methods, fetal heart rate monitoring has withstood the test of time.
Our continued use of fetal heart rate monitoring as a means of assessing the fetus's biochemical and biophysical status has contributed much to our understanding of fetal well-being, or lack thereof. More recently, efforts have been made to better correlate variations in fetal heart rate to fetal well-being.
It is well known that the fetus is the final arbiter of intrauterine stress and may respond with compensatory mechanisms that may thwart various types of stresses. In such a scenario, the fetus may not manifest a compromised state, despite potentially harmful stress conditions. On the other hand, another fetus facing similarly stressful intrauterine conditions may struggle, exhibiting fetal distress or worse.
In reality, what matters most is the response of the fetus and not the stressful condition per se. Every attempt to monitor fetal well-being has been focused, therefore, on the response of the fetus to various types of stress. Because we're unable to conduct biochemical testing on a real-time or continuous basis, fetal heart rate monitoring often has been used as a surrogate for the biochemical adaptations by the fetus to intrauterine stress conditions.
Fetal heart rate monitoring, thus, becomes a very important diagnostic tool because the decisions that physicians make and the interventions that they undertake often are based on their interpretation of the fetal heart rate tracings. Such decisions are critical to the overall outcome of the fetus.
It is in this light that we are dedicating a Master Class to the subject of fetal acidosis and fetal heart rate assessment and have invited Dr. Michael G. Ross to serve as our guest professor this month. Dr. Ross is the chair of obstetrics and gynecology at the Harbor-UCLA Medical Center in Torrance, Calif., and professor and vice chair of obstetrics and gynecology at the David Geffen School of Medicine at the University of California, Los Angeles.
Dr. Ross's exceptional article on this topic delineates the mechanisms of fetal metabolic acidosis and its effects on fetal well-being. He also offers valuable insights on how fetal heart rate tracings might be better utilized as a powerful tool for detecting and predicting where a fetus may lie along the acidosis spectrum during various stages of labor so that interventions may be implemented to prevent severe acidosis and associated injury to the fetus.
