| Table 2: | The FHR and patterns associated with fetal and neonatal distress |   |
| PATTERN | PROBLEM |
| Severe bradycardia (<80/min) and loss of variability | Fetal hemorrhage, asphyxia |
| Sustained tachycardia, no abnormal patterns | Infection, often with apnea |
| Late decelerations and loss of variability | Asphyxia |
| Severe, recurrent variable decelerations and loss of variability | Asphyxia, possible hypovolemia |
| Sinusoidal | Severe anemia with asphyxia |
If fetal distress is suspected following FHR monitoring, the assessment of fetal scalp pH is used as a secondary tool to determine fetal well-being. The acid-base balance of the fetus is determined by the viability of the placenta, and its ability to exchange oxygen and carbon dioxide between maternal and fetal blood. If that exchange is disrupted, either at the placenta or in the cord, the resultant drop in pH can be measured.
There are two reasons for the drop in pH:
- as blood gas exchange decreases, fetal PaCO2 increases, decreasing
the pH;
- facing hypoxia, the fetus begins to metabolize glycogen without oxygen, resulting in a dramatic increase in lactic acid. This metabolic acid, combined with increased PaCO2, causes the pH to drop.
The fetal scalp blood sample is obtained through the cervix between contractions. Normal fetal blood pH is considered to be above 7.25. A pH of 7.2 to 7.24 shows slight asphyxia, and a pH of less than 7.2 signifies severe asphyxia. Since maternal pH can influence fetal pH, it may also be necessary to determine the acid-base status of the mother concurrently. Fetal scalp pH is useful only in the presence of abnormal FHR tracings, since normal tracing indicates a healthy infant in most instances.