Patients
In the first three months of our clinical observational study, 1715 pregnant women delivered vaginally. Overall, there were 601 primiparas and 600 multiparas in the whole population. The median gestational age was 39 weeks, the median body surface area was 1,73 m2 and the median BMI was 26.22 kg/m2 (Table 1). From May 2017 to May 2020, 514 patients experience hemorrhage exceeding 1000 ml.
Different blood loss and percentages of total blood volume in different body surface quintile groups
All patients were classified into different quintiles according to the body surface area (Table 2). From quintiles 1 to 5, the body surface area was 1.40-1.64 m2, 1.65-1.70 m2, 1.71-1.77 m2, 1.78-1.84 m2 and 1.85-2.83 m2, respectively. Overall, the median total blood volume was 4639 ml (interquartile range 4445-4866 ml). Hence, 10% of total blood volume is 464 ml and 15% of total blood volume is 696 ml. We found that the median total blood volume increased with increasing body surface area, and different proportions of total blood volume increased accordingly. The median 24h blood loss for the whole population was 385 ml (interquartile range 245-565 ml). For each quintile of body surface area, the median 24h blood loss was 363 ml, 360 ml, 390 ml, 380 ml, and 440 ml, respectively (Figure 1). The median blood loss at 24h postpartum in quintile 5 was significantly higher than in quintile 1 (P<0.05), but the percentage of median 24h blood loss in total blood volume between the two groups was identical (P>0.05).
The incidences at each quintile across different groups
The prevalence of PPH at each quintile was 27.5%, 28.3%, 30.3%, 25.7% and 37.9%, respectively using the universal definition of PPH (cutoff value of 500 ml). The occurrence of PPH at quintile 5 was significantly higher compared to other quintiles. At quintile 1, we divided 174 patients with blood loss of less than 500ml into two different groups. Blood loss of total blood volume in Group 1 is less than 10%, while blood loss of TBV in Group 2 is more than 10%. The alterations in blood hemoglobin and hemocrit were significantly different (p<0.05). At quintile 5, we divided all 91 patients with blood loss more than 500 ml into two different groups. Blood loss of total blood volume in Group 1 was more than 10%, while blood loss of TBV in Group 2 is less than 10%. The alterations in cardiovascular system and blood routine examination were not significantly different.
Furthermore, we compared the consequent dynamics and laboratory changes according to the amount of blood loss or different percentages of total blood volume. Our study indicated that blood loss as a percentage of total blood volume was significantly correlated to changes in diastolic blood pressure, heart rate, hemoglobin and hematocrit.
We found that prevalence of PPH at each quintile was approximately 30% using the universal definition of PPH. When we altered the volume of blood loss to 700 ml or redefined blood loss at 10%, 13% or 15% in total blood volume as definition, the changes in blood pressure, heart rate and blood routine results were not significant different. The incidences of PPH at each quintile was 11.3%, 11.3%, 10.2%, 13.5%, and 20% when we used blood loss of 700 ml as definition. On the other hand, when we used 13% proportion of total blood volume as a new merit to define PPH, the occurrence for each quintile were 22.1%, 16.7%, 18%, 17.3%, and 21.3%, respectively. Additionally, the incidences of PPH at each quintile were 16.3%, 12.1%, 10.7%, 11%, and 17.5% when we used blood loss of 15% in total blood volume. There were no significant differences at each quintile when we used proportion of blood loss in TBA as definition, but there was significant difference at each quintile when we used the amount of blood loss as definition (Table 3).
Blood loss or blood loss in TBV ratio in predicting blood transfusion.
Considering the low incidence of bleeding over 1000 ml, we identified a total of 514 patients with blood loss over 1000 ml in three years. We investigated the value of either blood loss or blood loss in total blood volume in predicting blood transfusion using blood transfusion as the outcome variable. In the meantime, we grouped them by surface area and compared amount of blood loss or blood loss in total blood volume by forecasting blood transfusion at each group. Use of blood loss or blood loss in total blood volume has high specificity and sensitivity as the indicators to predict blood transfusion. However, cutoff values of blood loss predicted by different body surface area groups were significantly different, while the blood loss in total volume ratio was relatively stable (Table 4 and figure 2). The results were a good explanation for a high blood transfusion rate in the group with small body surface area, with a low blood transfusion rate in the group with large body surface area. As the same amount of blood loss in the group with small surface area takes up a larger proportion of blood volume, and is more likely to cause changes in blood pressure and heart rate, clinicians are more likely to administer blood transfusion.