The reliability of catheter-tip transducers for the measurement of intrauterine pressure in the third stage of labour

Authors


Correspondence: Dr S. Chua, Department of Obstetrics and Gynaecology, National University Hospital, Lower Kent Ridge Road, Singapore 119074.

Abstract

In order to assess the reliability of intrauterine pressure measurements in the third stage of labour, catheter-tip transducers were used in 20 women randomly allocated into two groups of 10. In each case in the first group two catheters were tied together and introduced transcervically into the uterine cavity after delivery of the placenta. In each case in the second group two catheters were inserted independently into the same uterine cavity. The active and cumulative active pressures recorded from the pairs of catheters within each uterine cavity were compared. Comparison of individual active pressure readings from separate transducers revealed good agreement whether the catheters were tied together or were seperate. Cumulative active pressure was very similar when assessed by each catheter in the same uterus. Intrauterine catheter-tip transducers can be used reliably to measure uterine activity in the third stage of labour although there may be minor contraction by contraction differences in recordings of individual active pressures.

The in vivo effect of many drugs used for prophylaxis and treatment of postpartum haemorrhage is usually assessed by using indirect parameters of blood loss like change in pulse rate, fall in blood presure, the need for additional drugs to stop excessive bleeding, or the need for blood transfusion, as well as laboratory investigations such as a fall in haemoglobin levels. Direct visual estimation of blood loss in the third stage is well known to be inaccurate, compared with the measurement of blood loss in the laboratory1. However, these laboratory methods are often cumbersome to use and time consuming.

Uterine atony is the most common cause of postpartum haemorrhage2, and many drugs have been formulated to treat this condition. In cases where the drug to be tested is meant to increase myometrial contractility, and hence prevent or treat postpartum haemorrhage, intrauterine pressure measurements in the postpartum period have been used in the in vivo model, to quantify myometrial activity.

In the first stage of labour intrauterine pressure measurements using catheter-tip transducers provide reliable information on cumulative pressure wherever the transducer was placed in the uterus3. These catheters have also been used in the third stage to assess the uterotonic effect of drugs4. The aim of this study was to assess the reliability of intrauterine pressure measurements in the third stage of labour using the Gaeltec® transducer (Gaeltec Ltd, Dunveyan, Isle of Skye, UK).

Methods

Twenty women admitted in labour to the National University Hospital, Singapore, were recruited for the study, which was approved by the departmental ethical committee. Informed consent was obtained from the women during the first stage of labour. The women were randomly assigned to two groups using a random number table. We decided to study 20 women on an empirical basis as it was considered that if major differences were not found with this number, then any differences which might be apparent with larger numbers would be sufficiently rare and would not be clinically important. There was no statistically significant difference in the mean age, gravidity and period of gestation between the women in the two groups. The catheters were calibrated before use according to instructions set out in the supplier's handbook (Oxford Medical, Abingdon, UK). The catheters were inserted transcervically within five minutes of delivery of the placenta. The accuracy of each pair of catheters was checked before insertion by simultaneous insertion to equal depths in sterile fluid; the reading from the second catheter was recorded when the first catheter read 0.0, 1.3, 2.7, 4.0, 5.3 and 6.7 kPa (0, 10, 20, 30, 40 and 50 mmHg). The calibration checks showed that the discrepancies between the catheters were: < 0.1 kPa (1 mmHg) 30%, 0.1 kPa (1 mmHg) 23%, 0.3 kPa (2 mmHg) 17%, 0.4 kPa (3 mmHg) 17%, 0.5 kPa (4 mmHg) 10%, and 0.8 kPa (6 mmHg) 3% (i.e. the difference was < 0.5 kPa (4 mmHg) on 97% of occasions).

The women in group 1 had two sterile Gaeltec catheter-tip pressure transducers inserted independently into the same uterus. One catheter (catheter A) was inserted such that the catheter tip was felt to impinge on the uterine fundus, and the other catheter (catheter B) was inserted similarly but withdrawn an arbitary distance outward and away from the tip of the first catheter. In group 2 the two catheters (A and B) were tied together with sterile catgut before insertion, so that the tips of both catheters were next to each other at all times. The two catheters were then inserted together transcervically into the uterine cavity, so that the tips were felt to impinge on the uterine fundus.

Each catheter in the pair was individually connected to a Sonicaid FM6 fetal heart rate monitor. The FM6 fetal monitor measures frequency, duration and amplitude of contractions. Pressures > 100 mmHg were not recorded on the double channel chart paper, but were shown on the digital display. The uterine activity in each successive 15 minute period was manually calculated as total active pressure. This was obtained by summation of the active pressure (pressure above the baseline) observed at 30 s intervals during this period. The catheters that were tied together were checked on removal and showed no displacement from each other.

In each woman, the mean difference in active pressure and cumulative active pressure (obtained by summation of all active pressures recorded in each woman) recorded by the two catheters was compared using the intraclass correlation coefficient, derived using ANOVA statistics. Limits of agreement were depicted on Bland and Altman plots.

Results

When two catheters were tied together and inserted into the uterine cavity, the intraclass correlation coefficient was 0.98 when active pressures recorded by each transducer were compared. The mean difference in active pressures recorded by individual transducers was 0.03 mmHg (95% CI -0.6 to 0.7). The limits of agreement were 13.2 and -13.2 mmHg (Fig. 1). When the cumulative active pressures in each of the 10 cases were compared, the intraclass correlation coefficient was 1.0 and the mean difference in cumulative pressure recorded by each of the two transducers tied together was 0.6 mmHg (95% CI -95.1 to 96.3); the limits of agreement were 268.0 and -266.8 mmHg. (Fig. 2). The plots in Fig. 1 and Fig. 2 show graphically that the difference in individual active pressure values as well as the cumulative active pressure recordings obtained by each transducers are not clinically significant.

Figure 1.

Difference against mean active pressure from two linked transducers. —= mean; —=±2SD.

Figure 2.

Difference against mean cumulative active pressure from two separate transducers. —= mean; —=±2SD.

When two catheters were inserted separately into the uterine cavity, the intraclass correlation coefficient was 0.89 when active pressure recorded by each transducer was compared. The mean difference in active pressures recorded by individual transducers was 0.2 mmHg (95% CI 0.5 to -0.8), and limits of agreement were 19.7 and -20.0 mmHg. Figure 3 shows graphically that while transducer B tends to give a reading which may be between 0.5 mmHg higher and 0.8 mmHg lower than transducer A, the limits of agreement are larger than if the two transducers were tied together. When cumulative active pressure in each of the 10 labours were compared, the intraclass correlation coefficient was 0.99, and the mean difference in cumulative pressure recorded by each of the two transducers inserted separately was 86.3 mmHg (95% CI -48.9 to 221.1); limits of agreement were 464.0 and -291-4 mmHg (Fig. 4).

Figure 3.

Difference against mean active pressure from two separate transducers. —= mean; —=±2SD.

Figure 4.

Difference against mean active pressure from two separate transducers. —= mean; —=±2SD.

Discussion

The postpartum uterus provides a good model for in vivo evaluation of the uterine effect of drugs used in labour and the puerperium, as the constraint of a fetus in utero is not present. Uterine atony is a major cause of postpartum haemorrhage2. When different drugs used to prevent or treat postpartum haemorrhage due to uterine atony are tested in a clinical trial, visual or laboratory estimation of blood loss, or other indirect estimates of blood loss (such as a fall in blood pressure, need for other oxytocics, blood transfusion, and change in haemoglobin level) are used. The more direct methods are either not accurate (as in visual estimation of blood loss)1 or cumbersome (as when laboratory methods are used to measure blood loss). Measurement of uterine activity or myometrial contractility would provide a direct and potentially simple method of evaluating the effects of different drugs on the uterus.

Intrauterine pressure measurements are known to reflect intramyometrial pressure deep within the myometrium5. Thus a transducer placed within the uterine cavity reflects changes in uterine contractility. Over the years a variety of methods have been used to characterise postpartum intrauterine pressure changes. These range from microbaloons to open-ended fluid-filled catheters5, and in more recent years, intrauterine pressure transducer4,6. The earlier systems were cumber-some and more difficult to use, and common problems included hydrostatic instability and damping in fluid filled catheters, as well as changing elasticity, wall contact and induced uterine activity associated with balloons. The electronic microtransducer catheters are simple to insert and may give more reliable readings.

The intraclass correlation coefficient was used to express reliability of the transducers, which amounts to determining that the transducers were measuring uterine activity in a reproducible and consistent fashion. Whether the transducers are tied close together in the uterine cavity near the fundus of the uterus, or separate within the uterine cavity, the intraclass correlation coefficient (R) derived when the active pressures recorded by two transducers were compared showed good correlation (R) = 0.98 when catheters were tied together and R= 0.89 when catheters were inserted separately). As with other studies5, we found that the magnitude of each contraction in the third stage to be much larger than that in the first stage of labour. Peak active pressures > 200 mmHg were not uncommon. The commercially available catheter-tip transducers were able to measure reliably the peaks, although pressures > 100 mmHg were not recorded on the recording paper. As tetanic contractions may occur in the third stage, we adapted a method of measuring uterine activity by summation of active pressure every 30 seconds over a given period of time. The online computer in the Sonicaid FM6 machine measures uterine activity online as area under the curve (uterine activity intergral) and in the third stage would have had difficulty finding the baseline during a long tetanic contraction.

Although not significant, there were more contraction by contraction differences when catheters were inserted separately (R= 0.89), as seen by the larger limits of agreement shown in Fig. 3 compared with Fig. 1. This could be explained by the fact that when the catheters are inserted separately, the second catheter was specifically pulled down towards the cervix away from the first catheter whose tip impinged on the uterine fundus. This could have resulted in some cases in the catheter tip being pulled near the cervix, and giving inaccurate readings because of the pressure leak from an open cervix. There was less difference in contraction to contraction difference when both catheters were tied together and inserted in the part of the uterine cavity near the fundus (R= 0.98). The greater contraction to contraction differences which occurred when the catheters were inserted separately did not result in larger bias or inconsistency when cumulative pressures obtained by two catheters inserted separately (R= 0.9) are compared with those obtained when two catheters are tied together (R= 1.0) (Figs 2 and 4).

The Gaeltec intrauterine catheter-tipped transducer is known to provide reliable information of the intramniotic pressure during the first stage of labour3. The results in this study show that they can be used to measure uterine activity in the third stage of labour. The uterine cavity has some blood between the apposed walls of the uterus in the third stage. Hence the transducer tip is likely to be in fluid medium recording the intrauterine pressures. To cut down on inaccuracies which may occur when the transducer is placed too close to the open cervix, the transducer should be inserted as far into the uterine cavity as possible, with the catheter tip impinging on the uterine fundus. When assessing the tonic effect of oxytocic drugs on the post-partum uterus, the woman may be used as her own control, and the calculation of the change in uterine activity after a drug is administered will give a good assessment of the uterotonic actin of the drug in question.

Acknowledgement

The authors would like to thank Dr F. Dong for help with the statistical analysis.

Ancillary