IUPCs and baseline uterine pressure
Ok folks – I gotta a braintwister for us all.
I was on call this morning and the nurse was letting me know that a patient with an IUPC had a baseline pressure of 40 mm Hg, and anything above 30 mm Hg was considered abnormal. I didn’t know what to say to that, because the whole idea of baseline pressure never made sense to me.
First, the IUPC is zeroed theroetically to the air, so the baseline pressure should be relative to air. But, we have no idea what intraabdominal pressure is! When we do urodynamics we need to subtract intraabdominal pressure from the bladder pressure to really understand how hard the detrusor is contracting. It seems to me that we would need the same information to say anything meaningful about the intrauterine pressure in an absolute sense. Otherwise, the baseline pressure could be influenced by things like patient position and position of the baby (making a different amount of direct pressure on the IUPC.) The whole concept seems sort of flawed to me.
Certainly if baseline uterine tone were really increasing, that might have some meaning. But are we really measuring what we think we are measuring?
So what did I tell the nurse? I said “don’t worry about it. the whole concept makes no sense.”
What would you have told her?
Academic OB/GYN 
We don’t use IUPCs, instead using external “toco” to pick up contraction on CTG (which we only use with selected higher-risk women). We know that it doesn’t actually measure the intensity of contractions (whatever the height of the peaks on the trace), but simply the presence, frequency and length of contractions. This is completely acceptable for the interpretation of the FH in relation to contractions, which is what is required. Intensity is a bit beside the point. For the purpose of titrating syntocinon, we are interested in the frequency/length of contractions and periods of resting tone- rather than intrauterine pressure. This routine disregard for intrauterine pressure in UK monitoring doesn’t appear to result in any mishaps (i.e. it has not been a issue flagged by the regular reports into perinatal mortality produced by CMACE and its forerunners).
What is the purpose of the more invasive form of monitoring (with some increase in risk of healthcare acquired infection?) given that the information it provides is not especially useful?
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Wow – a comment on a super old post. Thanks!
In America we do use IUPCs some, at least I do, but not for routine monitoring of patients on pitocin. I use them when a patient is on pitocin and has a protracted or arrested active phase, and seems to be contracting frequently. An IUPC in this situation indicates if the contractions are adequate, and thus whether or not more pitocin would be helpful. If a patient in this situation is adequate for 4 hours without cervical change, I would consider cesarean for an arrested active phase at that point.
Without IUPCs, how do you decide when to section a patient for arrest of dilatation?
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We wouldn’t be augmenting unless we had already diagnosed some kind of ‘slow progress’/’delay’ – which I suspect has slightly different parameters in UK than in the US. (We are viewing 4cms+ as established labour here, and then 2 cms/ 4hours as within ‘normal progress’ here (slightly different in parous women: 1cm in progress in that time, then return to normal labour pathway. If <1cm progress, then for oxytocin augmentation, increasing rate to achieve and maintain contractions 4-5:10 If the contractions are picked up by the external toco and/or are palpable (the old-fashioned way!) then they simply viewed as good enough. I have never placed or even seen an IUPC in our unit (I'm sure we have some knocking around somewhere for some special circumstances, but I wouldn't know where to lay hands on one!). If <2cm/4hours after then review for caesarean section. The obstetrician would take into account the strength of contrations (on palpation – any evidence less accurate than IUPC as a measure of 'adequacy'), frequency, how soon 4-5:10 had been achieved. If it had taken a long time to achieve 4-5:10 and/or contractions did not palpate as strong, maternal and fetal well-being confirmed, there was some progress (perhaps descent/rotation, not just dilatation) then he or she might want to try a higher rate – but if that increases length/frequency rather than strength, you are looking at having to turn it down to avoid hyperstimulation/FHR changes anyway.
The issue for me is not whether IUPC measures amplitude better than external toco (which I guess is easily measured in a lab setting). The 'proof of the pudding' would be whether, with external toco & palpating contractions, we are doing more caesarean section for 'failure to progress' using our pathways or otherwise having poorer outcomes overall than we would using IUPC. It's not something that has been properly tested (at least I'm not aware of literature on it), though I have just discovered there is a Cochrane protocol for a systematic review on the subject. Other variables (such as when you start your partogram and where you put your action line for intervention) may have a greater magnitude of effect than use of IUPC or external toco.
The other issue is whether IUPC increases risk of infection (I find it hard to believe it doesn't at population level) given that you are providing an excellent vehicle for the ascent of bacteria from the vagina into the uterus – especially given that oxytocin is not used with intact membranes in the UK.
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Oops, I managed to edit to make some of the above nonsense.
Different in parous women, in that <2cm/4hr OR slowing of progress = suspected delay.
We are expecting 1cm/2hr after amniotomy (indicated for suspected delay).
If <1cm/2h following amniotomy, then for oxytocin augmentation.
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I’m pretty sure that the amplitude of a external toco has little relationship to absolute contraction strength, as the toco is just a spring on an A to D converter. Lots of things will change how much that spring shortens, and thus how far the signal deflects. If its in the same place, you can see relative contraction strength, but you can’t tell actual pressure.
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I don’t doubt that external toco doesn’t show amplitude at all (in fact, that’s what all the research shows – that external toco bears no real relation to amplitude. In fact, that’s what I tell technology-obsessed partners when they say “oooh, that was a big one”.
As I said, the issue for me is *not* whether IUPC measures amplitude better than external toco, but rather – if you aim for contraction frequency of 4-5:10 and palpate contractions from time to time, do you actually need an objective measure of contraction strength, such as a precise mmHg of intrauterine pressure? Does having this information improve outcomes? I don’t have the answers, but I wouldn’t necessarily assume that more information = better outcomes. So, I will be interested in the Cochrane review on this subject.
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