M2Vascular access, haemodynamic monitoring and point-of-care ultrasound

Clinical overview

Access and monitoring are useful only when they accelerate a decision. A large peripheral cannula that delivers blood rapidly is more valuable during obstetric haemorrhage than a technically impressive multi-lumen central line with poor flow. An arterial waveform is valuable when it reveals beat-to-beat instability or permits repeated gases; it is not a treatment. Point-of-care ultrasound (POCUS) can move the team from “shock” to a working haemodynamic phenotype, but a poorly acquired image can also create false certainty.

The obstetric clinician’s role is to match the device to the immediate need, understand what the signal actually measures, recognise artefact and complications, and escalate to a trained operator where the procedure exceeds competence. Pregnancy changes anatomy, haemodynamics and thrombotic risk. Haemorrhage compresses veins, obesity impairs landmarks, pre-eclampsia increases pulmonary-oedema risk, and a gravid uterus makes lower-body venous return position-dependent. These factors favour early planning, ultrasound guidance and repeated interpretation in clinical context.

Core knowledge

Access is a flow problem

For laminar flow through a catheter, flow rises steeply with internal radius and falls with length and viscosity. The practical consequence is memorable: short and wide beats long and narrow for rapid resuscitation. Two functioning 14–16 G peripheral cannulae, if obtainable, usually deliver blood and warmed fluid faster than the individual lumens of a standard central venous catheter. Pressure-assisted infusion, rapid infusers and viscous blood products change actual performance, so inspect the device and test flow rather than trusting the label.

Use access according to purpose:

Do not delay life-saving treatment while pursuing the ideal line. In maternal cardiac arrest, upper-extremity IV or IO access is reasonable because aortocaval compression may impair lower-body drug delivery. During severe haemorrhage, begin transfusion through working peripheral access while advanced access is prepared.

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Figure M2.1 — Short, wide vascular access exploits the fourth-power effect of radius on flow; in major haemorrhage, use two large-bore upper-limb peripheral IVs when possible and escalate without delaying transfusion.

Central venous access

Central lines permit concentrated vasoactive medicines, multiple infusions, parenteral nutrition, difficult access and selected haemodynamic measurements. They introduce risks: arterial puncture, haematoma, pneumothorax, haemothorax, air embolism, malposition, arrhythmia, infection and venous thrombosis. Pregnancy and the puerperium increase thrombotic risk; severe thrombocytopenia or coagulopathy changes site selection and procedural planning.

Ultrasound guidance should be standard where equipment and competence exist. The operator confirms vein compressibility, course and relationship to the artery; maintains visualisation of the needle tip; confirms venous wire position before dilation where feasible; uses full barrier precautions and performs post-procedure confirmation appropriate to the site and technique. Never dilate unless the wire’s intravascular location is secure. A line that flushes is not proof of safe placement.

Site choice is contextual. Internal jugular access is ultrasound-friendly but may be difficult with airway work, obesity or inability to tolerate head-down position. Subclavian/axillary access may reduce infection burden in some ICU contexts but carries pneumothorax and non-compressible bleeding risk. Femoral access is rapid and avoids pneumothorax, but pregnancy anatomy, infection and thrombosis matter; it is not automatically forbidden when it is the safest emergency option. Remove emergency lines or exchange strategy only when the patient is stable and a safer plan is ready.