Overview

The Philips M1355A is the external tocodynamometer (Toco) transducer designed for use with the Philips Series 50 family of fetal monitors (cardiotocographs), including the Series 50 A (M1351A) Single Ultrasound model, the Series 50 A Dual Ultrasound Twins model, and the Series 50 IP-2 (M1353A) intrapartum model. It is also used as a service replacement on earlier Philips/HP M1350 series monitors. The transducer is strapped to the maternal abdomen — typically over the fundus uteri, the area where contraction-induced tension changes are most pronounced — and uses a strain-gauge pressure sensor with a movable base plate to convert mechanical force into an electrical signal. The host monitor processes that signal into a continuous Toco trace on the integrated thermal recorder, providing the “uterine activity” channel of a complete external cardiotocograph (CTG) study. The M1355A is intended for both antepartum and intrapartum external monitoring and is paired with the M1356A ultrasound transducer for FHR.

Connector, Cable, and Belt Attachment

The M1355A terminates in a keyed round multi-pin connector that mates with the dedicated Toco socket on the front face of the Series 50 monitor. This Toco socket is physically and electrically distinct from the US, US1, and US2/ECG sockets used by the M1356A ultrasound transducer, the M1357A DECG legplate, and the M1359A MECG transducer, so the monitor automatically recognizes the connected transducer type and selects the correct operating mode without operator action. On the M1353A Series 50 IP-2, the Toco socket can additionally accept the M1350A / 8040A compatible internal Toco transducer when intrauterine pressure (IUP) monitoring is being substituted for external Toco. The transducer body includes a snap-shut clip that secures it to the M1562A reusable abdominal belt and is designed to slide along the belt for repositioning during monitoring; an optional transducer knob adapter (M1356-43201) is available where the buttoned belt-attachment style is preferred.

Strain-Gauge Operation and Baseline Behavior

External tocodynamometry detects uterine contractions indirectly. As the uterus tightens during a contraction, the abdominal wall over the fundus stiffens and pushes outward against the Toco transducer, deflecting its movable base plate. A strain gauge inside the transducer housing converts that deflection into an electrical signal proportional to the relative contraction force. The Series 50 monitor displays the result in unitless relative pressure on the Toco channel and on the right-hand scale of the recorder paper. Because the signal is relative rather than calibrated absolute pressure, the Toco baseline key on the monitor is used to zero the trace to 20 units between contractions (or to 0 units when an internal IUP transducer is in use on the IP-2). Belt tightness, transducer placement over the fundus, and patient body habitus all affect signal amplitude, so brief repositioning of the transducer or re-tensioning of the belt is normal during a study. Unlike the M1356A ultrasound transducer, the M1355A does not use ultrasound coupling gel and is not affected by gel availability or contact medium quality.

Compatibility with the Series 50 Monitor Family

The M1355A is the standard external Toco transducer specified for the Series 50 A and Series 50 IP-2 monitors in the Philips Instructions for Use and Service and Installation Guide. The Series 50 A Single Ultrasound model, the Series 50 A Dual Ultrasound Twins model, and the Series 50 IP-2 (M1353A) all ship with one M1355A as a standard accessory. Philips supplied the transducer in two housing color variants — Brown (M1355-60001) and Blue (M1355-60011) — to color-match the monitor cabinet; the two variants are functionally identical and are interchangeable on any Series 50 Toco socket regardless of monitor housing color. The M1355A is also commonly used as a service replacement on the earlier Hewlett-Packard / Philips M1350 series fetal monitor lineage from which the Series 50 platform evolved. When monitoring uterine activity together with FHR on a Series 50 monitor, the M1355A and M1356A are typically applied simultaneously using two M1562A belts.

Use with Belts and the Series 50 Workflow

The M1355A is held in position with the M1562A reusable abdominal belt, fastened around the mother with the fixing button positioned to one side and the loose ends arranged so they remain at the patient’s side rather than under her body. Two belts are typically used when monitoring uterine activity and FHR simultaneously, and three belts are supplied with twin and IP-2 configurations. After the belt is in place, the operator palpates for the fundus, positions the M1355A over that area, snaps it onto the belt, tensions the belt to a comfortable but firm fit, and presses the Toco baseline key on the monitor to zero the trace between contractions. The recorder then prints the Toco channel on the right-hand side of the trace paper, alongside the FHR channel from the M1356A ultrasound transducer on the left, producing the standard external CTG layout. Belts are reusable and should be cleaned per the IFU between patients; the transducer body itself should be wiped clean and surface-disinfected per the Care and Cleaning chapter of the IFU and never immersed in liquid while connected to the monitor.

Service, Testing, and Preventive Maintenance

The Philips Series 50 Service and Installation Guide (M1353-9000J) defines a structured Toco parameter test that biomedical staff use to validate M1355A function in the field. The Series 50 monitor includes a built-in Self Test, Quick Test, and Parameter Test, and the dedicated Toco Parameter Test exercises the front-end Toco circuitry together with an attached M1355A to confirm correct strain-gauge response and baseline behavior. Common bench failures on aged Toco transducers include drifting baseline that cannot be zeroed (typically pointing to strain-gauge degradation or moisture ingress at the base plate), intermittent signal as the patient moves (often cable flex damage near the strain relief or at the connector backshell), and connector pin damage from repeated insertion and removal. The Service Guide identifies the transducer as a field-replaceable unit; component-level repair of the strain-gauge assembly is not recommended by Philips, and the standard remedy for a failed transducer is replacement of the assembly. Routine preventive maintenance is described in the Care and Cleaning chapter of the IFU and the Regular Maintenance section of the Service Guide.

Service and Maintenance Considerations

The M1355A is a passive strain-gauge transducer rather than a networked device and has no software, cybersecurity, or EMR integration of its own; data transmission, alerts, trend storage, and integration with obstetrical surveillance systems such as Philips OB TraceVue are handled by the host Series 50 monitor and its optional Combined Interface Module or Modem Interface Module. From a clinical engineering perspective the transducer should be inspected at each preventive maintenance interval for housing cracks, wear of the patient-contact base plate, cable jacket integrity, strain-relief integrity at both the transducer body and the monitor connector, and connector pin condition. Because the M1355A does not use coupling gel and has no ultrasound emitting element, its failure modes are different from the M1356A and tend to manifest as baseline drift, sluggish response to contractions, or amplitude loss rather than signal-quality faults. IPR Medical’s biomed team can support facilities that need bench testing, evaluation, or replacement of M1355A transducers as part of fleet maintenance on Philips Series 50 monitors.

FDA Device Corrections and Safety Bulletins

A Class 2 medical device recall record exists in the FDA Medical Device Recalls Database for the External Toco Transducer (M1355A) under Recall Event ID 175941. Class 2 indicates a situation in which use of, or exposure to, the device may cause temporary or medically reversible adverse health consequences, and the probability of serious adverse health consequences is remote. IPR Medical recommends that clinical engineering teams review the FDA’s official record for this recall directly to confirm the affected lot or date ranges, the recalling firm’s reference and reason, the specific corrective action requested, and the current status of the recall, because these details may be updated by FDA after the date of this listing. Facilities operating Philips Series 50 fetal monitors should also review the broader Philips fetal-monitoring transducer safety communications referenced on the Philips customer support portal, including any field correction or service notices that affect transducers refurbished beyond their useful life.

Because FDA device corrections and safety communications may be updated or expanded after the date of this listing, IPR Medical recommends that facilities also check the FDA’s official resources directly to ensure they have the most current information.

FDA resources for further review:
FDA Medical Device Recalls Database  |
FDA Medical Device Safety

IPR Medical — Buy, Sell, Repair, and Parts

IPR Medical buys, sells, repairs, and stocks parts and accessories for the Philips Series 50 fetal monitor family, including the M1355A external Toco transducer in both Brown and Blue housing variants. Whether your facility needs a single replacement Toco transducer to bring a Series 50 A or Series 50 IP-2 back into service, a bench-tested unit for a Labor and Delivery floating cart, or evaluation and repair of an existing M1355A that is producing baseline drift or amplitude loss, IPR Medical’s biomed team can support the request. We also buy back surplus Series 50 monitors and accessories from facilities decommissioning older fetal monitor fleets, and can supply matching M1356A ultrasound transducers, M1357A DECG legplates, M1359A MECG transducers, M1364A patient modules, and thermal recorder paper to keep existing units in service. Reach out for color confirmation, lot/serial verification against current FDA recall scope, and service or software questions on the host monitor.

IPR Medical buys, sells, repairs, and stocks parts for the Philips M1355A external Toco transducer. Contact IPR Medical at 855-477-8866 or cs@iprmed.com with service or software questions.