The SiPAP platform: driver, generator, gas module, monitoring, electronics and power

Name every module of the Infant Flow SiPAP driver, follow the gas from the wall outlets to the generator at the infant's nose, and know which configuration, board and part numbers apply to the unit in front of you.

What this machine is, and what it is not

The Infant Flow SiPAP is a neonatal nasal CPAP system with an additional bi-level function. It is not a ventilator in the sense of Level 1: it does not measure tidal volume, it has no expiratory valve and it does not control a breath. It mixes air and oxygen, delivers a controlled flow of that mixture to a fluidic generator at the infant's nose, monitors the pressure and the oxygen concentration that result, and, in the bi-level modes, adds a second, switched flow for a short time to raise the pressure. Level 2 lesson 6 described this class of drivers in general. Here we go to the specific driver, its modules, its boards and its parts.

The service manual we work from is the Infant Flow SiPAP Service Manual, document 675-120 Revision D, dated May 2005 and issued by VIASYS Healthcare for the driver referred to as the IFSD (Infant Flow SiPAP Driver), model M675. Many internal parts are labelled Mk3 in the parts list. The product line was later sold under the CareFusion and then the Vyaire names. Vyaire sold its ventilator business in 2024, and some resellers now list the driver under another maker's name, so ask your distributor who supports the product today. Whoever supports your unit today, check that the manual revision matches the driver's labels and firmware. Later units can differ in parts and screens.

The system has three physical parts on the bedside: the driver (the box on the stand), the patient circuit with the Infant Flow generator and the nasal prongs or mask, and, where fitted, the transducer interface with the abdominal respiratory sensor. Remember that the pressure is made at the generator, not in the driver. The driver controls flow; the generator converts flow into a stable pressure at the nose. That relationship is why so much of the fault-finding on this machine is about flow, leaks and the circuit.

Configurations and modes

Two configurations are described. The Plus offers nasal CPAP and time-triggered BiPhasic, each with or without breath rate monitoring from the abdominal sensor. The Comprehensive adds BiPhasic tr, in which the extra pressure pulses are triggered by the infant's own breaths and backup breaths are given during apnea. The manual notes that the Comprehensive was not sold in the United States, and US units use text labels such as LBR (low breath rate) where international units use symbols and the word apnea. Rate ranges also differ between US and non-US configurations, so read Appendix B of the manual for the unit you have.

Practically, before you start any job, confirm three things: the configuration (Plus or Comprehensive), the labelling (US or international) and whether a patient trigger board and transducer interface are installed. A complaint such as 'BiPhasic tr is missing' may simply describe a Plus unit. The configuration is stored in the unit and is cross-checked against hardware at start-up, which matters in lesson 4: a mismatch produces its own error code. Changing the feature set is done only by the manufacturer's procedure with a code from the manufacturer; it is not a service adjustment and this course does not cover it.

The gas module: from the inlet block to the patient outlet

Air and oxygen hoses connect to an interchangeable inlet block at the back, available in DISS, NIST and other national fittings. The air passes a water trap with its own filter element; the oxygen passes a cone filter held in the mounting block by a banjo screw. Both gases then reach a precision mechanical air-oxygen blender (a Bird blender in the parts list), with non-return valves at its inlets and an alarm and bypass module. If one supply fails, or the two inlet pressures differ by more than the limit in the manual, the bypass routes gas through a reed whistle to warn the staff and feeds the outlet from the higher-pressure gas. The whistle is purely pneumatic and works with the power off.

From the blender the mixed gas goes three ways. A small metered flow passes through the fuel cell filter and restrictor to the galvanic oxygen cell, which measures the delivered oxygen concentration. The main flow goes through the NCPAP Pres low flowmeter, which sets the CPAP flow and therefore the baseline pressure. A second path goes through the NCPAP Pres high flowmeter; when bi-level delivery is not active this flow is dumped to exhaust through a restrictor, so the operator can set it in advance, and when the timing calls for it a fast selector valve, the PA (pressure assist) solenoid valve, switches it to the patient. On some units an optional auxiliary outlet on the back supplies blended gas at a fixed flow.

Two protective devices sit on the patient side. A spring-loaded vent valve on the exhaust manifold, piloted by blended gas, is normally open so that large flows escape with little resistance; an electrically controlled dump valve is normally closed and opens on over-pressure while the vent valve closes, dropping the patient pressure towards zero within moments. A factory-set mechanical pop-off valve limits the maximum pressure the driver can deliver. Patient pressure is measured through a separate pressure line from the generator to a Luer port on the front, read by a pressure sensor on the valve/sensor PCB and regularly checked against atmosphere by a zero valve. Exhaust gas leaves the case through a manifold placed away from electrical parts, and the alarm whistle exhausts there too. The case itself is purged by a small case bleed filtered restrictor so that leaked oxygen cannot accumulate inside.

Electronics, firmware and the patient trigger

The electronics module consists of the power supply unit, the rechargeable battery, the main processor PCB, the LED PCB with the ambient light sensor, the valve/sensor PCB and the touch-screen LCD with its backlight inverter. The valve/sensor PCB is the key board for us: it carries the pressure, flow and oxygen measurement circuits and drives the zero, dump and PA valves. Each valve has a sense line, so the processor knows whether a valve is connected and whether it actually switched. The current to this board is limited by fuses to keep the energy low in an oxygen-enriched space, which is a design safety feature you must never defeat with a different fuse or a bridged link.

The firmware is the unit's embedded program in the main processor PCB, together with configuration and calibration data held in non-volatile memory. DIP switches on the main processor PCB set the configuration and give access to the diagnostics mode described in lesson 2. The patient trigger PCB plugs onto the main processor PCB, talks to it over a CAN bus and takes its power from it. It processes the signal from the abdominal sensor through the transducer interface and reports breaths and apnea. The manual warns that the abdominal sensor does not detect every form of apnea; independent monitoring stays mandatory.

Fault management follows a graded logic you will meet again. A software-detectable fault can lock the unit out of electronic operation while it remains usable as a purely pneumatic CPAP source; other faults restrict it to NCPAP modes or remove the triggered modes; a discharged battery prevents operation once mains is removed.

Power and battery

The driver is Class I equipment with type BF applied parts and an IPX1 rating. It accepts a wide range of mains voltages, draws little power and has a time-lag mains fuse in a twin fuse holder at the inlet; the exact values are on the rating plate and in the manual. Note that the power switch does not isolate the unit from mains: the green power light stays on whenever the cord is connected, and you must pull the cord before opening the case. If the protective earth of the outlet is in doubt, the manual tells users to run the unit on its battery.

The internal sealed lead-acid battery is specified for about two hours of operation from full charge and needs a long time to recharge fully; the manual gives a maximum of 16 hours. The switch-over to battery is automatic and gives an audible and visual alarm. When the battery is too low the unit shuts down in a controlled way; when it is very low the screen stays dark until external power is restored. Without any electrical power the driver still gives CPAP flow at the set oxygen concentration through the low flowmeter, with only the pneumatic gas failure whistle as a warning. Keep this in mind when you advise staff: pneumatic CPAP without monitoring is a fallback for transfers, not a normal way to run the machine.

On-site checklist

• Model, serial number, configuration (Plus or Comprehensive) and labelling (US or international) recorded
• Manual revision checked against the unit's labels and firmware version
• Inlet block fitting type matches the hospital's gas outlets
• Patient trigger board and transducer interface present or absent, as recorded
• Auxiliary outlet fitted or not, noted
• Mains cord removed before the case is opened; power switch is not an isolator

Check yourself