In a patient with a Brio ventricular assist device, should the flow meter on the drainage cannula be placed upstream or downstream of the clamp?

Flow Meter Placement on Drainage Cannula

The flow meter should be placed upstream (before) the clamp on the drainage cannula to accurately measure actual flow through the system, as flow measurement devices must be positioned where they can detect the pressure differential or flow characteristics without interference from downstream occlusions.

Fundamental Principles of Flow Measurement

The positioning of flow meters relative to clamps is dictated by basic fluid dynamics and measurement physics:

• Flow meters measure flow by detecting pressure differentials, velocity, or volume displacement that occurs when fluid passes through the sensor 1
• Placing a clamp downstream of the flow meter allows the sensor to detect the actual flow characteristics before any obstruction 1
• If the flow meter were placed after (downstream of) the clamp, it would measure zero or severely restricted flow when the clamp is engaged, rendering the measurement meaningless 2

Technical Rationale for Upstream Placement

Pressure-Flow Dynamics in Drainage Systems

• Drainage cannulae in ventricular assist devices and ECMO systems rely on accurate flow measurement to ensure adequate venous return and prevent complications 3, 4
• The proximal portions of drainage cannulae are responsible for the majority of flow extraction (>1.6 L/min in typical configurations) 4
• Vacuum-assisted drainage systems require precise flow monitoring, with measurements taken at specific negative pressures (-20 to -80 mmHg) to optimize venous return 2

Flow Meter Sensor Requirements

• Flow transducers must be positioned where they can sense bidirectional flow without obstruction 1
• The sensor must have low resistance to flow and minimal dead space to avoid interfering with the measurement 1
• Accurate calibration is essential and must be performed with the flow meter in its operational position before each use 1

Clinical Application to Brio VAD Systems

Optimal Configuration

• Position the flow meter on the drainage cannula between the patient and the clamp (upstream position) 5, 2
• This allows continuous monitoring of actual blood flow from the patient, even when the clamp is partially or fully engaged 5
• The flow meter can then provide real-time feedback for zero compensation and flow rate adjustments 5

Monitoring Considerations

• Flow measurements should account for the pressure-flow characteristics specific to blood (hematocrit ~27%), which differ significantly from water-based calibrations provided by manufacturers 3
• Drainage pressure is consistently 10-350% higher with blood compared to water at equivalent flow rates 3
• Zero compensation should be performed during initial driving conditions to establish accurate baseline measurements 5

Critical Pitfalls to Avoid

• Never place the flow meter downstream of the clamp, as this will only measure post-clamp flow and cannot detect actual patient drainage 2
• Do not rely solely on manufacturer pressure-flow charts, as these are typically generated using water and do not reflect clinical performance with blood 3
• Ensure the flow meter is calibrated at the correct reference level (mid-thoracic line in supine position) to avoid measurement errors 6
• Avoid excessive vacuum pressure (>80 mmHg) during vacuum-assisted drainage, as this can cause hemolysis and measurement artifacts 2

Practical Implementation Algorithm

1. Position the drainage cannula with TEE guidance to ensure optimal placement in the SVC 1
2. Install the flow meter on the drainage line between the cannula connection and any clamps 5
3. Calibrate the flow meter to zero with the system primed but before initiating flow 5
4. Place the clamp downstream of the flow meter for emergency flow control 2
5. Monitor flow continuously, with measurements taken during stable conditions to avoid artifacts from cardiac output variations 1