Adult Circulatory Support
Vascular access cannulas are described by lengths and outside circumference (French size). However, wall thickness, side holes, and tapering diameter affects P and Q characteristics, and therefore cannula performance cannot be adequately defined from length and diameter measurements alone. We have previously described the M number, a single dimensionless number that is based on a Reynolds factor correlation and that accurately characterizes the pressure flow characteristics of vascular access cannulas.1–3 In addition, the original M number is arbitrary and requires a nomogram to ascertain the diameter and length of the tubing, which impeded clinical utility. Since the original publication of the M number, many new cannulas have become available with significant design changes (side holes, flexible septum for double lumen cannulas (DLCs), etc). Therefore we tested a variety of new cannulas and described the pressure flow characteristics in a single relevant number; the updated M number, termed the UM number. The UM number is designated as the flow (Q) achieved at a 100 mm Hg pressure gradient. It can be used to describe the full pressure-flow characteristics for a given cannula.
Figure 1 shows a circuit that was created using a novel cannula-testing circuit: centrifugal pump (Terumo, Somerset, NJ), Tygon tubing, and a reservoir. Similar to previous studies, a solution of glycerin and water was used as a blood analog.4–7 The solution (approximately 33% glycerin by volume) had a measured viscosity of 3 cP at room temperature and was used to mimic blood at physiological temperatures with a hematocrit of approximately 40%. Arterial (n = 17), venous (n = 43), a standard dual lumen (n = 7), and a flexible septum DLC (n = 7) ranging from 6 to 50 Fr were studied. Cannulas were submerged horizontally in the reservoir with their distal tips left open and inlets connected to the circuit outflow (pump inflow drained the reservoir) and evaluated by controlling flow incrementally over a cannula’s performance range while measuring the pressure gradient across the cannula with a BIOPAC MP150 data acquisition system (Biopac Systems, Inc., Goleta, CA) and ultrasonic ⅜″ tubing flow probe (Tansonic, Ithaca, NY). After flow stabilized at each increment for at least 10 seconds, measurements were taken at a sampling frequency of 250 Hz for 10 seconds and then averaged into a single data point. The pressure over a full range of flow was measured. Flow was directed progradely for arterial infusion cannulas and retrogradely for venous drainage cannulas. Flow in standard dual lumen cannulas (DLCs) was measured for each lumen independently. For flexible septum DLCs, pump outflow was connected to the arterial lumen and inflow to the venous lumen; pressure was measured in each lumen simultaneously and independently for comparison. Using regression analysis, full-pressure flow curves were calculated and plotted for each cannula over a full range of potential pressures and flows encountered during clinical usage.
The calculated pressure flow curves for adult (>16 Fr) and pediatric (≤16 Fr) cannulas are shown in Figures 2 and 3 respectively. Positive pressure drops denote arterial cannulas, whereas negative pressure drops denote venous cannulas. Figure 4 shows the pressure versus flow curves for a variety of DLCs with rigid septa; arterial lumens are shown with positive pressure gradients, whereas the venous lumens are displayed with negative pressure gradients. Figure 5 displays the results for flexible septum DLCs; solid lines denote measurements of these lumens independently and dashed lines represent measurements of the arterial and venous lumens taken simultaneously. The flow achieved at 100 mm/Hg pressure gradient was identified for each cannula, and this flow is designated the UM number (this is technically the opposite of resistance; the flow coefficient). The UM number for specific cannulas, along with other cannula properties, is listed in Tables 1–3. Resistance in the conduit tubing between the cannula and the pump is always much less than the cannula itself.
Cannula selection is the first and one of the most important steps for successful extracorporeal support. Clinical selection of cannulas is a decision made at the time of cannulation, generally to select the largest cannula that will fit in the vessel and deliver the desired flow for an individual patient; the blood flow required for partial or total support is known (adults 60 cc/kg/min, children 80 cc/kg/min, infants 100 cc/kg/min). When planning for extracorporeal circulation, access cannulas are chosen to achieve the desired flow at acceptable pressure gradients (typically less than 100 mm Hg for venous drainage cannulas and less than 300 mm Hg for arterial infusion cannulas). This is very important for extrathoracic cannulation for extracorporeal life support (ECLS, extracorporeal membrane oxygenation [ECMO] ) where the size of the access vessels limits the size of the access cannulas. It is less important in cardiopulmonary bypass for cardiac surgery because direct access to the heart and great vessels is possible.
The original M number was described as an arbitrary number to select cannulas for extracorporeal support and to assess function during support. However, the clinical use of the original M number was not practical. This report describes an easier and clinically useful metric to assess cannula performance: Q at 100 mm Hg P, or the UM number. The full pressure flow characteristics of a vascular access cannula are described by a single curve. The relationships are not linear, and the resistance (pressure per flow) increases as the flow increases. The resistance of any cannula is specific, and not described by the length or outside diameter.8 In clinical practice, the factors determining the selection of cannulas are the size of the cannula related to the size of access vessels, and the amount of flow that can be expected with acceptable drainage or infusion pressures. During extracorporeal circulation, if the drainage flow is less than expected for a specific cannula, the cause may be hypovolemia, distortion of the drainage vein or atrium, or cannula occlusion. If the inflow pressure is higher than expected, the cause may be malpositioning of the cannula or partial occlusion. Knowing the characteristics of an individual cannula makes it possible to diagnose problems of inadequate flow or excess pressure.
The cannulas described in this report include some discontinued models (which are still in inventory at some centers). All available access cannulas are not included, and we invite manufacturers to report the UM number for all cannulas in their product list.
The original M number was an arbitrary scale ranging from 1 to 10 to describe the pressure flow curves. In this report, we describe the curves as the flow at a 100 mm Hg gradient. We chose this point on the curve because it represents the flow under typical conditions. Knowing the UM number, the full range of pressure flow can be easily determined from the graphs in Figures 1–5.
DLCs are used for simultaneous drainage and infusion into the right atrium and vena cava for gas exchange in venovenous ECMO. In models with flexible septa, the resistance of each lumen may be different when tested independently or simultaneously, as shown in Figure 5. To mimic clinical situations, we used the simultaneous measurements when calculating the UM number for DLCs with flexible septa. With double lumen catheters, venous drainage is the limiting factor and therefore the UM number is based on venous drainage and the corresponding infusion pressure is listed.
The pressure flow characteristics of a variety of vascular cannulas were tested under standardized conditions. Each cannula can be described by a single number, the UM number, which can be used to select and evaluate cannulas for vascular access.
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Keywords:Copyright © 2013 by the American Society for Artificial Internal Organs
cannula selection; extracorporeal support (ECS); pressure flow; UM number