Insufficient vascular access and heart drainage during cardiopulmonary bypass (CPB) may disturb the physiological flow of blood, giving rise to an intermittent caval collapse and consequently interruption of venous drainage, resulting in non-physiological pressure variations and shear stresses.1–4 The amount of venous blood drained from the patient determines the flow rate that can be achieved during cardiopulmonary bypass (CPB), and end organ perfusion. Venous drainage defines also the amount of blood that stays in the patient’s cardiovascular system during CPB.5 Major efforts have been made to develop minimal invasive heart surgery.6–8 Adequate venous drainage has the potential to simplify the surgical procedure for the patient and the surgeon; this is of prime interest in view of the minimal invasive surgery aspect.5–11
There are numerous factors that can influence the quality of venous drainage during CPB including venous cannula design and venous cannula positioning.4 Improved flow of blood drainage can be achieved if better cannula designs are used as we have previously demonstrated for the Smartcanula (Smartcanula LLC, Lausanne, Switzerland), which is based on the “collapsed insertion and expansion in situ” principle.2, 12, 13 With its self-expanding design, this device acts also as a spacer preventing the vein from collapsing, and therefore allows all collateral blood to be drained directly towards the pump oxygenator. A new, simplified, plastic self-expanding Smartcanula is designed for central insertion (trans-atrial insertion into the inferior vena cava), and prevention of caval collapse (Figure 1).
Evaluation of the hydrodynamic behavior of the venous cannulas is a valuable technique for the analysis of their performance capability during cardiopulmonary bypass (CPB).14–17 We developed a new fully automatic method and reported on the determination of the flow rates (Q) of water through the cannula itself and the corresponding pressure drop (ΔP) over different height differentials using cannulas of different lumen diameter and length.18–20 Simulation of caval collapse over the entire caval axis, right atrial, hepatic, renal vein, and iliac vein is realized in reversed capillary drainage tubes (TAUT, Research Triangle Park, North Carolina) with opposite holes at 5 cm distance intervals. Various lengths of the plastic self-expanding Smartcanula for central insertion are compared to the commercially available two-stage control cannula. Thus, the main goal of this study is to assess the influence of the new design with mesh structure on atrial chatter and to validate the performance of the new self-expanding Smartcanula for central insertion.
Material and Methods
The water pressure drop values (ΔPH2O) versus different water flow rates (QH2O) over six different height differentials for Smartcanulas of different lengths and the commercially available two-stage reference cannula were measured using the in vitro circuit18–20 with the following modifications: The in vitro circuit comprised an upper hard shell reservoir (Res # 1) that was 65 cm long, 47 cm wide, and 30 cm high (KAISER+KRAFT/St-Sulpice, Switzerland), connected to a lower hard shell reservoir of the same size (Res # 2) by a 2 m long silicone test tubing (Figure 2). The reservoirs are needed to assess the drainage load in this open system.
Simulation of caval collapse over the entire caval axis, right atrial (= position 1), hepatic (= 2), renal vein (= 3), and iliac vein (= 4) is realized in reversed (inside out) 42-cm capillary drainage tubes (TAUT) with 2, 4, 6, or 8 opposite holes of 1-cm diameter at distance intervals of 5 cm (Figure 3). The test cannula is first inserted into the capillary drainage tubes at the particular position and then the connection site of the cannula is connected to the upper end of the test tubing within the Res # 1. The 10-cm length of the two-stage Medtronic reference cannula (Medtronic World, Minneapolis, Minnesota), is eliminated from the connection side to avoid tilting of the drainage capillary tubes.
Various lengths (26 cm [= right atrium], 34 cm [= hepatic], 43 cm [= renal), and 53 cm [= iliac]) of the plastic self-expanding Smartcanula for central insertion were compared with the two-stage Medtronic reference cannula (= right atrial + hepatic).
Precision and Accuracy
Imprecision was defined by the coefficient of variation (CV). Within-assay precision was determined by six repeated measurements within the same assay.
Results were presented as means ± standard deviation, unless stated otherwise. The unpaired Student t test was used to compare two cannulas. The standard two-way ANOVA test was used to compare more than two cannulas. The significance level was p < 0.05.
Effects of the New Smartcanula Design on Inhibition of Caval Collapse Model
The effects of the new self-expanding Smartcanula design on inhibition of caval collapse model was characterized by the relationship between ΔP and Q and by the cannula lumen resistance, defined as the slope of the linear relationship of ΔP versus Q2 (ΔP/Q2 ratio). The six points of each curve were determined at six height differentials: 15, 24.8, 37.1, 53.2, 66.3, and 88.9 cm. Expanded Smartcanulas of four different lengths (26, 34, 43, and 53 cm) were each first inserted into the capillary drainage tube at the particular position (Figure 3), and then the hydrodynamic parameters were determined. The results clearly demonstrated higher Q values (Figure 4, upper panel) and lower ΔP/Q2 ratio (Figure 4, lower panel) for fully expanded 26-cm Smartcanulas as compared with Medtronic two-stage cannula when both cannulas were first inserted in the drainage tube at the position =1, right atrium. Smartcanulas of 34 cm (Figure 5), 43 cm (Figure 6), and 53 cm (Figure 7) lengths were inserted in the drainage tube at positions = 2 (hepatic), = 3 (renal), and = 4 (iliac) respectively, and compared with the two-stage Medtronic cannula inserted at position 2 (= hepatic).
The fully expanded 26-cm and 34-cm Smartcanulas showed 51% and 33% more flow rate (Table 1), and 22% and 15% less lumen resistance (ΔP/Q2 ratio) respectively as compared with the two-stage Medtronic cannula. The fully expanded Smartcanulas at 43 cm and 53 cm lengths showed 16% and 14% more flow rate and 33% and 25% less lumen resistance respectively as compared with the two-stage Medtronic cannula (Table 2).
Traditional two-stage and multistage central venous cannulas are known for arterial chatter due to caval collapse, as shown in Figure 8, upper panel. Thus, the shape and size of the cannula is believed to be the major factor for impeding venous drainage during CPB. The new plastic self-expanding Smartcanula, with its mesh structure, was designed for central insertion and prevention of caval collapse, as reported previously21 (Figure 8, middle panel). In addition, removal of Smartcanula is easy. Simple traction between two fingers reduces the cross-sectional area of the Smartcanula to a fraction of its expanded diameter. Hence, the collapsed Smartcanula can be easily pulled back through the access vessel in combination with the control of blood exit from the vascular system. Von Segesser et al.22 previously demonstrated that a small access (30 F) aperture to the right atrium is enough for smart cannulation of the inferior vena cava and allows for an adequate venous drainage with gravity alone. The cannula access orifice size is 30 F; however, the cannula body within the target vessel expands to 45 F in functional model. In a small person, a 30 F access is preferable to a 50 F access orifice. Furthermore, the self-expanding portion of the study cannula will open up to the target vessel diameter. To understand the potential significance of the new self-expanding design of the Smartcanula for central insertion, we conducted an in vitro study to compare the hydrodynamic characteristics of the new Smartcanula and the traditional two-stage Medtronic venous cannulas, and their effect on the collapse of the drainage tubing. The latter was reversed inside out to bring the outer smooth surface inside the tubing to mimic the smooth internal side of vena cava. Normalized intervenous drainage distances are based on standardized CT-scans in routine adult patients (personal communication, L. K. von Segesser). The opposite holes of 1 cm diameter were made at 5 cm distance intervals in our model to provide a linear model of drainage. ΔP, Q and ΔP/Q2 ratio for the 43-cm and 53-cm Smartcanulas were compared with those for the two-stage Medtronic reference cannula inserted at position 2 (= hepatic level), because this is the maximum insertion distance for this cannula in clinical routine surgery for adult patients. In this study, the 10-cm version is removed from the connection side of the two-stage Medtronic reference cannula to avoid tilting of the drainage capillary tubes in the upper bath of the experimental setup. We recently published that a minor modification in the design of the extracorporeal part of cannulas can generate significant modifications in the liquid flow and in the cannula lumen resistance.20 Therefore, ΔP, Q values and ΔP/Q2 ratios were determined and repeated six times for the cued two-stage Medtronic cannula and an intact one. The means were compared and the values were corrected for each point at each height differential for the cued cannula with regard to those of the intact one. The results demonstrated that longer self-expanding cannulas provided better flow rate and less cannula lumen resistance than shorter ones in spite of the insertion distance. This is somewhat against traditional understanding, which implies that longer cannulas have higher resistance and therefore lesser drainage capacity. The explanation for these results obtained here is that longer traditional cannulas are, in fact, long narrow tubes, and therefore the resistance increases in linear fashion with cannula length. In contrast, the longer self-expanding venous cannulas did not act as less flow limiting as compared with the shorter one because of their mesh structure, the so-called open wall design.
We conclude that self-expanding venous cannulas for central insertion preventing caval collapse outperform the commercially available two-stage reference cannula. They provide unmatched venous drainage and less cannula lumen resistance, and then less arterial chattering. In addition, self-expanding cannulas did not need a guide wire to insure the proper positioning.
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