Article ID: CJ-24-0083
Background: The Micra leadless pacemaker was developed to fit inside the right ventricle, thereby reducing overall complications by 48% compared with a historical control group. The current labeling restricts implants to the femoral approach. In this article we used 3-dimensional computer models of human hearts to demonstrate why implants can be difficult in small patients and how using the jugular approach reduces these difficulties.
Methods and Results: Cardiac computed tomography scans were made of 45 pacemaker patients, 26 in the US and 19 from a single center in Japan. Dimensional measurements were taken in all 45 hearts, and these dimensions were compared between patient cohorts and between the Micra delivery tool dimension and patient heart dimensions. Hearts were smaller among patients in the Japanese than US cohort. In addition, the tool dimension exceeded heart dimensions in a larger percentage of hearts from Japanese patients. Three dimensions were identified that most likely limit navigating across the tricuspid valve to the right ventricle in smaller hearts and for which the jugular approach improved navigation.
Conclusions: Although the femoral procedure today maintains an excellent safety profile and procedure experience for most global implants, this study provides the rationale as to why the jugular approach may improve the ease of the Micra implant in small hearts, namely by reducing the tortuosity of the navigation across the tricuspid valve.
Medtronic’s Micra leadless pacemaker was developed to fit inside the right ventricle (RV) and requires a 23-Fr introducer implanted via the femoral vein. By placing the Micra in the heart, the complications related to the pocket and the lead are eliminated, thereby reducing the overall complication rate by 48% compared with a historical control group.1
To minimize barriers to adoption with such a large-bore introducer and a novel implant procedure, the developed implantation route was focused on the familiar femoral vein access. With such a large-bore catheter, proper venous access is a key procedural step to avoid access site complications. The familiarity and comfort of femoral venous access for the electrophysiologist were thought to be important to minimize complications. Since the launch of Micra, the use of ultrasound to guide venous access for various procedures has become standard, and implanters have become more accustomed to implanting large-bore sheaths.
Since the market release of Micra over 8 years ago, the unique benefits of implanting Micra via the jugular vein have been articulated in numerous published cases.2–10 In one single-center series, 82 patients were implanted with Micra via the jugular vein with a 2.4% complication rate.10 The benefits of implanting Micra via the jugular vein, include avoiding known femoral vein access issues (e.g., inferior vena cava [IVC] filter, artificial hip implants, deep vein thrombosis), faster ambulation time, and easier navigation across the tricuspid valve (TV) towards the septum.7–10 The jugular approach is often the only possible access approach in the pediatric population.2–6 Although the current femoral procedure safety profile is excellent, the jugular implant approach has the potential to reduce specific procedure-related risks even further in certain patients.
One example of a femoral Micra procedure-related risk is the fact that the average procedure time in Japan was 39.3 min, compared with 34.5 min for the rest of the world, in the investigational device exemption study of Micra.11 It is likely that the increased implantation time was due to navigation challenges through a more tortuous path from the femoral route in smaller anatomies. This increased procedure time also likely translates to a higher perforation risk due to longer interaction time between the cup and the heart tissue.
For these various reasons, implanting the Micra via the jugular vein in some patients may be easier and safer, or make the implant of Micra possible. In this paper, we present the anatomical measurements of a group of 45 hearts, comprising 19 relatively smaller hearts from a Japanese cohort and 26 hearts from a US cohort. The purpose of this study was to determine what is different in the smaller hearts that may cause more difficult navigation across the TV.
A literature search was conducted of the PubMed (National Library of Medicine, Bethesda, MD, USA) database to identify studies that reported any dimensions of the jugular vein. These studies used various measurement techniques, including computed tomography (CT), magnetic resonance imaging, and ultrasound.
We also searched the literature for articles related to assessing the maximum diameter or area of the internal jugular vein using the Valsalva maneuver to dilate the internal jugular vein. The goal was to provide a non-invasive method for ensuring the vein was of adequate size to accommodate the introducer.
Selection of HeartsThe heart scans used in this study came from 2 sources. The first group of 26 patients (12 female) were part of a US Medtronic study of His bundle pacing with a bradycardia indication.12 Patient height and weight were not collected as part of that study. In this study, the hearts in this group are referred to as “typical” in terms of size. The second group of 19 Japanese patients were from a single center in Japan. Most were indicated for bradycardia pacing, and some were indicated for Micra specifically. The Japanese hearts selected were from smaller patients to amplify the traits that may increase the difficulty of the implant and are referred to as “small” hearts.
All data used in this study were deidentified. In the Japanese cohort, the Kyorin University School of Medicine Ethics Committee waived the requirement for informed consent because the data were anonymized. For the American cohort, 3 separate ethics committees (Geisinger Institutional Review Board [IRB], Western IRB, Indiana University IRB) granted approval for the study, and the consent form included a statement that the data could be used for future studies.
Converting ScansDigital Imaging and Communications in Medicine (DICOM) images from cardiac-gated, contrast-enhanced CT scans were imported into Mimics Innovation Suite 24 (Materiaize). A clear image closest to complete diastole was selected for analysis. Threshold Hounsfield values were selected for each patient to create 2-dimensional masks of the region of interest. The masks were edited to remove artifacts and fill holes. Three-dimensional models were created from the segmentation masks to generate blood volume models of the right atrium (RA) and RV.
How Measurements Are MadeThe key measurements that were made on the 3-dimensional computer models of the heart scans are shown in Figure 1. The Mimics Analyze toolbox was used to manually place landmark points within the right heart blood pool. Posterior points were placed at the ostium of the superior vena cava (SVC) and 2–3 cm cranial of the SVC ostium. Similarly, posterior points were placed at the ostium and at the diaphragm level of the IVC. Vessel axis lines were created by connecting the SVC or IVC ostium points with the cranial or caudal posterior point, respectively.
Description of measurements taken. (A) Measurement 1, distance from the inferior vena cava (IVC) to tricuspid valve annulus superior (TVS) superior; Measurement 2, distance from the superior vena cava (SVC) to the tricuspid valve annulus inferior (TVI); Measurement 3, IVC to tricuspid valve center (TVC) angle; Measurement 4, SVC to TVC angle. RAO, right anterior oblique. (B) Posterior offset of the IVC (Measurement 5) and SVC (Measurement 6) shown in the right lateral view.
A spline was drawn around the TV annulus (TVA) and the centroid of the spline was calculated to mark the TV center (TVC). Additional points were placed at the superior and inferior edges of the TV (TVS and TVI, respectively).
The Mimics Measurement toolbox was used to measure the distance between many of these landmark points. A subset of these measurements in a single heart are shown in Figure 1. The distance was measured between the IVC ostium and the TVS (Measurement 1, Figure 1A), and the SVC ostium and the TVI (Measurement 2). The Angle tool was used to measure the angle between the IVC (Measurement 3) and SVC (Measurement 4) vessel axis to the TVC, respectively (Figure 1A).
The posterior offset of the IVC (Measurement 5) and SVC (Measurement 6) were calculated as the distance in the anterior-posterior direction between the SVC or IVC ostium and the TVC (Figure 1B). The IVC (Measurement 7) and SVC (Measurement 8) ostium diameters were measured as the mean of the maximum and minimum distances in a cross-section of each vessel near the atrium.
Comparison of Delivery Tool Dimensions to Heart DimensionsThe Micra delivery tool is designed to hold the Micra in a cup at the distal end. This cup has a nominal outer diameter of 7.67 mm (23 Fr) and a nominal length of 35 mm. The articulating shaft connects into the proximal end of the cup and adds 30 mm to the rigid length of the cup when pulled to a 90° angle. Therefore, the combined rigid section of cup plus shaft length is 65 mm. The rigid length of the tool was compared with the various dimensions of the hearts to understand how the tool may navigate through the heart.
Eleven studies were found that reported internal jugular vein dimensions for ages 1–93 years.13–23 The compiled data from these journal articles are presented in Table 1. Note in the younger patients how the mean diameter and cross-sectional area (CSA) are smaller, yet the distensibility is much greater. Across all ages, most patients will be able to accommodate the Micra introducer into the right internal jugular vein.
Mean Diameters and CSA of the Internal Jugular Vein With and Without the Valsalva Maneuver
| No. patients |
Age (years) |
Right internal jugular vein | Left internal jugular vein | ||||||
|---|---|---|---|---|---|---|---|---|---|
| Mean diameter (mm) |
Mean diameter Valsalva (mm) |
Mean CSA (mm2) |
Mean CSA Valsalva (mm2) |
Mean diameter (mm) |
Mean diameter Valsalva (mm) |
Mean CSA (mm2) |
Mean CSA Valsalva (mm2) |
||
| 203 | 1–10 | 8.4±1.1 | 14.6±1.5 | 37.0±6.2 | 131.4±17.5 | 5.5±3.8 | 13.0±1.7 | 36.8±5.9 | 118.9±28.4 |
| 33 | 10–20 | 8.75±3.1 | 15.91±4.2 | 36.64±28.8 | 135.66±72.8 | 4.82±2.1 | 9.82±3.4 | 35.84±29.3 | 118.39±72.7 |
| 46 | 20–40 | 9.5±0.6 | 15.2±2.6 | 9.7A | 11.9A | ||||
| 11 | 70–75 | 13.0±2.0 | 16.5±2.0 | ||||||
Data are presented as the mean±SD. AMean value only. These data are a summary of the data found in numerous articles.13–23 Note how Valsalva increases the vein diameter more in younger patients, but the older patients have larger baseline vein diameters. Also note that most patients can accommodate the Micra introducer into the right internal jugular vein based on the Valsalva measurement. The diameter and cross-sectional area (CSA) of the Micra sheath were 9 mm and 64 mm2, respectively. Note: although minimal, there is some overlap between the groups.
Heart Measurements: Small vs. Typical
The measurements taken on the 19 small and 26 typical hearts are presented in Table 2. Student’s t-test revealed that, compared with the typical hearts, the small hearts had significantly smaller IVC and SVC distance to the TVC, posterior offset for the TVC to the IVC ostium and SVC ostium, and IVC ostium diameter. Although differences in the angle of the IVC and SVC to the TV were similar between cohorts, the SVC angle was 40% larger than the IVC angle for the small hearts, compared with 38% larger for the typical hearts.
Mean Values of the Measured Dimensions in the Small and Typical-Sized Hearts
| Measurement parameter |
Small hearts (n=19) |
Typical hearts (n=26) |
Measurement parameter |
Small hearts (n=19) |
Typical hearts (n=26) |
|---|---|---|---|---|---|
| 1. Distance from IVC outlet to TVS (mm) |
58.4±9.0* | 65.9±10.8 | 2. Distance from SVC outlet to TVI (mm) |
62.8±9.7* | 68.9±8.8 |
| 3. Angle from IVC to TVC (°) | 97.0±25.3 | 99.9±12.3 | 4. Angle SVC to TVC (°) | 136.8±13.1 | 137.6±11.1 |
| 5. Posterior offset of IVC ostium to TVC (mm) |
38.4±9.5* | 48.5±8.8 | 6. Posterior offset of SVC ostium to TVC (mm) |
25.0±8.3* | 32.6±6.4 |
| 7. SVC ostium diameter (mm) | 17.4±5.1 | 27.7±4.1 | 8. IVC ostium diameter (mm) | 23.1±3.3* | 27.7±4.1 |
Data are presented as the mean±SD of the measurements of the 26 typical-sized hearts and 19 relatively smaller hearts from Japanese patients. *P<0.05 compared with typical-sized hearts (Student’s t-test). IVC, inferior vena cava; SVC, superior vena cava; TVC, tricuspid valve centroid; TVI, tricuspid valve inferior point; TVS, tricuspid valve superior point.
The percentage of hearts in which the tool will fit inside the rigid computer model of the RA is presented in Table 3 for the IVC and SVC to TV measures (Measurements 1 and 2 in Figure 1A and Table 2). From these models, there appears to be a large difference in the percentage of patients in the small vs. typical heart groups that can accommodate the cup and shaft. From the femoral approach, only 16% of patients in the small heart group, compared with 46% of patients in the typical heart group, can accommodate the cup and shaft. Using the jugular approach, these numbers increase to 37% and 69%, respectively. These numbers inaccurately assume a rigid heart, which is why the implant success rate is much higher in the clinical setting. However, these dimensions demonstrate how the jugular approach likely makes the navigation to the RV much easier and explains why femoral Micra implants in small patients can be more challenging.
Mean Dimensions of Small and Typical-Sized Hearts and the Percentage of Hearts That Will Accommodate the Delivery Tool
| Small hearts (n=19) |
% Accommodating delivery toolA |
Typical hearts (n=26) |
% Accommodating delivery toolA |
|
|---|---|---|---|---|
| Distance from IVC outlet to TVS (mm) | 58 | 16 | 66 | 46 |
| Distance from SVC outlet to TVI (mm) | 63 | 37 | 69 | 69 |
AThe percentage of hearts that could accommodate the Micra delivery cup and adjacent curved shaft. Note that a much smaller percentage of patients in the small-heart group that could accommodate the Micra delivery tool. These percentages are based on a simple comparison of the tool length to the measured parameter in the hearts. The rigid model makes these percentages much smaller than is found clinically. The percentages simply show that the smaller hearts have a more difficult pathway. RA, right atrium. Other abbreviations as in Table 2.
Another way to visualize this issue is shown in Figure 2, in which one of the small hearts is shown in the right anterior oblique and lateral views, with the cup against the annulus and the rigid portion of the shaft within the atrium.
Image of one of the smaller Japanese hearts with the overlaid cup. The deflected catheter is overlaid from the inferior vena cava (IVC) and the undeflected catheter is overlaid from the superior vena cava (SVC) approach. Note the limited space for the rigid portion of the cup, especially from the IVC. This limited space will restrict the curvature of the catheter and make navigating the corner from the IVC to the tricuspid valve center (TVC) very difficult. In contrast, the curve from the SVC appears to be more spacious and requires a shallower curve, potentially making navigation easier. (A) Right anterior oblique (RAO) view showing that the cup from the femoral approach is too long medial laterally, whereas from the jugular approach the cup fits within the atrium. (B) The lateral view shows how the cup must first course anterior and then lateral in a space that is smaller than the cup and shaft. Note that navigating via the SVC is a less tortuous path.
Comparison of the Femoral and Jugular Paths to the RV
First Consideration: Curve From the IVC/SVC to the Atrium The first curve to consider is that from the IVC (or SVC) into the atrium. The IVC tends to be a posterior structure in the thorax, and it must course anterior to enter the RA. In many hearts the IVC enters the atrium very posterior. If it enters the atrium posterior, then the delivery cup must first enter the atrium in an anterior direction, and then curve medially towards the TV. Figure 3 shows a heart with a large posterior offset (Figure 3A) and a heart with little to no posterior offset (Figure 3B). In Figure 3A, the atrium constrains the cup both anteriorly and laterally, making the path from the IVC to the RV quite tortuous. In Figure 3B, with little posterior offset, the curve is mostly in a single plane, making the navigation much easier, despite the short IVC to the TVC distance (dashed line).
Effect of large posterior offset of the inferior vena cava (IVC). (A) A smaller heart with a tortuous path. Note how the delivery tool must first course anterior and then medial to cross the valve. In addition, the IVC inlet is very medial, forcing the tool to be directed anterior first and then clocked medial with little room to make the curve. (B) A small heart with little to no posterior offset of the IVC. In this heart only a single curve is required to cross the tricuspid valve (TV), which is likely why navigating across the TV in this heart was easy. RA, right atrium; RV, right ventricle; SVC, superior vena cava.
Second Consideration: Angle Between the Vena Cava and TVC The second consideration in navigating to the RV is the angle between the line along the IVC or SVC and the TVC. The smaller this angle, the more the tool must curve and the harder it can be to cross the TV. The mean angle between the IVC and TVC for small and typical hearts is presented in Table 1. Although the angles for the small vs. typical hearts are similar, the angle from the SVC is 38–40% larger than the angle from the IVC, indicating the tool must curve more from the femoral approach (Figure 2). The combination of a small atrium and the smaller IVC to TV angle clearly makes the femoral approach more difficult.
Third Consideration: Heart Size The third consideration is heart size. If the atrium is large with respect to the rigid tool dimension, then navigation is much easier. With room enough for the cup and shaft to make the curves, even a relatively tortuous path can be navigated easily. An example of a larger atrium with a multiturn path is shown in Figure 4 from both an anteroposterior and full lateral view. Although the tool must still course anterior to reach the atrium and then turn medial to cross the TV, there is ample room to do so.
A “typical” heart with dimensions significantly larger than those of the rigid dimensions of the tool. (A) Anterior-posterior (AP) view. Note how the entire deflected cup and shaft can fit within the atrium and inferior vena cava (IVC). (B) Right lateral view. Note how the tool is pointing anterior because the IVC is offset to the posterior. However, the atrial dimensions are large with respect to the delivery tool, allowing easy navigation through the tricuspid valve in this heart.
Fourth Consideration: Angle to the Septum The femoral approach also allows the delivery cup to be brought perpendicular to the septum more easily. This concept is best shown in in Figure 5, where it can be seen that, from the jugular approach, the tool is already pointing in the direction of the septum. By applying more curve or pushing the tool further into the introducer, the cup can be made more perpendicular to the septum. This contrasts with the femoral approach, in which the tool is more parallel to the septum. In this situation, you must rotate the tool handle significantly clockwise to create a more perpendicular angle between the tool and the septum. It is also sometimes necessary to “catch” the cup tip on the septum to allow the tool to pivot around the cup tip when the tool is pushed into the introducer.
Comparison of Micra delivery tool angle while crossing the tricuspid valve (TV) and images of the tool crossing the TV from the (A) femoral and (B) jugular approaches. (A) Femoral approach with right anterior oblique (RAO) X-ray and camera view showing the cup pointing more inferior with the natural curve. (B) Jugular approach with RAO X-ray showing how the cup naturally points towards the septum.
In this study, CT scans of 45 hearts were used to gain an understanding of the reasons for the difficulty of Micra implants in patients with small hearts and to analyze whether the jugular approach would make these implants easier. A group of patients with small hearts were compared to a group of patients with typical-sized hearts.
The typical hearts were based on a Medtronic study of His bundle pacing using the 3,830 lead.12 These patients were typical bradycardia patients and not heart failure patients. The small hearts in this study were provided by a single center and chosen based on their smaller size. Some of the patients with the smaller hearts were Micra indicated, and not all were bradycardia patients. The choice of smaller hearts was driven by a desire to understand why, in Japan Micra, implant times are, on average, longer.11
Using segmented CT scans, computer models were created of each heart and multiple parameters were measured in these models. From this analysis, it was determined that the primary hurdle when navigating to the RV in smaller hearts appears to be making the turn from the IVC outlet to the TV.
There are 3 anatomical parameters that seem to affect the ease of crossing the TV. These are the distance from the IVC or SVC outlet to the TV, the angle between the central line of the SVC/IVC and the center point of the TV, and the anterior-posterior offset of the central line of the IVC/SVC and the central body of the TV.
However, the size of the atrium is likely the most important parameter. The heart shown in Figure 4 had a significant IVC offset, yet it was as easy to navigate to the RV from both the femoral and jugular approaches. However, heart size is not the only parameter affecting the ease of crossing the TV. In the small heart shown in Figure 3B, it was very easy to access the RV from the femoral approach despite the heart being very small. This was likely due to the fact there was little or no offset to the IVC. This meant the tool only had to make a single medial turn to enter the RV. It appears that the combination of a posterior offset, a small angle, and a small atrium makes implants most difficult.
The Micra delivery tool has a rigid distal cup and softer articulating shaft that collectively make a 65-mm rigid section at the distal end of the tool when fully curved. Changing the delivery tool to have a sharper angle at the tip will not affect this rigid length, and it is navigating this rigid cup through tight corners that makes navigation difficult.
Another concern about placing the Micra introducer in the internal jugular vein is the potential for flow restriction and increased intracranial pressure. There are a few mitigating factors related to this concern. First, the procedure is very short, so any increased pressure will be of minimal duration. Second, there are many collaterals from the internal jugular veins, and these are known to expand to accommodate flow in the event of partial or total occlusion of the jugular veins.24 Third, as seen in Table 1, the internal jugular vein in most patients is larger than the Micra introducer after using the Valsalva maneuver. Therefore, complete occlusion of the jugular vein is unlikely, thereby reducing the risk of increased intracranial pressure.
One significant limitation of the present study is that cardiac measurements were taken from CT scans, which are an expensive and time consuming prescreen for a pacemaker patient. Alternatively, it may be possible to get these measurements from other imaging modalities. First, the overall heart dimensions could be taken from a chest X-ray. Both posterior-anterior and lateral heart dimensions would give a simple measure to determine whether the heart is small and may therefore warrant either further measurements or a jugular approach. The IVC offset and the angle towards the TV would likely require a CT image, although it is possible that echocardiography may provide these measures using a subxiphoid image.
Knowing the cardiac measurements that may influence procedural ease prior to implanting the Micra device could help inform which access site approach would be preferable. More work needs to be done to further characterize these metrics, as well as how to implement them in clinical practice. An important learning from this analysis was that even if the tool curve were modified for a “size small” version, the same challenge of navigating across the TV from the femoral would be experienced due to the rigid cup length.
Although the femoral procedure today maintains an excellent safety profile, this study validated that the jugular approach may improve the ease of implant, particularly in patients with small hearts. In geographical locations with disproportionately small anatomies (e.g., Japan and other Asian countries), the jugular approach has the potential to reduce the number of patients with difficult navigation across the TV.
Although other studies have demonstrated the safety of this approach in clinical practice,2–10 the present study has provided the rationale as to why the jugular approach is feasible and may improve the ease of the Micra implant in small hearts, namely by reducing the tortuosity of the navigation across the tricuspid valve.
The authors acknowledge Samantha Kohnle and William Schindeldecker for their considerable effort in converting so many DICOM files to CAD models and then making all the distance and angle measurements in the CAD models.
This study did not receive any specific funding.
K.H., M.L.S., and R.L.T. are employees of Medtronic. K.S. and M.D.B. are consultants to Medtronic, Inc. K.S. is an Associate Editor of the Circulation Journal.
All data used in this manuscript was deidentified. In the Japanese cohort, the Kyorin University School of Medicine Ethics Committee waived the requirement for informed consent because the data were anonymized. For the American cohort, 3 separate ethics committees (Geisinger Institutional Review Board [IRB], Western IRB, Indiana University IRB) granted approval for the study and the consent form included a statement that the data could be used for future studies.
The anonymized participant data will not be shared.
