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Good afternoon, and welcome to TransMedics First Quarter 2024 Earnings Conference Call. [Operator Instructions] As a reminder, this call is being recorded for replay purposes. I would now like to turn the call over to Laine Morgan from the Gilmartin Group for a few introductory comments.

Thank you, operator. Earlier today, TransMedics released financial results for the quarter ended March 31, 2024. A copy of the press release is available on the company's website. Before we begin, I would like to remind you that management will make statements during this call, including during the question-and-answer portion of the call, that include forward-looking statements within the meaning of federal securities laws. Any statements contained in this call that relate to expectations or predictions of future events, results or performance are forward-looking statements. All forward-looking statements, including, without limitation, our examination of operating trends, the potential commercial opportunity for our products and timing of new clinical programs and our future financial expectations, which include expectations for growth in our organization and guidance and/or expectations for revenue, gross margins and operating expenses in 2024 and beyond are based upon our current estimates and various assumptions. These statements involve material risks and uncertainties that could cause actual results or events to materially differ from those anticipated or implied by these forward-looking statements. Accordingly, you should not place undue reliance on these statements. Additional information regarding these risks and uncertainties appears under the heading Risk Factors of our Form 10-K filed with the Securities and Exchange Commission on February 27, 2024, our subsequent form -- and our subsequent Form 10-Q filings and the forward-looking statements included in today's earnings press release, which are available at www.sec.gov and on our website at www.transmedics.com. TransMedics disclaims any intention or obligation, except as required by law, to update or revise any financial projections or forward-looking statements, whether because of new information, future events or otherwise. This conference call contains time-sensitive information and is accurate only as of the live broadcast today, April 30, 2024. And with that, I will now turn the call over to Waleed Hassanein, President and Chief Executive Officer.

Thank you, Laine. Good afternoon, everyone, and welcome to TransMedics First Quarter 2024 Earnings Call. As always, joining me today is Stephen Gordon, our Chief Financial Officer. For the past 2 years, TransMedics has delivered exceptional revenue growth while making transformational investments in our business. 2024 represents another crucial year not only for exceptional growth, but also for broadening our infrastructure and product pipeline to drive further growth, profitability and importantly, increased transplant volumes. Specifically, we are focused on 3 verticals: first, completing the initial build-out phase of our TransMedics aviation fleet and transplant logistics infrastructure; second, preparing for the launch of 3 major new clinical programs to accelerate OCS Lung and OCS Heart adoption and expand our clinical indications for OCS Heart in the U.S.; and finally, growing the overall national transplant volumes even further through our one-of-a-kind NOP program. On every front, we have started the year with very strong momentum towards achieving these goals. With 1Q results representing a new high watermark for our business, let me review the key highlights for the first quarter performance. Total revenue for Q1 grew to $96.9 million, representing 133% growth over Q1 2023 and a 19% sequential growth from Q4 2023. This growth was achieved through increased utilization of both OCS product across lung, heart and liver as well as TransMedics transplant logistics service. I want to highlight the diversified nature of our growth to dispel any potential misperception that our growth is only driven overwhelmingly by transplant logistics revenue growth. Said differently, we fully expect -- I repeat, we fully expect our future growth to be driven by both increased product and transplant logistics adoption. TransMedics transplant logistics service revenue for Q1 was $14.5 million, up from $9.2 million in Q4 of last year, representing approximately 58% growth quarter-over-quarter. We are continuing to demonstrate that our integrated and cost is such efficient, TransMedics NOP and logistics service infrastructures are delivering real value to transplant programs across the U.S. We remain focused on expanding our operational capabilities for TransMedics logistics throughout 2024, which I will detail further later in the presentation today. Our overall gross margins for Q1 was 62%, up from 59% last quarter and in line with our expectations. We are extremely confident that we will be able to further improve the gross margin over the next 12 to 18 months as we achieve more leverage of scale in both product and service operations. The strong growth in revenue and gross margins enabled us to deliver GAAP operating profit of $12.4 million, which represents 13% of total revenue. Net income was $12.2 million. We are very proud to have achieved these profitability metrics while still investing heavily in future growth. We remain laser focused, however, on delivering sustainable positive operating cash flow over the next several quarters. Before moving on to our momentum beyond our financial performance, I'd like to take a moment to recognize the entire TransMedics team, which had -- which has worked tirelessly to achieve these results. We are focused on execution to build upon the Q1 result. Now with that background, let me provide more detail across key operating metrics. As I stated above, we set a new high watermark for case volume across all 3 organ markets in Q1. Overall, NOP contribution remains at 98-plus percent of our case volume, a trend which we expect will continue throughout the foreseeable future. Turning now to the key TransMedics transplant logistics metrics. Through Q1, we continue to expand our fleet of owned aircraft reaching 14 owned aircraft by end of the quarter. Meanwhile, the daily average number of active TransMedics aviation planes were 9 planes in Q1 compared to 7 in Q4 of 2023. We expect this number will continue to increase throughout the year as we strive to reach 15 to 20 operational aircraft by year-end. Our owned aircraft covered approximately 49% of our NOP flight missions in Q1 compared to 35% in Q4 of '23. This further underscores the potential long runway to drive additional growth and maximizing efficiency across our transplant logistics operations. As we stated before, at scale, we fully expect to cover 80-plus percent of the total NOP missions using our TransMedics logistics services for both air and ground transport. We will continue to use carefully selected, highly reliable and safe operators for supplemental lifts to support our additional missions. From a customer footprint perspective, we have also continued to grow the number of programs that are using our transplant logistics services. In Q1, approximately 105 U.S. transplant programs used TransMedics logistics compared to approximately 97 in Q4 of 2023. As we have rapidly achieved this critical mass of users, we are now focusing on going deeper within these programs and meeting more of their transplant logistics needs going forward. Overall, we are very pleased with the early success of our transplant logistics services and are confident that transplant programs are seeing the significant cost efficiency and reliability of TransMedics logistics compared to historical model. We look forward to expanding further throughout the year and into 2025 as we scale our air fleet and ground operations. We are also encouraged by our growing base of clinical evidence from real-world outcomes and the growing excitement around our offering across clinical transplant users. We saw this excitement firsthand in April of this year as we attended the International Society of Heart and Lung Transplant Conference in Prague. At the meeting, several scientific presentations by transplant academic experts, demonstrating the value of OCS Heart and OCS Lung were presented. Here are the key highlights. Dr. Jacob Schroder from Duke presented the OCS Heart Perfusion, or OHP, registry experience with DCD heart transplants in the U.S. The data demonstrated that OCS Heart was used in approximately 3/4 of all DCD heart transplanted at OHP registry centers. The data also demonstrated that OCS DCD heart transplants have superior patient survival outcomes compared to NRP DCD transplants in high-risk recipients. This provides evidence -- this provided evidence that OCS Heart affords better protection of the DCD donor hearts as compared to NRP. During his presentation, Dr. Schroder commented that the overall OCS NOP cost is more favorable to NRP costs when factoring in the cost of dry runs, the clinical support overhead and the hardware costs. Importantly, Dr. Schroder highlighted that the OCS NOP enhances the ability for any heart transplant program in the U.S. to offer the clinical service of DCD heart transplantation to their patients without the burden of overhead costs and clinical learning curves giving the standard or unified procurement and management of donor hearts by the TransMedics NOP staff. Next, Dr. Mani Daneshmand from Emory University Medical Center presented the outcomes of OCS DCD compared to standard of care DBD hearts in the U.S. The data showed that OCS DCD hearts were transported nearly double the distance from donor to recipients and had double the cross-clamp time. This signifies the broader access to DCD donors afforded by OCS NOP. The data also showed that despite higher risk donor factors, OCS clinical outcomes were similar to standard criteria DBD outcomes in the U.S. This further validates the safety profile of the OCS Heart. Simply stated, the OCS enabled a DCD heart transplant to have similar survival outcomes to the U.S. national DBD heart transplant outcomes, which are the best in the world. Dr. Daneshmand also highlighted that the increased use of OCS NOP has led to significant reduction in moderate and severe primary graft dysfunction, or PGD, after OCS DCD heart transplants. Severe PGD is the most severe early post-heart transplant clinical complication and historically has been associated with worst, short- and long-term patient survival. Next, Dr. Mauricio Villavicencio from Mayo Clinic presented the OCS Heart DBD experience from the OHP registry. The data showed that OCS NOP resulted in excellent post-transplant clinical outcomes from DBD donors compared to standard criteria donors preserved with static cold storage despite having 3x longer distance travel and double the cross-clamp time in the OCS NOP arm. Again, this data validates the broader access to distant donors and potential for improved workflow afforded by the OCS NOP. Next, Dr. Gabe Loor from Baylor St. Luke's presented the OCS Lung expand trial 5-year clinical results. The data showed that the OCS Lung expand lungs from extended criteria DBD and DCD donors had similar survival and freedom from chronic rejection at 5 years post transplant compared to routine, standard criteria DBD lung transplanted at the same program over the same time period. These results support the huge clinical potential of increasing donor lung utilization for transplants using extended criteria DBD and DCD donors in the U.S. Finally, Dr. Steve Huddleston from University of Minnesota shared the latest data from the thoracic OCS perfusion registry, or the TOP registry. The data showed that the OCS Lung enabled the use of extended criteria donor lungs from DBD and DCD donors and resulted in post-transplant survival outcomes that are similar to standard criteria lung transplant despite nearly having double the cross-clamp time, again, further validating the huge clinical impact on expanding the donor pool and the potential growth of lung transplant volumes in the U.S. Collectively, these presentations once again highlighted our ever-growing body of positive clinical evidence as well as the exceptional clinical outcomes enabled by OCS and NOP. Now let me shift gears and talk about our plans to further grow OCS adoption and the overall national U.S. transplant volumes even further. Specifically, I want to discuss 3 new major clinical programs designed to grow adoption of our OCS Lung and OCS Heart as well as expand our OCS Heart FDA clinical indications in the U.S. Pending FDA approval, we expect that all 3 programs will initiate enrollment within the next year. Let me start with detailing the OCS Lung program. As we stated many times, we believe that the clinical stakeholders across the United States lung transplant market need to be reintroduced to the potential positive clinical value of the OCS Lung perfusion and assessment. More specifically, we believe the ability of the OCS Lung and NOP to increase their transplant volumes, improve their post-transplant clinical outcomes and enhanced workflow remain underappreciated. Our goal is to replicate the successful outcomes achieved with OCS Liver where 62% of transplant volumes at OCS NOP programs are now done in the morning working hours compared to middle of the night and replicating that with the OCS Lung. Said differently, we want to have lung transplant programs and clinical and surgeons experience firsthand the value of OCS NOP to enable morning transplants while growing their overall transplant volumes and improving their post-transplant clinical outcomes. To do this, we're planning to launch a new clinical program to achieve the following. First, we will target a minimum of 12- to 24-hour plus of OCS Lung perfusion using the NOP model to increase access to transplantable donor lung and optimize work hours for transplant program staff. Importantly, we aim to prospectively randomize between OCS NOP versus controlled cold static storage to assess a clinical value. We also plan to use newly developed near physiologic OCS perfusion solution combined with blood to minimize the impact of longer perfusion on lung edema and potentially eliminate any clinical concerns of lung perfusion times on lung function. We will also use next-gen perfusion circuitry and ventilation modality to maximize the protection for the donor lungs during prolonged OCS perfusion and ex vivo ventilation. We expect the entire clinical program to be managed by NOP to increase the rate of enrollment and adoption during the trial phase. From a timing perspective, we are targeting initiation of this program sometime around the end of 2024. Now let me move on to our planned OCS Heart programs. We are also actively working on 2 distinct large OCS Heart programs in the U.S. that will be also managed exclusively via the OCS NOP model. The first is OCS Heart therapeutic warm perfusion for DBD hearts. This program is aimed at increasing utilization of DBD hearts from both standard and extended criteria donors to increase the overall heart transplant volumes in the U.S. We intend to target 12-hour plus of OCS Heart perfusion using the NOP model to increase access to donor hearts and optimize the working hours for our transplant program staff. We will also aim to prospectively randomize OCS NOP versus controlled cold static storage to assess the clinical value. We're planning not only to use our newly developed near-physiologic OCS perfusion solution combined with blood, but in this particular program, we're adding a new proprietary metabolic-enhancing therapeutic agents to maximize protection of the donor heart and improve its post-transplant clinical performance. From a timing perspective, we are targeting initiation some of this program sometime around the end of 2024. Finally, our second heart program is a new program that will require a new technology from the ground up. It's aiming at OCS Heart cold oxygenated perfusion for DBD hearts that are preserved for less than 6 hours. This program is designed to support a new FDA clinical indication for OCS Heart in the U.S. that will allow us to perfuse and preserve standard criteria DBD hearts for less than 6 hours, which is not our current clinical indications in the U.S. To do this, we're planning to offer a new lower-cost product that utilizes cold oxygenated blood-based perfusion technology. More specifically, we're developing our new pulsatile, fully portable cold perfusion technology and cold perfusion circuitry to achieve easy-to-use system for use within our existing NOP model. Again, we'll aim to prospectively randomize to cold controlled -- a controlled cold static storage to assess the clinical value. And we are targeting early 2025 to initiate this important clinical program. As you can see, we are advancing a very strong pipeline of clinical programs designed to drive significant growth in OCS case volume and the overall national cardiothoracic transplant volume in the U.S. However, we're not stopping here. We are also continuing to invest heavily in our next-gen OCS technology platform for all 4 -- for all 3 organs that will be highly automated, optimized for NOP workflow and designed to streamline the clinical support workload to allow us to continue to deliver the highest clinical quality of care and achieve better product leverage. We plan to share more details on this initiative later this year. To summarize, we are highly encouraged by our Q1 performance and are focused on several initiatives designed to further propel growth for TransMedics products and services. Given our strong performance in Q1, we are increasing our annual revenue guidance to $390 million to $400 million, which represents 61% to 66% growth over full year 2023 revenue. With that, let me turn the call to Stephen to cover the detailed financial results for the quarter.

Thank you, Waleed. I will now provide some additional detail on the Q1 results and other financial information for the quarter. So starting with revenue. For the first quarter of 2024, our total revenue was $96.9 million. This is an increase of 133% from the first quarter of 2023 and a 19% sequential increase from last quarter. The $96.9 million included $0.9 million related to our flight school. We have now exited all of the Summit Legacy Charter business. So other than this $900,000 from the flight school, all revenue is transplant-related. In the U.S., transplant revenue was $91.9 million. U.S. revenue increased 145% from the first quarter of 2023 and 22% sequentially from last quarter. And as Waleed said, Q1 2024 revenue included $14.5 million of logistics revenue. The organ breakdown on U.S. revenue was $67 million of liver, $20.2 million of heart and $4.7 million of lung, all organs growing substantially over Q1 2023 and sequentially from Q4 2023. Ex U.S. revenue was $4.1 million, a 1% increase from Q1 of 2023 and a 16% sequential increase from last quarter. The ex U.S. breakdown was $3.1 million of heart and $1 million of lung. Next, on the product and service revenue. As a reminder, our service revenue includes the added amounts we charge for the NOP clinical service of surgical procurement and organ management and also includes the logistics revenue. The flight school is also included in service revenue. In Q1, product revenue was $61.3 million, and service revenue was $35.5 million. So the service portion was 36.7% of the total. Gross margin for the first quarter of 2024 was 62%. This is down from 69% in the first quarter of 2023 and up from 59% last quarter. In comparison to Q1 last year, this reflects the higher service component of our business, which did not include logistics in the first quarter last year. Product margin was 77% in Q1, recovering as expected to more normalized product margins from the 73% we saw in Q4, which included a onetime unfavorable item. Service margin was 36%, improved from 35% last quarter as we continue to gain efficiency in our service offering. And as a reminder, all costs related to aviation, including fuel, pilots, maintenance and depreciation are included on our service COGS. Total operating expenses for the quarter were $47.5 million, 54% above Q1 2023 OpEx. This expense growth was driven by 94% growth in R&D related to investments in new product development, NOP tools and product quality and regulatory resources. SG&A grew 45%, primarily related to higher personnel costs and overall corporate infrastructure. I want to point out that our operating expenses grew significantly throughout the year last year. So the year-on-year growth comparison next quarter should not be as pronounced as it was this quarter. Given the strong revenue and margin performance, we were able to deliver GAAP operating profit of $12.4 million or 13% of revenue. Net income was $12.2 million. These compared with an operating loss of $2.6 million and also a net loss of $2.6 million in Q1 of 2023. And basic earnings per share in the quarter was $0.37, and diluted earnings per share in the quarter was $0.35. Total cash at the end of the quarter was $350.2 million as of March 31, 2024. This is down $44.6 million from December 31, 2023. $39 million of cash was used to purchase 3 additional jets in Q1, bringing our total number of owned jets to 14. Basic weighted average common shares outstanding for the quarter were 32.8 million, and diluted weighted average common shares outstanding for the quarter were 34.7 million. In summary, Q1 was a very successful quarter financially for TransMedics. We grew our revenue both annually and sequentially, improved our gross margin and showed good drop-down to profitability. All of this continues to validate our strategy of leveraging our NOP clinical service and logistics service to increase utilization of the Organ Care System and to increase the number of transplants in the U.S. Finally, just to repeat Waleed's earlier comment, we are updating our annual revenue guidance to be in the range of $390 million to $400 million, which represents 61% to 66% growth over the full year 2023. Now I'll turn the call back to Waleed for closing comments.

Thank you, Stephen. Overall, we are humbled and proud of our Q1 results as we simultaneously drove continued revenue growth, expanded our infrastructure and achieved profitability while advancing our clinical and R&D pipelines. We're looking forward to continuing to execute on all the major initiatives throughout 2024 to drive broader adoption of OCS NOP and growth of the overall transplant volumes to help patients in need of an organ transplant. With that, I will now turn the call to the operator for Q&A. Operator?

[Operator Instructions] Today's first question comes from Allen Gong with JPMorgan.

Congratulations on a really strong quarter out of the gate. I understand that aviation likely helps support the beat in the services, but I think it was the beat in disposables that might be a little bit more surprising given the fact it kind of to be on a dollar basis, at least relative to my forecast growth more of the upside. So I guess other than pull-through of some of the NOP cases that you were maybe previously losing due to the limitations of outside logistics, what else kind of went right in the quarter for you to drive these additional volumes?

Thank you, Allen, for the question. A lot of things went right in the first quarter, and we hope to continue to execute in the same tone going forward. The most important thing is the outcomes. The outcomes that are being achieved across the board are now more transparent to the clinical users. Specifically, the liver continues to grow. But specifically for heart and lungs, there was -- the lung outcomes are getting better. Our team has been working very hard at educating the market, demonstrating the better outcomes achieved with our newer use model, and it's resonated in the quarter. Also, we're seeing the outcomes in heart is really helping growing the heart market. And certainly, the discouraging results we heard at the ISHLT from the cold perfusion study may have fueled that. But it's really the outcomes that is driving our growth, and we plan to continue to lean on outcomes and that's why we are investing in these 3 major cardiothoracic programs. The liver is already there and continues to grow, and we will continue to add centers and go deeper within existing center. So everything went right. Also, the growth in the logistics business was important to help us get access to the cases that we couldn't get access to that also helped. But the fundamental growth from product is basically based on clinical outcomes.

Got it. And then a follow up just kind of on seasonality and how should we think about that strength carrying forward. If we kind of take the quarter you just put up, back it out of your updated guide, it really looks like you're setting what should hopefully be a very achievable bar for the balance of the year, especially as you're adding more planes, you're going to be starting the quarter with more planes than you had on average in first quarter. So why is this kind of the right target to go with? And how should we think about the seasonal cadence implied by that guidance? Should it be relatively flat? I guess like while that'd be the case, I shouldn't view the growth sequentially.

Thanks, Allen. I think there's many layers to answering that question, Allen, and Stephen, please comment as well from your perspective. I think we always are cognizant of what potential operational challenges in front of us. For example, we are very proud to have operating 14 planes hopefully in Q2. But we know that in the second half of the year, we have some of these planes are due for some annual service. So they're not going to be accessible to us. So we factored that into the guidance. We also factored in some of the -- any potential seasonality from summer vacations coming up for the holidays. So we always are prudent. When it comes to guidance, we want to -- when we issue guidance, we take it very seriously. So that's layered into our expectations here. Stephen?

And Allen, I would just say, look, we don't expect a down quarter sequentially. We expect modest growth quarter-over-quarter. And that's the way we've modeled it, and I would expect that's the way we'll come in.

Yes. Also, finally, Allen, to put a bracket around that, we're operating from a much bigger starting point now. So we have to be cognizant of that.

And our next question today comes from Josh Jennings with TD Cowen.

It's great to see such impressive start to the year. I was hoping that it'll lead to -- to circle back on the discussion we had earlier in the quarter, just about you have a lot of -- you announced a lot of pipeline initiatives both on the technology front and on the clinical development front. But just how should we be thinking about the OCS system potentially reducing the percentage of DCO donors -- DCD donors that do not progress in heart, liver and lung? And is that something that could happen in the next 12 to 24 months?

Josh, that's exactly our goal. As we discussed, this is the only system that we're aware of that exists out there that could help that picture is the OCS. So that's something we're planning to leverage over the next 12 to 24 months for sure. And we're hoping that once we launch these clinical programs, that becomes an opening to the next program being focused on specifically growing the DCD utilization.

Excellent. And another topic, just with ILTS kicking off this week, I wanted to just ask about -- just get a better understanding on the benefits and advantages of using OCS warm normothermic perfusion in fatty livers and just the percentage of donors that have fatty livers and how dramatic a difference there is in preservation for OCS versus cold storage or even cold hyper oxygenated perfusion.

Thank you. Thank you, Josh. Thank you for asking the question. It's a very important question. Without running the risk of burning some of the key plenary session presentations at the upcoming ILTS, the community should be expecting that we will reveal data that shows clinical superiority of fatty livers using warm perfusion compared to any other modality. And I'll leave it at that. It doesn't make sense to put fatty livers on ice, whether for perfusion or controlled or noncontrolled static cold storage. It just doesn't make any sense because fat cells with cold storage or any cold form of preservation congeals and then the liver becomes more of a foreign object than a physiologic body. So we're looking forward to our investigators and lead users to be presenting this data at the plenary session on Saturday.

And sorry to sneak a follow-up then, but just any help just thinking about the percentage of donor livers that are fatty. I imagine it's a sizable chunk of the donor pool.

It's a very sizable chunk. And again, the definition of fatty, it's varied. Some people consider fatty liver anything greater than 15%. We'll be presenting data on fatty liver greater than 25% or 30% even. So we experienced the full gamut. And again, there's a tremendous evidence supporting warm perfusion on the OCS platform having superior outcomes to any other modality for preservation of fatty livers. And I'll leave it at that, Josh.

And our next question comes from William Plovanic with Canaccord.

It's John on for Bill tonight. I just wanted to first touch on aviation. You said 80% is probably the terminal rate of U.S. cases that to be supported by you. What services and what level of jets are needed to reach that 80%? And when could we see that?

Thank you. Thank you, John. We think that -- at the current estimates, we think somewhere between 25 and 30 planes will get us there. But we fully expect to increase those estimates beyond 10,000. So that's our expectation. And the key for us is to build enough in this phase to continue to demonstrate the growth. And as we need more, we will have more planes. But right now, we're hoping to end this year around 20, between 15 and 20 planes, and hopefully, by end of next year to be between 25 and 30. And then we'll assess from there.

Great. Maybe more for Stephen, but any operating profit, cadence or guidance for the remainder of this year?

John, this is Stephen. I have -- I'm not prepared to give any guidance other than we're pleased with where we came out in Q1. And we hope we're on path to having sustainable profit going forward because we're a little bit ahead of where we thought we'd be. So -- but that's about all I can say at this point.

Great. And maybe to just squeeze one more in here. But while we [indiscernible] cold option perfusion for heart for only 6 hours, especially with the competitor cases that are notably going much longer than that.

Thank you, John. John, you heard the outcomes with me. They failed a trial in Europe. So why would I subject us to bad outcomes? And we are more sensible where we want to protect the outcome for the patient. So -- and we're providing this as a lower-cost solution for this small segment of the market that is below 6 hours. For longer hours, we hope to prove it based on the new heart program that warm perfusion is a better solution than cold perfusion. That's the rationale for why we're limiting it, at least based on an indication standpoint. And remember, all of the data that we heard at the ISHLT is not an FDA-level data. They're all few centers, a handful of cases, except for the European multicenter trial that failed the primary effectiveness end point.

And the next question comes from Suraj Kalia with Oppenheimer.

Gentlemen, congrats again on a blockbuster quarter. So Waleed, I just want to go back on one of the points that you made at our conference a month or so ago. And even on this call, you were talking about the next generation trial. So Waleed, stratify for us the standard criteria DBD hearts that are technically off-label for you all today, just so that people can compare and contrast as to what the denominator should be in terms of market penetration. Also, Waleed, the trial that you mentioned, that would be beginning, I believe, you said next year -- early next year, that is cold perfusion, but would it also have physiologic beats?

Thank you, Suraj, for the question. So let me address this in multiple points. First, right now, our FDA-approved indication does not cover standard criteria DBD hearts. Our plan is to have a new indication to cover that. Is it 4 hours? Is it 6 hours? The market segment of between -- the less than 4 hours is about 900. If you go down to -- up to 6 hours, about maybe 1,200 transplants, plus or minus. At least that's based on last year's number. The reality is we want to access this segment of the market, no matter how big or how small it is. We want to be, 2 years from now, every heart transplanted in this country should be preserved on a TransMedics technology. Whether cold perfusion or warm perfusion, it will be a TransMedics technology. And we want to have the full gamut of FDA indications like we have it for lung and we have it for liver. So that's number one. Number two, we have 2 heart programs, 1 warm focusing on therapeutic and optimization modalities for DBD donors; and 1 cold. The warm we expect to start before year-end this year. The cold, because it requires a full-blown new system and full new circuitry, will start in the first half or beginning of 2025. And that is the one that is focused in the new FDA clinical indication. I hope I addressed the question.

Yes. Fair enough. And...

And there was one segment -- I apologize, Suraj. Yes, it will be pulsatile. The cold perfusion will be pulsatile, which is a distinguishing factor that we have that nobody else has.

Fair point. Okay. Stephen, one question for you, and I'll hop back in queue. One of the questions that frequently comes up in investor discussions is -- and maybe you can quantify this a little better for everyone's consumption. And the question that comes up is, "Hey, how does TransMedics depreciate its planes? What are its all-in cost per hour for aviation? How are the margins where they are?" I would love for you to take all of these and wrap it up into some numeric or numbers that people can slice and dice. Gentleman, congrats again.

Thanks, Suraj. Well, the question I can answer is how we depreciate. So we depreciate the planes over 10 years with a 50% residual value. So that has been -- we've been clear from day 1. That's in our Qs and Ks. We haven't talked about the margin of aviation versus the margin of the service. But all in, we're at that 36%, and we expect some improvement. Certainly, I can say the aviation is a bit on the lower side versus the service, which is a bit on the higher than that side, but we haven't talked about anything more details on that. So that's what I can answer to that question.

And our next question today comes from Ryan Daniels with William Blair.

Yes. This is Jack Senft for Ryan Daniels. Congrats on the strong start to the year. Can you share any general feedback from customers that have used TransMedics aviation? And maybe if or how that feedback has changed since you began integrating the aviation segment?

Thank you for the question. I think the only thing that I can share publicly is just -- is I point out to the results. I point out to the -- their rapid pace by which we went from 0 to 105 customers using our TransMedics logistical services. And we expect to go deeper within these accounts. I'll leave it at that. I think centers are beginning -- or are actually witnessing the better structure, the more efficient cost structure and the availability that is afforded by TransMedics logistics. And again, I point to the results.

Understood. Can you just provide an update here on what you're seeing in the international markets and kind of what the expectations are there? And just as a quick follow-up then to, are there any like encouraging opportunities following the ISHLT meetings that took place?

Excellent question, and thank you for asking it. There is a tremendous focus on the success of NOP in the United States. There are many major European countries are coming to TransMedics and offering to collaborate on establishing NOPs across Europe. We're seeing similar behavior in the Middle East, specifically in Saudi Arabia. We had several discussions at the ISHLT. The way I want to characterize it is, absolutely, we're focusing on replicating the success of the NOP because we believe the problem that the NOP solves for in the U.S. is exactly the same problem ex U.S. However, we want to prioritize securing reimbursement first to make sure that our services will get reimbursed. And one final qualifier. When I talk about NOP ex U.S., we're talking only on the clinical support service, no logistics and no surgical procurement, just for clarification purposes. So yes, there's a huge momentum around NOP replication OUS, and TransMedics fully expects to be ready to implement those once we are confident that our services will be reimbursed.

Congrats again.

Thank you.

[Operator Instructions] Our next question comes from Matthew O'Brien with Piper Sandler.

This is Samantha on for Matt. I guess just to start off, if you could talk a little bit more about guidance -- your guidance for the rest of the year and kind of what's baked into the low end and the high end of that range?

Samantha, this is Stephen. Yes. I mean we think there's opportunity to continue to kind of grow sequentially, as I mentioned in an answer to the earlier call. And if we're able to add or go deeper in a few of our centers, we should be able to get to that high end. I mean some of these things will come to fruition. And so the low end is just being a little bit more conservative about the pace of how we do that. So it's a pretty narrow range, and we feel confident that we'll be able to meet it.

Great. And then just one more from us. I know you've talked a little bit in the past about the expected product-service mix. And how can we expect that to change throughout the year particularly [Audio Gap] do more, yes, your costs throughout the year?

Yes. So it's a good question. We've been kind of keeping an eye on the product and service mix. It ended up 36.7% service. I think it's going to get a little bit more than that. It might be between -- say, between 37% to potentially 39%. I think that's probably the top end. So it's a little higher than I had given a view earlier in the year based on the outcomes we're seeing. But we still think we're going to see overall gross margin continue to improve.

And this concludes our question-and-answer session. I'd like to turn the conference back over to Waleed Hassanein for closing remarks.

Thank you so much, operator. Thank you so much, everybody, for joining us on this call this evening, and we're looking forward to our next call. Have a wonderful evening, everyone.