The clinical problems

Orthostatic intolerance is the inclusive term for symptoms, ubiquitously including lightheadedness/dizziness but also often tachypalpitations, syncope, fatigue, nausea, and headaches, that occur while upright and are relieved by recumbence. ^{Reference Freeman, Wieling and Axelrod1} Orthostatic hypotension, which occurs in older adults, is not typical in paediatrics, and instead, orthostatic tachycardia and its chronic form, postural orthostatic tachycardia syndrome, predominate. Orthostatic intolerance is quite common—in this calendar year it has been a more common new referral complaint than chest pain or heart murmur to our paediatric cardiology practice. Typical patients are adolescent girls, usually between 12 and 16 years old at time of presentation. ^{Reference Freeman, Wieling and Axelrod1} Interestingly, a high percentage of these patients will have hypermobile joints, and some even meet criteria for hypermobile Ehlers-Danlos syndrome, which helps to explain the physiology. ^{Reference Wallman, Weinberg and Hohler5,Reference Roma, Marden, De Wandele, Francomano and Rowe6} Specifically, hypermobility in the joints marks one as having laxity in connective tissue, and thus laxity in the support of blood vessels, leading to increased gravitational pooling of venous blood in dependent parts of the body (leading ultimately to decreased cerebral perfusion, the stimulus for orthostatic symptoms).

As gravity potentiates venous pooling in the pelvis and lower extremities, there is decreased preload to the heart and therefore decreased cerebral perfusion. ^{Reference Raj, Biaggioni and Yamhure2,Reference Jacob, Biaggioni, Mosqueda-Garcia, Robertson and Robertson3} This causes lightheadedness in the patient. ^{Reference Roma, Marden, De Wandele, Francomano and Rowe6} In response, the sympathetic nervous system is activated to address the drop in cerebral perfusion, specifically by inducing peripheral vasoconstriction and increasing cardiac output. This often is insufficient, however, so lightheadedness is not alleviated and instead the patient experiences the unpleasant effects of this “adrenaline surge,” both in the moment and throughout the day depending on how often the necessary circumstances are met. ^{Reference Raj, Biaggioni and Yamhure2–Reference Hebson, McConnell and Hannon4} . The excessive sympathetic activation causes the additional bothersome symptoms—palpitations, fatigue (excessive daily adrenergic activity), nausea and other symptoms of gastrointestinal intolerance (gut underperfusion in a hyperadrenergic state), and eventually syncope if a vasovagal reflex occurs. ^{Reference Garland, Raj, Black, Harris and Robertson7,Reference Thieben, Sandroni and Sletten8} Teenagers are particularly at risk for this due to their higher metabolisms and thus higher needs for fluid and salt intake to maintain adequate blood volumes. This all begins with inadequate preload back to the heart—preload failure. Fortunately for human beings who must remain upright throughout the day, we have a skeletal muscle pump (particularly the muscles of the legs) that can augment preload back to our central cardiovascular system. Unfortunately for the most affected patients, this is often a weakness due to deconditioning as a result of feeling poorly for months or even years prior to presentation. ^{Reference Joyner and Masuki9} It is not unheard of to see previously healthy patients now presenting to clinic in a wheelchair due to severe leg weakness and excessive sympathetic activity. Fortunately, through dedicated exercise, the skeletal muscle pump can be restored, which leads to significant improvement in symptoms.

The Fontan operation is the final palliative procedure for patients with single ventricle heart disease, designed to route systemic venous return directly to the lungs to allow the heart to solely supply blood to the systemic arterial circuit. ^{Reference Fontan and Baudet10,Reference Rychik, Atz and Celermajer11} Once completed, pulmonary blood flow passively flows following a favourable pressure gradient, augmented by negative inspiratory pressure and the peripheral skeletal muscle pump. Survival after Fontan has improved over time; however, morbidity in many forms, including Fontan failure, remains. ^{Reference Rychik, Atz and Celermajer11–Reference Khairy, Fernandes and Mayer13} Fontan failure is a term describing deterioration of clinical status in these patients due to various causes, but often characterised by elevated central venous pressure and decreased cardiac output. ^{Reference Rychik, Atz and Celermajer11,Reference Deal and Jacobs14–Reference Book, Gerardin, Saraf, Marie Valente and Rodriguez16} Pathology underlying this derangement can be cardiac, pulmonary, hepatic, or lymphatic; often, a combination of these pathologies is present and additive to overall poor status. A common theme, however, is that augmentation of cardiac output is difficult, given the lack of a subpulmonary ventricle to offload elevated venous pressure or improve pulmonary blood flow—preload failure. This is particularly noteworthy during exercise, as cardiac output to exercising muscles is known to be limited due to inadequate systemic ventricular preload. ^{Reference La Gerche and Gewillig17,Reference Goldberg, Avitabile, McBride and Paridon18} It is here that augmentation of the skeletal muscle pump is advantageous, as strengthening this pump specifically combats the deleterious effects of gravity on these patients’ main limiting factor in improving cardiac output. ^{Reference Cordina, O’Meagher and Karmali19} Unfortunately for many Fontan patients, the skeletal muscle pump is not a strength. Patients are often less active than other children, not participating in sports, or frankly sarcopenic. ^{Reference Tran, D’Ambrosio and Verrall20,Reference Ouimet-Grennan, Guerrero-Chalela and Therrien21} Again, this can be addressed through dedicated exercise with focus on the lower extremities. ^{Reference Cordina, O’Meagher and Karmali19}

In the sections that follow, we will discuss the physiology and function of the skeletal muscle pump and how exercise focused on lower extremity muscles improves physiology and outcomes. The accompanying Figure 1 dramatises these findings. Finally, we will discuss practical ways to implement exercise counselling and participation in these seemingly divergent but therapeutically congruent groups.

Figure 1.

Figure is the original artwork of Ms. Julia Moore, created specifically for this manuscript and edited further for this purpose.

The skeletal muscle pump

Tall, upright animals such as giraffes and horses have tight leg fascia, which prevents venous pooling and dependent oedema. ^{Reference Hargens, Millard, Pettersson and Johansen22–Reference Ahmed, Kulikowska, Ahlmann, Berg, Harrison and Elbrønd24} Humans rely on a different mechanism, a well-developed skeletal muscle pump. Physiologists in the early twentieth century had already recognised the dynamic nature of venous return to the heart related to changes in body position, respiratory muscle use, splanchnic vascular tone, exercise, and even digestion. ^{Reference Sewall25} It was not until the 1940s that academic description of the skeletal muscle pump (and its effects on venous return) began to appear. ^{Reference Hellebrandt, Katharine Cary, Neall Duvall, Jane Houtz, Skowlund and Apperly26,Reference Barcroft and Dornhorst27} The term skeletal muscle pump refers to the mechanism, present at rest but particularly important during exercise, whereby muscle contraction enhances venous return by squeezing local blood centrally toward the heart. ^{Reference Joyner and Casey28} When a person is standing, postural muscles in the legs alternately contract and relax to keep the body in balance. This muscle activity promotes venous return and helps to maintain central venous pressure while also keeping venous pressure low in the lower legs and feet. ^{Reference Folkow, Gaskell and Waaler29–Reference Miller, Pegelow, Jacques and Dempsey31} The force of this pump exceeds the effects of gravity—consistent with this finding is that exercise performance as measured by peak oxygen uptake is 5–10% higher in the upright versus supine position. ^{Reference Diaz, Hagan, Wright and Horvath32} In addition, skeletal muscle relaxation causes vascular suction that increases blood flow from arteries and more distal veins into that muscle prior to the next contraction. This vascular flow pattern maintains a favourable perfusion gradient in the legs during upright exercise by emptying the distal veins and keeping venous pressure low. ^{Reference Pollack and Wood33–Reference Sheriff38}

To understand the skeletal muscle pump, it is important to understand what it must overcome. Compared to arteries, veins are thin-walled vessels with less smooth muscle and elastin. ^{Reference Rothe39} Venous compliance is up to 30 times that of arteries, ^{Reference Guyton, Armstrong and Chipley40} which explains why even as 2/3 of total blood volume is present in the systemic veins, central venous pressure remains low. ^{Reference Hall41} Gravity further exacerbates resting conditions, as 70% of the circulating blood volume is below the level of the heart when a person is upright. ^{Reference Rowell42} The anatomy of the veins aids unidirectional blood flow. One-way valves, present particularly in peripheral veins located in large skeletal muscles, help direct flow away from the limb and toward the heart by preventing backflow. As evidence of their importance, studies show that patients with venous incompetence demonstrate inferior leg blood flow response to exercise during upright posture. ^{Reference Padberg, Johnston and Sisto43–Reference Nådland, Wesche, Sheriff and Toska45}

During exercise, physiology is further altered—while skeletal muscle comprises around 40% of the body mass, muscle-specific blood flow can increase almost 100-fold during intense exercise. ^{Reference Gliemann, Vestergaard Hansen, Rytter and Hellsten46} This increased muscle perfusion occurs through multiple overlapping mechanisms. Adrenaline leads to increased oxygen demand, which is met by augmentation of skeletal muscle perfusion via local vasodilators, including nitric oxide and adenosine triphosphate, and blunting of local sympathetic vasoconstriction in the contracting muscles. ^{Reference Joyner and Casey28,Reference Gliemann, Vestergaard Hansen, Rytter and Hellsten46} It is against this cascade of hyperaemic recruitment that the skeletal muscle pump must work. To begin, a particular physiologic consequence occurs that indirectly improves venous return. Specifically, as skeletal muscles are engaged and blood flow recruited, this necessarily means that perfusion of “less recruitable” venous beds, for example the splanchnic venous plexus, decreases. As the skeletal muscle pump can augment venous return more readily, overall net venous return increases. Increased adrenergic tone during exercise improves muscular contraction, which promotes more effective squeeze of venous blood back toward the heart. A single peripheral muscle contraction can move 40% of the local intramuscular venous blood volume centrally. ^{Reference Stewart, Medow, Montgomery and McLeod47}

Contraction of the leg musculature may provide upwards of 30% of the energy needed to pump blood while running. ^{Reference Stegall34} While thigh and even foot muscles contribute to the skeletal muscle pump, the calf muscles are particularly important. The calf is comprised of two muscles, the gastrocnemius and soleus, with the soleus having the larger volume and surface area. During contraction, the intramuscular pressure within the calf can exceed 200 mmHg. ^{Reference Pollack and Wood33,Reference Ludbrook48} While the gastrocnemius muscle (and its tendons) spans the ankle and knee joints, allowing it to contract only during knee extension, the soleus muscle contracts regardless of knee angle. Thus, it provides consistent compression of the lower leg veins and is the predominant muscle augmenting venous return. ^{Reference Padberg, Johnston and Sisto43} Calf raises, running, and jumping all engage and strengthen the soleus muscle, improving its ability to ultimately provide preload.

Exercise training improves the strength of the skeletal muscle pump and thereby venous return to the heart. ^{Reference Padberg, Johnston and Sisto43,Reference Zion, De Meersman, Diamond and Bloomfield49–Reference Ercan, Çetin, Yavuz, Demir and Atalay51} Verma et al. ^{Reference Verma, Garg and Xu52} published a statistical confirmation of the causal relationship of augmentation of skeletal muscle strength to improvement in preload, with the intent of highlighting the importance of preventing sarcopenia in patients at risk for orthostatic hypotension. ^{Reference Verma, Garg and Xu52} As exercise capacity increases in response to training, there is increased muscular capillary density, increased blood volume in general and locally, and increased muscle content of mitochondria. Conversely, patients with sarcopenia and decreased calf muscle mass will have worse venous return, placing them at risk for complications including orthostatic hypotension and syncope. ^{Reference Ercan, Çetin, Yavuz, Demir and Atalay51,Reference Soysal, Kocyigit, Dokuzlar, Ates Bulut, Smith and Isik53}

Augmentation of the skeletal muscle pump benefits patients with orthostatic intolerance

Muscle deconditioning is a key and perpetuating component of orthostatic intolerance, which makes exercise particularly difficult. ^{Reference Joyner and Masuki9} In studied patients, there is low calf blood volume as well as low muscle mass. ^{Reference Stewart, Medow, Montgomery and McLeod47} Furthermore, the reduced calf blood volume lessens calf muscle size and is associated with decreased muscular capacity to pump venous blood out of the legs. ^{Reference Hargens, Millard, Pettersson and Johansen22} Tendency toward venous vasodilation in these patients exacerbates lower extremity blood pooling and thus intolerance of position change—this is why more effective exercise counselling emphasises recumbent or semirecumbent position during exercise and focus on resistance training during the early stages. ^{Reference Joyner and Masuki9,Reference Chen, Du, Jin and Huang54}

While researchers have reported smaller left ventricular muscle mass in patients with postural orthostatic tachycardia syndrome ^{Reference Fu, Vangundy and Galbreath55} , it must be emphasised that it is not a cardiac-centric diagnosis, and instead noted cardiac changes occur secondary to chronic intravascular and thus intracardiac volume depletion, deconditioning, and lack of physical exercise. Terms such as “Grinch heart” ^{Reference Fu, Vangundy and Galbreath55} are harmful to a patient’s self-image and miss the mark on the causative physiology. Instead, it is primarily preload failure that leads to decreased cardiac stroke volume. ^{Reference Chen, Du, Jin and Huang54} This is deleterious over time, as the heart remodels within the setting of chronically low intracardiac volume. As these cardiac changes solidify, orthostatic intolerance is potentiated. Cardiac muscle deconditioning and reduced filling capacity then further lower stroke volume, contributing to tachycardia as a reflex response when the patient stands. ^{Reference Fu, Vangundy and Galbreath55} Low resulting stroke volume in these patients has also been associated with reduced peak oxygen uptake and delivery. ^{Reference Joyner and Masuki9} All of this creates negative feedback within the patient, as the above changes often lead to further exercise avoidance and thus more significant deconditioning.

The benefits of exercise in orthostatic intolerance have been demonstrated. In a report on military recruits with postural orthostatic tachycardia syndrome, those assigned to a 3-month exercise regimen were significantly less likely to report orthostatic symptoms subsequently. Additionally, more than half the patients in the exercise group no longer met criteria for postural orthostatic tachycardia at follow-up. ^{Reference Winker, Barth and Bidmon56} In a study of 100 participants meeting postural orthostatic tachycardia syndrome criteria who completed a programme of mild-to-moderate intensity progressive endurance training, 71% significantly improved to the point that they no longer met the diagnostic criteria at follow-up. ^{Reference George, Bivens and Howden57} Lower resting heart rates, improved stroke volume, and faster heart rate recovery after exercise compared to baseline were all reported. The effects were even noted in a 6–12 month follow-up after intervention. Finally, Fu et al. ^{Reference Fu, VanGundy, Shibata, Auchus, Williams and Levine58} performed a double-blind drug trial (n = 19 patients) comparing propranolol treatment to placebo for 4 weeks, followed by exercise training for 3 months, in patients meeting criteria for postural orthostatic tachycardia syndrome. Upright haemodynamics, symptoms, renal-adrenal responsiveness, and quality of life improved with exercise training, but not after propranolol.

Additional studies are worth highlighting. A small study involving 17 patients with orthostatic intolerance showed that short-term exercise training decreased upright hyperadrenergic state and resting heart rate, in part due to increasing sensitivity of the baroreceptor reflex. ^{Reference Galbreath, Shibata, VanGundy, Okazaki, Fu and Levine59} A larger study of 77 patients showed a significant reduction in the number of patients that had orthostatic tachycardia after a 6-month unsupervised outpatient cardiovascular exercise programme. ^{Reference Gibbons, Silva and Freeman60} This programme also reduced frequency of syncope and improved perceived quality of life based on EuroQol scaled assessment. Finally, Wheatley-Guy et al. ^{Reference Wheatley-Guy, Shea and Parks61} compared outcomes in orthostatic intolerance patients undergoing a 3-month period of semi–supervised exercise sessions to standard of care. The exercise sessions consisted of a graded exercise regimen building to a target of 150 minutes/week of exercise that started with recumbent exercises. Utilising exercise testing at follow-up, the exercising orthostatic patients demonstrated significant improvement in oxygen consumption at peak exercise, workload time, and time until onset of symptoms. The exercising patients also tested superiorly in orthostatic intolerance domain score in the COMPASS tool, which assesses autonomic symptoms. ^{Reference Wheatley-Guy, Shea and Parks61}

Exercise training benefits patients after Fontan palliation

Exercise limitation is exceedingly common after Fontan palliation. ^{Reference Shafer, Garcia, Babb, Fixler, Ayers and Levine62} Numerous studies have demonstrated that Fontan patients have decreased maximal exercise capacity compared to the general population. ^{Reference Goldberg, Avitabile, McBride and Paridon18,Reference Cordina, O’Meagher and Karmali19,Reference Shafer, Garcia, Babb, Fixler, Ayers and Levine62,Reference Goldberg, Zak and McCrindle63} Furthermore, the extent of limitation is an important predictor of morbidity and mortality. ^{Reference Goldberg, Avitabile, McBride and Paridon18,Reference Goldberg, Zak and McCrindle63–Reference Scheffers, Berg, Ismailova, Dulfer, Takkenberg and Helbing65} In counselling these patients, it is not hyperbole to state that maintaining exercise capacity does much to maintain life itself, even during the young adult years after Fontan.

A prevailing explanation for exercise limitation is a restricted ability to augment cardiac output given the absence of a subpulmonary ventricle, a specific form of preload failure. ^{Reference Shafer, Garcia, Babb, Fixler, Ayers and Levine62} During exercise in patients with biventricular physiology, cardiac output is augmented to match metabolic demand by both increasing heart rate and increasing stroke volume from both the left and right ventricle. ^{Reference Shafer, Garcia, Babb, Fixler, Ayers and Levine62} Furthermore, a subpulmonary ventricle offloads systemic venous pressure and maximises pulmonary blood flow, thereby recruiting additional pulmonary vasculature and matching perfusion to ventilation. ^{Reference Khoury and Cordina66} In Fontan patients without a subpulmonary ventricle, there is increased systemic venous pressure at rest and exercise and a “starved” pulmonary vascular bed for blood flow, especially during exercise; this must be overcome as able with the “lesser” pumps including the skeletal muscle pump. ^{Reference Khoury and Cordina66} Investigators have specifically examined the role of both the skeletal muscle pump and the ventilatory pump (decreasing intrathoracic pressure during inspiration) in augmenting Fontan cardiac output. In a case-control study, Shafer et al. ^{Reference Shafer, Garcia, Babb, Fixler, Ayers and Levine62} utilised exercise testing protocols systematically to isolate the relative contribution of the respective pumps and reported that much of the increase in stroke volume and cardiac output was due to systemic muscle pump augmentation, with a small additional contribution from the respiratory pump.

Cordina et al. ^{Reference Cordina, O’Meagher and Karmali19} examined whether patients with Fontan circulation have altered muscle mass and metabolic dysfunction compared to healthy controls. Subjects underwent both cardiopulmonary exercise testing and magnetic resonance spectroscopy to assess skeletal muscle activity, as well as dual X-ray absorptiometry scans to assess lean mass. The study found that Fontan patients had significantly reduced lean mass, decreased aerobic muscle activity, and diminished exercise capacity. ^{Reference Cordina, O’Meagher and Karmali19} Furthermore, muscle mass positively correlated with peak exercise capacity and stroke volume—undergirding recommendations for physical fitness in these patients. ^{Reference Cordina, O’Meagher and Karmali19} Avitabile et al. ^{Reference Avitabile, Leonard and Zemel67} also reported this link via a prospective analysis comparing 50 Fontan patients to 992 healthy controls to identify risk factors for decreased lean mass and its link with exercise capacity. Not surprisingly, the Fontan patients had significantly lower total body lean mass and leg lean mass. Furthermore, leg lean mass was positively associated with peak oxygen consumption and exercise capacity. ^{Reference Avitabile, Leonard and Zemel67} Avitabile et al. ^{Reference Avitabile, Goldberg and Leonard68} separately evaluated this relationship in an additional study comparing leg lean mass and exercise performance. By utilising exercise testing and cardiac magnetic resonance imaging, the authors reported a positive relationship between leg lean mass and change in cardiac output from rest to exercise. Finally, the lack of pulsatility in the Fontan circuit is theorised to lead to less recruitment of the pulmonary vasculature compared to biventricular physiology. ^{Reference Khoury and Cordina66} However, a case study by Cordina et al. ^{Reference Cordina, Celermajer and d’Udekem69} suggests that pulsatile flow (demonstrated by pulsed wave Doppler) can be generated in the Fontan circuit, specifically by engaging the skeletal muscle pump during lower leg exercise.

While there is much evidence demonstrating that patients with Fontan circulation have diminished exercise capacity, this is not universally true. In fact, some so-called “Super Fontans” have normal to supraphysiologic testing results. ^{Reference Tran, Celermajer and Ayer70} This has prompted further exploration to explain the contributing mechanisms. Tran et al. ^{Reference Tran, Celermajer and Ayer70} compared so-called “Super Fontan” patients to Fontan patients with impaired exercise performance. While there was no difference in level of self-reported activity as adults, there was a significant difference in self-reported childhood physical activity, with Super Fontan patients having higher activity levels. ^{Reference Tran, Celermajer and Ayer70} This serves as a warning to paediatric cardiologists—encourage physical activity in your patients unless there is a compelling reason for restriction.

Given the evidence that greater lean muscle mass is associated with improved exercise capacity, researchers have examined whether a weightlifting regimen might improve exercise capacity after Fontan. Cordina et al. performed a prospective study involving adult Fontan patients and controls enrolled in a resistance training programme with the goal of improving calf muscle mass and exercise capacity. At follow-up, lean mass was significantly higher in the weightlifting exercise Fontan group. Interestingly, the proportion of respiratory dependent venous inflow (during inspiration) decreased in the patients who underwent the exercise programme, meaning flow occurred more favourably at other times presumably due to improved musculovenous propulsion. Peak oxygen consumption was significantly higher in the patients who went through the training programme as well. ^{Reference Cordina, O’Meagher and Karmali19} Scheffers et al. ^{Reference Scheffers, Helbing and Pereira71} evaluated the effects of 12 weeks of resistance training on Fontan patients. At follow-up, peak oxygen consumption and 6-minute walk test improved significantly. ^{Reference Scheffers, Helbing and Pereira71} Additionally, stroke volume, aortic flow, and inferior caval venous flow all improved after exercise participation. Interestingly, superior caval vein flow was not significantly different, suggesting that lower leg muscles were the major driver in the improvements seen. Sutherland et al. ^{Reference Sutherland, Jones and Westcamp Aguero72} conducted a prospective study in teenage Fontan patients who were divided into home-based versus hospital-based exercise groups. Both groups participated in 2 months of aerobic activity and resistance training. The authors reported improvement in exercise capacity as measured by oxygen consumption and 6-minute walk test in all patients after training. ^{Reference Sutherland, Jones and Westcamp Aguero72} Importantly, there was no difference in exercise testing between the groups, suggesting that home-based exercise programmes can be as effective as hospital-based routines. ^{Reference Sutherland, Jones and Westcamp Aguero72} Finally, Scheffers et al. ^{Reference Scheffers, Berg, Ismailova, Dulfer, Takkenberg and Helbing65} performed a systematic review of exercise training after Fontan palliation, which included 22 articles with a total of 264 patients. There were 16 cohort studies, seven of which were paediatric. Described training regimens included aerobic and resistance training, and three of the studies involved only inspiratory muscle training. Peak oxygen consumption increased significantly in 56% of the studies after the ascribed training programme. This is impressive considering these exercise interventions were not uniform and variable in efficacy and implementation. All the studies that analysed 6-minute walk times reported improvement, half of the studies looking at estimated cardiac output by magnetic resonance imaging or echocardiogram showed benefit, and all the studies looking at parent report of quality of life were positive. None of the studies reported negative outcomes associated with an exercise intervention. The authors concluded that exercise training in Fontan patients has positive effects on exercise capacity, cardiac function, and quality of life and therefore should be encouraged. ^{Reference Scheffers, Berg, Ismailova, Dulfer, Takkenberg and Helbing65}

Exercise counselling, training, and prescription

Effective exercise counselling depends on prudent explanation of the benefits of exercise, as well-informed patients make the best decisions for their health. The myriad health benefits of exercise—improved bone health, reduced adiposity, improved psychosocial well-being, reduced risk of depression, etc., are true for our patients as well. ^{Reference Watts, Beye and Siafarikas73–Reference Rodriguez-Ayllon, Cadenas-Sánchez and Estévez-López77} Beyond strengthening the skeletal muscle pump, consistent exercise is particularly beneficial for orthostatic patients due to the favourable vascular adaptations that take place. Endurance training increases plasma volume, thus mitigating the risk of volume depletion. ^{Reference Lavie, Arena and Swift78} Improved preload to the heart is also achieved by this adaptation, leading to improved cardiac output which is matched by more rich capillary development in the exercising muscles, allowing better oxygen delivery. ^{Reference Murias, Kowalchuk and Paterson79,Reference Hackney and Schumann80} With this knowledge in mind, a provider does well to convey this information to patients with a positive tone and expectation. Many orthostatic patients have tried to exercise previously and failed to see success—making it clear that this time will be different, because particular strategies that will be employed, can help patients overcome their trepidation to try again. Next comes specific advice to help patients navigate around pitfalls. For patients with orthostatic intolerance, the most important advice is dizziness during exercise should be avoided. Patients may be counselled that symptoms such as heart racing, shortness of breath, and fatigue can be “pushed through,” as these often go with physical conditioning; however, lightheadedness should not be ignored. As mentioned previously, dizziness signals decreased cerebral perfusion, which leads to a disproportionate increase in adrenaline production and effect. This is deleterious to progress, as patients will report excessive fatigue after exercise and thus not follow a consistent exercise plan to improve. Instead, the plan should be to increase fluid and salt intake significantly ∼ 30 minutes prior to exercise, stop exercising when importantly dizzy, further increase fluid and salt intake if dizziness is occurring too soon into exercise, and try to do exercise that is easiest orthostatically when starting out. This is where aquatic type exercise and aquatic physical therapy are particularly helpful.

Exercising in water is advantageous to orthostatic intolerance patients and those after Fontan palliation. This advantage comes from the effect of water pressure on blood pooling—when standing or exercising in water, less blood can pool in the lower extremities compared to standing on land. ^{Reference Gabrielsen, Bie and Holstein-Rathlou81–Reference Bissacco, Mosti and D’Oria83} This is particularly helpful for patients after Fontan who have lower extremity venous stasis and orthostatic patients with excessive venous compliance. While older studies have theoretically cautioned Fontan patients from diving, recent reports evaluating Fontan haemodynamics during swimming or diving found similar physiology to controls without adverse events. ^{Reference Paech, Gebauer and Weidenbach84,Reference Paech, Gebauer and Weidenbach85} As the skeletal muscle pump is augmented, preload is increased which will increase cardiac output and importantly cerebral perfusion. This translates ultimately into further ability to exercise. We see this in practice often, particularly in orthostatic intolerance patients, who finally make progress through aquatic exercise. Exercising in water can take the form of swimming, water aerobics, or in those who need additional guidance, aquatic physical therapy.

Aquatic physical therapy

Aquatic physical therapy is offered through many physical therapy providers. Patients can check with local practices to see where this is offered, and insurance coverage is typically obtained utilising ICD-10 codes Q24.0 for congenital heart disease, R55 for syncope, R53.1 for weakness, and G90 for autonomic nervous system dysfunction, depending on the situation. A typical prescription reads:

Frequency: 2 times weekly

Duration: 30–60 minutes

Number of sessions: 12–18

Goals: improve aerobic conditioning, strengthening of calf muscles

Activity limitations: stop exercise if significant dizziness

In patients who are hesitant to utilise aquatic physical therapy, counselling regarding its benefit is important. From a cardiac standpoint, it has been shown to favourably affect heart rate variability, which is an important marker of cardiac health, among postmyocardial infarction patients. ^{Reference Jug, Vasić, Novaković, Avbelj, Rupert and Kšela86} From a musculoskeletal standpoint, the benefits include reduced pain in exercising joints, easier balance, and consistent resistance training as extremities are moved through the water. ^{Reference Becker87,Reference Barker, Talevski, Morello, Brand, Rahmann and Urquhart88} A recent meta-analysis and systemic review concluded that aquatic exercise benefits these domains, as well as overall quality of life and physical function compared to “no exercise” or land-based exercise. ^{Reference Wang, Wang, Chen, Ruan and Dai89} Typically, patients can make progress with physical therapy and then eventually transition to independent exercise involving swimming or other aquatic activities going forward.

For patients and physical therapists that need a more structured plan, Children’s Hospital of Philadelphia has modified the Dallas protocol for orthostatic intolerance patients and provided detailed guidelines on types of exercise and calendars of advancement. ^{Reference Fu and Levine90} Again, even if using a protocol, therapists should be instructed that pushing patients through dizziness is counterproductive and can even lead to vasovagal syncope. Specific exercise prescriptions can also be utilised. Within these prescriptions, calculated maximal steady state heart rate, heart rate-based training zones, and exercise modes (i.e. recumbent activity, upright stationary bike, treadmill with and without incline) can be explicitly written for an individual patient and followed for progress. ^{Reference Wang, Wang, Chen, Ruan and Dai89–Reference Miranda, Boris, Kouvel and Stiles93} It must be remembered, however, that heart rate in these specific populations is often confounded by factors other than training and effort during exercise (i.e. beta-blocker use, chronotropic incompetence in Fontan patients, or suboptimal hydration status in those with orthostatic intolerance). For this reason and for ease of use in young patients, using symptomatic limitation (dizziness) is often preferred. For Fontan patients, building up exercise capacity dutifully might be a superior strategy, as studies have shown benefits of even submaximal exercise training. ^{Reference Hedlund and Lundell91} Fontan exercise protocols have been described. ^{Reference Herrmann and Selamet Tierney92} As mentioned previously, these range from closely supervised hospital-based interventions to telemedicine-supported home-based sessions, and high-intensity interval training to deliberate submaximal limitation. While the best regimen to follow is up for debate, the salient point is to exercise, to exercise often, and to benefit from this exercise in all the ways described.