Management of acute intradialytic cardiovascular complications: Updated overview (Review).

An increasing number of patients require renal replacement therapy through dialysis and renal transplantation. Chronic kidney disease (CKD) affects a large percentage of the world's population and has evolved into a major public health concern. Diabetes mellitus, high blood pressure and a family history of kidney failure are all major risk factors for CKD. Patients in advanced stages of CKD have varying degrees of cardiovascular damage. Comorbidities of these patients, include, on the one hand, hypertension, hyperlipidemia, hyperglycemia, hyperuricemia and, on the other hand, the presence of mineral-bone disorders associated with CKD and chronic inflammation, which contribute to cardiovascular involvement. Acute complications occur quite frequently during dialysis. Among these, the most important are cardiovascular complications, which influence the morbidity and mortality rates of this group of patients. Chronic hemodialysis patients manifest acute cardiovascular complications such as intradialytic hypotension, intradialytic hypertension, arrhythmias, acute coronary syndromes and sudden death. Thus, proper management is extremely important. Copyright: © Timofte et al.

1. Introduction

Chronic kidney disease (CKD) represents a public health concern as it affects over 50 million people worldwide, and is more and more commonly encountered, especially due to the increased incidence of high blood pressure (HBP) and diabetes mellitus (DM). Over 1 million CKD patients require renal replacement therapy (RRT) through dialysis and renal transplantation (1). DM, HBP and a family history of kidney failure are all major risk factors for CKD (2-4). Official data in the USA reported over 661,000 patients with advanced stage CKD, out of which 468,000 receive RRT through dialysis and 193,000 have a functional kidney transplant (5-7). The acute complications occur quite frequently during dialysis, and are caused by complex mechanisms, which are insufficiently known (8). Among these, the most important are cardiovascular complications, which influence the morbidity and mortality rates in this group of patients (8).

2. Acute cardiovascular complications of hemodialysis

Advanced stage CKD is associated with the increased risk of cardiovascular affectation. Thus, in the case of chronic dialysis patients, cardiovascular disease is identified in a large percentage of patients. An important role in its onset, in addition to factors such as mineral bone disease and patient comorbidities (e.g., HBP, hyperlipidemia, hyperglycemia, homocysteine, hypeuricemia), is chronic inflammation (9-12).

Research particularly describe a smaller total antioxidant capacity (TAC) in healthy controls than in diabetic hemodialysis patients; oxidative stress is one of the main factors leading to the onset of CKD in this group of patients (13).

Acute intradyalitic cardiovascular complications besides chronic cardivascular affectation are identified in chronic hemodialysis patients. These are summarized in Table I and are: intradialytic hypotension (IDH), HBP, arrhythmias, acute coronary syndrome (unstable angina/myocardial infarction) and sudden death (1).

Table I

Acute cardiovascular complications of hemodialysis (1).

Cardiovascular complications
1. Intradialytic hypotension
2. High blood pressure
3. Arrhythmias
4. Acute coronary syndrome (unstable angina/myocardial infarction)
5. Sudden death

3. Intradialytic hypotension

IDH is quite commonly encountered. It has an impact on the quality of lives of these patients, on the cost of dialysis and is associated with mortality. There is no clear definition of IDH; two factors are taken into account in clinical practice: The decrease in systolic pressure under 90 mmHg (14) or the symptomatic intradialytic decrease in systolic pressure by more than 20 mmHg compared to the value from the beginning of dialysis (15). Studies have demonstrated the strong association between the decrease in systolic pressure under 90 mmHg during dialysis in over 30% of treatments and an increase in mortality (15).

Epidemiology

IDH reporting differs according to the defining criteria. Thus, IDH episodes can vary between 5 and 30% of all hemodialysis treatments (16-18). A study that analyzed a number of 44,801 hemodialysis treatments performed on 1,137 patients found IDH present in 75% (16).

Risk factors and physiopathological particularities

There are several groups of risk factors for IDH onset, and they concern the patient, the dialysis machine or the medical manoeuvres (iatrogenic factors). Hemodialysis patients with direct or indirect cardiovascular affectation, that is to say elderly patients undergoing dialysis for a long time, diabetic patients, patients with low arterial pressure prior to dialysis, patients with systemic infections, arrhythmias, valvulopathy, myocardial infarction, hemorrhage, or patients with hypoalbuminemia are predisposed to IDH (16,17,19,20) (Fig. 1).

Figure 1

The impact of cardiac flow decreases in chronic hemodialysis patients (14). IDH, intradialytic hypotension.

Dialysis parameters, such as acetate dialysis, the dialysate composition and temperature (20-22), the ultrafiltration rate and the total ultrafiltration volume (23), the rapid reduction in plasma osmolality, incorrect determination of the dry weight, antihypertensive medication before dialysis and food ingestion pre- or intradialysis can also represent risk factors for IDH onset.

IDH may occurs in the case of patients with acute hemodialysis, air embolism or allergic reaction to the dialysate (17,18,20,24). Table II summarizes the factors leading to IDH.

Table II

Intradialytic hypotension: Etiologic factors (24).

Intradialytic hypotension
Decline in circulating volume	Hemorrhage
	The decrease in vascular filling rate
	Excessive ultrafiltration
Reaction to dialysate
Cardiac factors	Myocardial infarction
	Organic cardiac disease
	Arrhythmias
	Cardiac tamponade
Hemolysis
Defective vasoconstriction	Patient-related factors
	Neuropathy
	Hypertensive medication
	Insufficient norepinephrine plasma level
	Decrease in RAAS sensitivity
	Splanchnic vasodilation secondary to food ingestion
	Tissue ischemia
	Sepsis
	Anemia
	Inflammation
	Hemodialysis-related factors
	Vasodilation secondary to acetate dialysate
	Low calcium concentration in the dialysate
	Complement activation
	Generation of cytokines

RAAS, renin-angiotensin-aldosterone system.

Intradialytic ultrafiltration causes the decrease in venous return, with the subsequent decrease in cardiac flow. This phenomenon is emphasized in patients with cardiac damage in whom the ventricular allure or the myocardial contractility do not increase to compensate (25,26). Several studies have shown that the optimal ultrafiltration rate is 10 ml/kg/h; an ultrafiltration rate higher than 13 ml/kg/h is associated with an increased IDH risk and an elevation in mortality (27).

The decrease in blood volume in hemodialysis patients takes place along with peripheral vasodilation (25,28-30). There are several mechanisms which may produce this phenomenon, such as the release of adenosine in response to tissue ischemia, the increase in the synthesis of the vasodilating endogenous substances (nitric oxide) and the inadequate decrease in the vasopressin plasma levels (31-37).

Clinical presentation

IDH patients can be asymptomatic or can suffer from dizziness, muscle cramps, nausea, vomiting and dyspnea at rest. Vagal symptoms such as yawning, ‘sighing’ or hoarseness can occur before a decrease in BP (17).

Management

IDH management involves two aspects: Emergency treatment and the prevention of relapses.

Emergency IDH treatment

In emergency IDH treatment, the rate of ultrafiltration is decreased/stopped, the patient is placed in the Trendelenburg position, and oxygen (O2) is administered (38,39). If blood pressure does not increase following these techniques, hyperosmolar solutions or albumin is administered in bolus; these include isotonic saline solutions (40,41) (Fig. 2).

Figure 2

Intradialytic hypotension: Emergency management (38-41). UF, ultrafiltration.

IDH management to prevent relapse

The general measures for the prevention of IDH relapses involve a re-evaluation of the dry weight (42-46), avoiding intradialytic food ingestion, avoiding the administration and diminishing salt intake). Patients should also be advised regarding caffein consumption as studies have shown that 3 or more coffees daily increase the risk of a higher diastolic BP, potassium and interdialytic weight gain (IDWG). IDWG in hemodialysis patients and caffeine may alter the cardiovascular response even in healthy people (47-49).

The composition of the dialysate (Ca ≥2.25 mEq/l, Mg ≥1.0 mEq/l and Na) (50-54) and the dialysate temperature (low dialysate temperature increases hemodynamic stability) (55-68) must be reassessed, as complementary measures to diet adjustment.

Patients exhibiting recurrent IDH may be administered midodrine (2.5 or 5 mg) 15-30 min prior to dialysis (63,69-73). Sertraline, vasopressine and carnitine can also be administered (74,75).

The evaluation of the cardiac function and the treatment of anemia with erythropoiesis-stimulating agents (which increase cardiac flow) should be performed in patients with recurrent IDH. It should also be mentioned that chronic dialysis patients who are prone to recurrent IDH should have a longer hemodialysis session, and another RRT method, such as hemodiafiltration or peritoneal dialysis, respectively, should be considered (76) (Fig. 3).

Figure 3

Recommended measures for the prevention of IDH. IDH, Intradialytic hypotension.

4. Intradialytic hypertension

Intradialytic hypertension (HBP) has been defined as an increase in intradialytic systolic pressure by ≥10 mmHg compared to pre-dialysis systolic pressure and it has been confirmed to be associated with increased mortality in dialysis patients (77). Some patients develop HBP during the last part of the dialysis session, a moment when the hydric excess has been ultrafiltered. The frequency of intradialytic HBP varies, even in the same patient, and the mechanisms are not clear; there are proofs related to the alteration of the nitric oxide/endothelin-1 balance and/or endothelial dysfunction (78,79).

Intradialytic HBP occurs in 10-15% of hemodialysis patients (80). The frequency of intradialytic HBP episodes varies; a study undertaken during a 6-month period showed that 90% of the patients experienced at least one HBP episode (77). The 5-year analysis of a large number of hemodialysis treatments (n>100,000) showed an increase in intradialytic systolic pressure by at least 10 mmHg in 10% of the cases; the same study concluded that the mortality risk occurs once the intradialytic pressure begins to increase, irrespective of value, and rises with a higher BP value (81). Other studies have demostrated that an increase in systolic pressure by 5 or 10 mmHg during dialysis is associated with an increase in patient mortality (80).

Studies have shown that the onset of intradialytic HBP is associated both with a volume overload between dialysis sessions and with elevated values of intradialytic pressure (82,83). Patients with IDH generally have a smaller dry weight than most hemodialysis patients, have a smaller IDWG and lower pre-dialysis blood pressure (81). These patients do not have clinical signs of hyperhydration, which causes the prescription of a smaller ultrafiltration volume than the one needed, without the lowering of the arterial pressure (80). The intensive intradialytic ultrafiltration for several weeks resulted in the decrease in intradialytic pressure, emphasizing the fact that the expansion of the extracellular volume, even in the absence of clinical signs of volemic overload can mediate HBP (82,84). The intradialytic osmolar changes contribute to the arterial pressure changes, irrespective of the calculated ultrafiltration rate (85).

The composition of the dialysate in establishing blood pressure values plays an important role, alongside volume overload in the chronic dialysis patients. In this respect, the clinician focuses on the sodium, potassium and calcium concentrations of the dialysate (86-88).

Sodium

A retrospective study undertaken on 113,255 hemodialysis patients over 5 years, highlighted the fact that patients with intradialytic HBP have a series of common characteristics, such as malnutrition markers, the lack of correct feeding or hydration, and lower pre-dialysis values of urea and creatinine, serum albumine, and normalized protein appearance (nPNA); they have smaller interdialysis body weight and weight gain compared to most dialysis patients (81). Sodium in the dialysate represents a key element in modulating intradialytic blood pressure values; it has been found that the sodium concentration in the dialysate is higher compared to the serum concentration of pre-dialysis sodium in intradialysis HBP patients (increased sodium gradient) (86).

Potassium

In patients who received a low-level potassium dialysate, blood pressure values were decreased after the first hour of the dialysis session (87); low-level potassium dialysate is also associated with rhythm disorders in dialysis patients (89).

Calcium

Calcium levels in the dialysate influence myocardial contractility and vascular tone (90). A high calcium level in the dialysate is associated with vascular hyperactivity and possibly with intradialysis HBP (88,91). Literature data show that intradialysis hypertension is associated with an increase in vascular resistance and less with extracellular volume overload (78,92). An increase in vascular resistance is likely related to the method of dialysis per se but, on the other hand, patients with interdialysis HBP have certain common comorbidities, including ischemic coronary disease, heart failure, a history of vascular accident, and peripheral vascular disease (81).

Dialysis patients with intradialysis HBP have endothelial dysfunction, with an imbalance between vasoconstrictor substances [endotheline 1 (ET-1) and asymmetric dimethylarginine (ADMA)] and vasodilator substances [nitric oxide (NO)]. Studies have shown that ET-1 diminishes or increases during dialysis along with blood pressure (78,93,94) and that patients with intradialysis HBP have high levels of ET-1 post dialysis and a low NO/ET-1 ratio (95,96).

The direct involvement of the stimulation of the sympathetic nervous system in intradialysis HBP has not been demonstrated; recent research has shown that blood pressure increases during dialysis when the cardiac rhythm increases and the baroreflex activity is supressed, indicating an increased activity of the sympathetic nervous system (80).

Patients experiencing an increase in intradialysis blood pressure can be asymptomatic or can complain of headaches, profuse perspiration, thoracic discomfort, dispnea, palpitations or anxiety (78).

There is no optimal therapeutic approach for HBP. Taking into account the association of HBP with volemic overload, it is necessary to accurately establish the dry weight (97).

Considering the role of ET-1 in causing IDH, carvedilol may play a beneficial role, since it is an inhibitor of ET-1 release. A pilot study, which lasted for 12 weeks, found that administration of carvedilol (50 mg twice/day) was asociated with a decreased frequency of HBP episodes from 77 to 28% during hemodialysis sessions (98). Similarly, a reduction in sodium concentration in the dialysate under the patient's serum sodium level can trigger a decrease in blood pressure values during dialysis sessions (99).

5. Arrhythmias

Definition

Hemodialysis patients quite frequently present with hydroelectrolytic and acid base imbalances both during and between treatment sessions, which can cause heart rhythm disorders (100).

In 2013, United States Renal Data System (USDRS) reported a mortality rate of 198/1,000 patients/year, 40% of the deaths having a cardiovascular cause. Among the cardiovascular causes, 26% were cardiac arrhythmias (101). In addition, atrial fibrillation (AFi) was the most commonly found heart abnormality in clinical practice and affected more hemodialysis patients than the general population (102), with percentages varying between 14% (103) and 27% (104). The Framingham Study reported an incidence of 0.2% per year for AFi in the general population, for 20 years. In comparison, the Afi incidence in the hemodialysis patients reaches 1.25 episodes/100 patient-year (105).

Chronic hemodialysis patients have a higher risk to develop arrhythmias, taking into account the special context of the disease: The presence of certain structural and functional myocardial defects (interstitial fibrosis, decrease in coronary perfusion reserve, endothelial dysfunction), rapid hydroelectrolytic and blood pressure dynamic changes, as well as the use of certain drugs (100).

Intradialytic arrhythmias are generated by hydroelectrolytic and acid base disorders which occur quite frequently in the dialysis patients; all of these, along with the composition of the dialysate, create an ‘arrhythmogenic environment̓ (106). On the other hand, dialysis patients present cardiovascular comorbidities, such as myocardial ischemia and secondary anemia, which increase the risk for intradialytic arrhythmias (107).

A range of acid-base (pH) and electrolytic (especially in potassium, calcium and magnesium) changes, causing prolongation of the QT interval and associated with an increased risk of arrhythmias occur in the dialysis patients, both during and post-dialysis (108). Dialysis patients can develop atrial fibrilation during dialysis. The risk factors for the onset of Afi in these patients include ischemic coronary disease, old age, enlarged left atrium, the value of systolic pressure before the beginning of dialysis and the presence of peripheral vascular disease (104,105).

The clinical picture is influenced by the rapidity of the onset of rhythm disorder, the cardiac rhythm and the pre-existing cardiovascular pathology. Patients can be totally asymptomatic when the rate of ventricular contractions is within normal limits or can suffer from cardiac failure to collapse. If the ventricular rhythm is rapid, patients complain of palpitations, precordial pain, dizziness, nausea, and syncope. Cardiorespiratory arrest and sudden cardiac death (SCD) may also occur in very severe cases (100).

It is important to reduce structural myocardial changes, especially hypertrophy of the left ventricle, which predisposes to ischemia and arrhythmias. It is also necessary to optimize the dialysis parameters to ensure hemodynamic and electrolytic balance, to evaluate the drug treatment and its impact on the incidence and seriousness of malignant arrhythmias and SCD (100).

AFi treatment aims mainly to maintain the ventricular rate by administring antiarrhythmic medication or cardioversion, to improve symptomatology and to increase effort tolerance. AFi treatment also focuses on lowering the CVA risk, discontinuing the anticoagulant treatment, increasing the quality of life and the survival rate. An accepted alternative, although often secondary to antiarrhythmic medication is the strategy to simply control the rate of ventricular response of AFi by using node blocking agents in association with continuous anticoagulation (109).

Class 1A and 1C arrhythmic medications can ensure the rapid conversion of AFi to the sinus rhythm; for example, propafenone can be administered successfully both for paroxistic AFi and for the prevention of relapses. Taking into account that it is eliminated through the liver, it can be safely used to treat dialysis patients (102). As in most cases, caution is necessary when administered to patients with concomitant liver disease (110).

Digoxin is sometimes administered also in patients with AFi and cardiac failure. Taking into consideration that digoxin half-time is extended; this can cause arrhythmias in the presence of arrythmogenic factors such as hypopotassemia or class 1A arrhythmic medication. Digoxin toxicity causes bradycardia, different degrees of atrioventricular block, junctional tachycardia, ectopic ventricular activity and ventricular tachycardia. Oral administration of digoxin in dialysis patients at doses of 0.125 mg 3 or 4 times a week seems safe and efficient. A 0.125 mg/day dose may easily lead to toxic levels and 0.25 mg/day can be life threatening. For these reasons, digoxin administration in dialysis patients should be carried out with extreme care (100).

There are few studies that show that angiotensin-converting enzyme (ACE) inhibitor treatment in dialysis patients is associated with a lower number of AFi episodes as compared to this incidence in the general population.

AFi hemodialysis patients run the risk of bleeding and thrombosis (104). Because of their renal pathology, chronic hemodialysis patients have a high risk of bleeding, thus treatment with oral anticoagulants increases the risk by 3- to 10-times in this group of patients compared to the general population (111-113). On the other hand, the rate of thromboembolic events is much higher in these patients. Randomized studies have shown that oral anticoagulant prophylaxis is more efficient than aspirin in reducing the CVA risk associated with AFi in dialysis patients, although it increases the risk of bleeding and/or vascular calcifications (114). Until new data become available, treatment with oral anticoagulants and monitoring are recommended in hemodialysis patients with chronic AFi (105).

Implantable cardioverter defibrillator (ICD) implantation in dialysis patients who were resuscitated after a cardiorespiratory arrest significantly improved the survival rate (the risk of death decreases by 42%) (115). On the other hand, a meta-analysis of the existing data in the literature showed that the mortality rate in ICD dialysis patients is 2.7% higher than in non-dialysis patient (116). The use of ICD in dialysis patients has a series of adverse effects. It is associated with an increased risk of bleeding and infection. Positioning the ICD on the same side with the vascular approach is associated with a higher rate of stenosis and occlusion of the subclavian vein. Factors that should be taken into consideration in these patients include: Performing hemostasis with special care, avoiding post-implantation anticoagulation, placing of intravascular leads on the contralateral side of dialysis access and the use of high-output devices with left-sided prepectoral generator placement (116).

6. Acute coronary syndrome

CKD is associated with a high death risk of cardiovascular pathology (117-121). Most often patients with CKD have an acute myocardial infarction as the initial manifestation of ischemic heart disease, without previous signs of stable angina (122).

Cardiovascular diseases account for about 45% of the death in dialysis patients (123,124). Among these, approximately 10% are caused by ischemic coronary disease/coronary heart disease (CHD). Dialysis patients have a higher CHD incidence with a rate of death through myocardial infarction higher than the general population (125). The data reported by the 2018 Annual Data Report of the United States Renal Data System (USRDS) showed a 15.3% prevalence of acute myocardial infarction in the hemodialysis population (https://adr.usrds.org/2020/end-stage-renal-disease/8-cardiovascular-disease-in-patients-with-esrd) Similarily, in 2016, the adjusted mortality rate was 166 in 1,000 patients-year for hemodialysis patients; 37% of the deaths had cardiovascular causes, and 11% were due to myocardial infarction and CHD (125).

There are several types of CHD risk factors in the dialysis patients. In this respect, CHD onset can be favored by ‘traditional̓ risk factors or by uremia-related risk factors. The ‘traditional̓ risk factors include: DM (54%), low serum high-density lipoprotein (HDL) cholesterol (33%), HBP (96%), HVS diagnosed by electrocardiographic criteria (22%), sedentary life style (80%), old age (125), and smoking (126-128).

CKD is an independent CHD risk factor (129-131). Uremia and hemodialysis as treatment methods increase oxidative stress and the production of proinflammatory factors (132), and so creates a favorable envirnonment for the fast development of atherosclerosis (133-139).

Chronic hemodialysis patients exhibit the increased production of nitric oxide (NO) inihibitors, which cause vasoconstriction and HBP and augument the risk of acute cardiovascular events. ADMA, an endogenous NO inhibitor, is significantly elevated in chronic hemodialysis patients and is an important predictor for cardiovascular mortality in these patients (140-142). The amplification of oxidative stress in these patients represents an extra aggravating factor (143) and can be evaluated by determining the activity of certain antioxidant enzymes (144).

Hemodialysis patients present extensive vascular and valvular calcifications, associated with mineral and bone anormalities. These patients have been found to have increased phospho-calcium product, secondary hyperparathyroidism and increased calcium intake through the treatment with calcium-based phosphorus binders. In chronic hemodialysis patients, calcium is identified at the levels of vascular media and the intima, in atheroma plaque (145,146). The calcification of the vascular media is associated with an increase in arterial stiffnes, but not with atherosclerosis or the narrowing of the arterial lumen. Even in the absence of atherosclerosis or luminal narrowing, coronary media calcification can cause a decrease in diastolic filling, while the peripheral medial calcification increases cardiac afterload (145,146).

Hemodialysis patients are most often asymptomatic or have atypical symptoms which can delay the diagnosis and choice of therapeutic approach (147,148). Angina can occur during dialysis, and is precipitated by the exchange of fluids and by the episodes of IDH. Myocardial ischemia and the effort angina are covered in this group of patients because they are generally sedentary, or the level of their effort is very low. Patients with serious coronary lesions can suffer from acute coronary syndrome (ACS)-unstable angina, non-ST elevation myocardial infarction, or ST elevation myocardial infarction. The classic diagnostic triad (angina, increased biological markers and EKG changes) cannot be found in hemodialysis patients (147). EKG can show left ventricle hypertrophy in HD patients, which can mask the ST segment depression. The cardiac lesion markers (creatine kinase MB isoform and troponin I) can be elevated in dialysis patients in the absence of myocardial necrosis, as a reflection of cellular apoptosis or small vessel disease (149).

The prognosis of CKD and ACS patients are unfavorable, in spite of the present medical therapies and the revascularization techniques (150). Platelet antiagregants in ACS patients decrease the mortality risk, although they increase minor bleeding. Thus, clopidogrel administered to non-ST-segment elevation patients to prevent relapses (CURE Trial) proved beneficial (151,152). The PLATO Study (Platelet Inhibition and Patient Outcomes) showed that ticagrelor, an oral purinergic receptor inhibitor cleared by extrarenal mechanisms, reduced mortality and major cardiovascular events, being more efficient than clopidogrel in CKD and ACS patients (153). A recent meta-analysis showed that antiplatelet agents reduce the probability of myocardial infarction in CKD patients, but have unclear effects on vascular accidents and mortality and can increase the risk of bleeding (152).

Glycoprotein IIb/IIIa inhibitors or clopidogrel, in association with the standard ACS treatment, have a minimal or no effect on mortality, myocardial infarction or coronary revascularization and can heighten the risk of major bleeding in CKD and ACS patients or in patients with high-risk coronary artery intervention. Aspirin is essential in CKD and ACS patient treatment (154). The benefits of antiplatelet agent treatment are not known in CKD and ACS patients (154).

Statins decrease the risk of cardiovascular events and cardiovascular death in dialysis patients (155). The results reported in studies performed to date do not explain the impact of the treatment with statins in CKD and ACS patients (154).

The therapy of cardiovascular revascularization, including percutaneous coronary intervention (PCI) and coronary artery bypass grafting (CABG) is used also in dialysis patients. Studies have demonstrated that SCA dialysis patients treated with PCI can have a lower mortality risk compared to those patients who only receive medication (156). Comparing various strategies of coronary revascularization, dialysis patients who received CABG surgery have a prolonged long-time survival vs. the ones who received PCI (157-159). Several studies have aimed to ascertain whether dialysis patients benefit from aggressive SCA therapy more than from conservative therapy.

7. Sudden death

Sudden death refers to the sudden arrest of cardiac activity, with hemodynamic collapse, generally caused by sustained ventricular arrhythmia (ventricular tachycardia or ventricular fibrillation). These events occur in patients with preexisting cardiac diseases, particularly ischemic coronary disease (160).

Data reported by DOOPS (Dialysis Outcomes and Practice Patterns) show a high SCD prevalence among hemodialysis patients in the US (33% of all deaths) compared to other countries, such as Japon (23%), Australia/New Zealand (19%), and Canada (18%) (161). Hemodialysis patients who suffered sudden cardiac arrest and were resuscitated present smaller chances of long-term survival (8%) (162,163).

Hemodialysis patients have a particularity concerning the predisposition for SCD, because of the myocardial affectation and due to the risk factors for fatal arrhythmias (164). In the general population, the main SCD physiopathologic mechanism is the rupture of atheroma plaques, with acute secondary ischemia and reduction in the left ventricle ejection fraction. The association of ventricular fibrilation causes cardiac arrest and death takes place in about 80% of cases (165,166). The mechanism is different for hemodialysis patients. Thus, these patients present with arterial wall stiffening, valvular and vascular calcifications, affecting especially the vascular media, not the intima (167). A study on 1,200 patients showed that a reduction in left ventricle ejection fraction occurs in only 13% of the cases (168). On the other hand, it seems that hemodialysis patients with SCD and left ventricular hypertrophy present diastolic dysfunction. Studies show that left ventricular hypertrophy (LVH) is a risk factor for sudden death in this group of patients (169). More than 70% of SCD patients had LVH (170,171).

In chronic hemodialysis patients there is a series of factors which trigger arrhythmias: Low content calcium of the dialysate, aggressive ultrafiltration, hyperkalemia and rapid potassium elimination, especially in patients who receive hemodialysis three times a week, during the session following the longest interdialysis pause (Monday and Tuesday) (164). The use of a high bicarbonate concentration in the dialysate causes metabolic alkalosis, associated with hypocalcemia, hemodynamic instability and the elongation of the QT interval (164).

Another risk factor for SCD is the overexpression of angiotensin II. There is a range of angiotensin II mechanisms of action, such as stimulation of fibrosis and inflammation, increased activity of the sinus node and of the His-Purkinje system, alteration of Ca2+, K+ and Na+ exchange at the cell level, increased sympathetic nervous system activity, and aldosteron release (109).

SCA patients lose consciousness within seconds or minutes because of insufficient cerebral irrigation. These patients do not generally have warning symptoms or they may have unspecified signs, such as discomfort in the thorax, palpitations, dyspnea and fatigability. Ventricular tachyarrhythmias are the most common and are associated with cardiorespiratory arrest, both in the general population and in dialysis patients (172). Studies have shown that the indexed left ventricular mass is the most powerful predictor for ventricular arrhytmia in CKD patients (173). A study of 75 chronic hemodialysis patients who had a portable defibrillator showed that 79% of cardiac arrests were caused by ventricular tachycardia or ventricular fibrilation (174).

There are studies showing that supraventricular rhythm disorders can lead to cardiorespiratory arrest in hemodialysis patients. SCD patients can suffer from bradycardia (26.3%), asystole (15.8%) and electromechanical dissociation (15.8%) (175). There are few data regarding fatal supraventricular arrythmias, which do not respond to the classical ressuscitation measures, electrical defibrilation included. In order to obtain more knowledge in this respect, the Monitoring in Dialysis Study reports on the use of implantable loop recorders employed to analyze the type and frequency of arrythmias on the traces obtained in a 6-month period (176). The final results of the study have not been published yet, but the preliminary results for 66 enrolled patients show the presence of atrial arrythmias (57.4%), bradycardia (15%) and of ventricular arrythmia in only 9.1% of the cases, mainly in the postdialysis period (177).

Management Primary prevention of SCD

Taking into consideration the frequency and the importance of this phenomenon, identifying the risk factors for SCD proves to be significant. SCD risk occurs in the first three months following the onset of hemodialysis and builds up directly proportional with the period of dialysis, which means both new and old patients can be considered at risk (164). In addition, at risk for SCD are hemodialysis patients who suffer from large IDWG, extreme variations in serum potassium (hypo/hyperpotassemia), uncorrected mineral or bone deficiencies or malnutrition (89,178).

SCD hemodialysis patients are generally diabetics, with preexisting cardiac pathology and a history of cardiac arrythmias (179-181). There are studies that emphasize the strong association between SCD and inflammatory markers including interleukin (IL)-6(181), C reactive protein (CRP) (182) and adiponectin (183), but also between SCD and nutrition markers: Serum albumin (182) and predialysis serum creatinine (89).

Medication

Several drugs have proven useful in lowering the risk of SCD. In this respect, β adrenergic blockers were found to reduce SCD risk following myocardial infarction (184). A study of 200 hemodialysis patients assessed the efficiency of lisinopril vs. atenolol in reducing left ventricle hypertrophy and reported a significantly lower number of hospital admissions for cardiovascular events and cardiac failure in a group of patients who were treated with atenolol (185). Patients treated with atenolol had fewer episodes of arrythmia and cardiorespiratory arrest. On the other hand, the HEMO study did not show an association between β-blockers and the decrease in SCD risk (186). However, the initiation of treatment with β-blockers in hemodialysis patients to prevent SCD cannot be recommended, based on present data.

There is no clear evidence that treatment with cholesterol-lowering medication (statin therapy) or renin-angiotensin-aldosterone system blockers, which is beneficial in the general population in lowering cardiovascular risk, would prove equally beneficial in hemodialysis patients (164).

Adjusting the hemodialysis prescription

The parameters of dialysis can be adjusted so as to prevent SCD. Thus, a low potassium level in the dialysate (<2 mEq/l) in patients with predialysis serum potassium within a normal limit increases the risk of SCD (89,107,161). A study on 30 hemodialysis patients who received potassium modeling vs. a fixed potassium dialysate demonstrated a decrease in ventricular arrythmias, which suggests that the gradual elimination of potassium excess has a protective effect compared to its linear elimination, the latter one with aritmogenous effect. Unfortunately, potassium modeling is not widely available in hemodialysis centers (164).

The decrease in the Ca2+ level in the dialysate is associated with elongation of the QT interval and ventricular arrhytmias (187,188). Research has shown that exposure to low-calcium dialysate (<2.5 mEq/l) is associated with an increase in the risk of SCD by 40% (189). On the other hand, other studies have shown that the use of high-calcium dialysate is correlated with an increase in mortality, by acceleration of the vascular calcification and by increasing the myocardium vulnerability to arrhythmias (190-192).

In addition to the role of calcium in the dialysate, further studies are necessary to explain the role of vitamin D analogues, of phosphate binders and of calcimetics in SCD. Furthermore, it is necessary to control the phosphate serum levels, taking into account that the relationship between hyperphosphatemy and mortality has been demonstrated, probably because of myocardial calcifications and hemodynamic changes in microcirculation (193).

In addition to the electrolytic exchanges in hemodialysis patients, the relationship between cardiovascular mortality and high rate of ultrafiltration has been demonstrated (194). An ultrafiltration rate over 10 ml/kg/h is associated with increased mortality (195). It is necessary to train the patient to respect dietary recommandations (to limit salt and fluid intake), to increase the frequency and duration of the dialysis sessions and to maintain a small gradient between serum sodium and the sodium in the dialysate (196). The temperature in the dialysate influences blood pressure and coronary circulation, a decrease in the dialysate temperature causing a decrease in IDH and myocardial ischemic injury, and the risk of cardiovascular death (197).

8. Conclusion

Acute intradialytic cardiovascular complications are commonly encountered in clinical practice and influence the quality of life, such as morbidity and the mortality rate of dialysis patients. In order to have detailed knowledge concerning the risk factors and the pathogenic mechanisms and to ensure an optimal management of these complications, more studies must be conducted.