Energy transfer is the single most important factor for the difference in vascular response between isolated ultrafiltration and hemodialysis

Why is Intradialytic Hypotension the Commonest Complication of Outpatient Dialysis Treatments?

Intradialytic hypotension (IDH) is the most frequent complication of hemodialysis (HD) treatments with a frequency of 10% to 12% for patients with chronic kidney disease attending for outpatient treatments and is associated with both temporary ischemic stress to vital organs, including the heart and brain, and increased patient mortality. Although there have been many different definitions of IDH over the years, an absolute nadir systolic blood pressure (SBP) has the strongest association with patient outcomes. The unifying pathophysiology is one of reduced effective blood volume, resulting in lower plasma tonicity, and if this cannot be adequately compensated for by activation of neurohumeral systems, then arteriolar tone and blood pressure fall. The risk factors for developing IDH are numerous, ranging from patient-related factors, including age and comorbidity with reduced cardiac reserve, to patient compliance with dietary and lifestyle advice, to reactions with the extracorporeal circuit and medications, choice of dialysate composition and temperature, setting of postdialysis target weight, ultrafiltration rate, and profiling. Advances in dialysis machine technology by providing real time estimates of the effective circulating volume and adjusting dialysate composition to maintain vascular tonicity are being developed, but currently require more sophisticated biofeedback loops to be clinically effective in preventing IDH. While awaiting advances in artificial intelligence, the clinician continues to rely on patient education to limit interdialytic weight gains, frequent assessment of the postdialysis target weight, adjusting dialysate composition and temperature, introducing convective therapies to increase thermal losses, and altering dialysis session duration and frequency to reduce ultrafiltration rate requirements.

Hypocholesterolemia is a risk factor for reduced systemic vascular resistance reactivity during hemodialysis

Intradialytic hypotension (IDH) is associated with high mortality. Peripheral vascular resistance and circulating blood volume are important factors in IDH; however, the effects of hemodialysis (HD) on vascular resistance in IDH remain unclear. We herein performed a retrospective observational cohort study to investigate changes in and factors related to vascular resistance during HD. A total of 101 HD patients were divided into two groups (Decreased blood pressure (BP) during HD group: N = 19, Nondecreased BP group: N = 82), and cardiac output was measured with electrical velocimetry (AESCLON) for 3 h. The systemic vascular resistance index (SVRI) was significantly decreased in the Decreased BP group, while the cardiac index was similar in both groups. A multivariate regression analysis identified hypocholesterolemia as a predictor of reduced vascular resistance reactivity during HD. Furthermore, a correlation was found between changes in the SVRI and cholesterol levels in patients with a higher Geriatric Nutritional Risk Index (GNRI) but not in those with a lower GNRI. The present results suggest that hypocholesterolemia contributes to reducing systematic vascular resistance reactivity during HD, which is an important predictor of a reduction in BP during HD. The relationship between hypocholesterolemia and vascular resistance may involve mechanisms other than malnutrition.

The oxygen cascade in patients treated with hemodialysis and native high-altitude dwellers: lessons from extreme physiology to benefit patients with end-stage renal disease

Patients treated with hemodialysis (HD) repeatedly undergo intradialytic low arterial oxygen saturation and low central venous oxygen saturation, reflecting an imbalance between upper body systemic oxygen supply and demand, which are associated with increased mortality. Abnormalities along the entire oxygen cascade, with impaired diffusive and convective oxygen transport, contribute to the reduced tissue oxygen supply. HD treatment impairs pulmonary gas exchange and reduces ventilatory drive, whereas ultrafiltration can reduce tissue perfusion due to a decline in cardiac output. In addition to these factors, capillary rarefaction and reduced mitochondrial efficacy can further affect the balance between cellular oxygen supply and demand. Whereas it has been convincingly demonstrated that a reduced perfusion of heart and brain during HD contributes to organ damage, the significance of systemic hypoxia remains uncertain, although it may contribute to oxidative stress, systemic inflammation, and accelerated senescence. These abnormalities along the oxygen cascade of patients treated with HD appear to be diametrically opposite to the situation in Tibetan highlanders and Sherpa, whose physiology adapted to the inescapable hypobaric hypoxia of their living environment over many generations. Their adaptation includes pulmonary, vascular, and metabolic alterations with enhanced capillary density, nitric oxide production, and mitochondrial efficacy without oxidative stress. Improving the tissue oxygen supply in patients treated with HD depends primarily on preventing hemodynamic instability by increasing dialysis time/frequency or prescribing cool dialysis. Whether dietary or pharmacological interventions, such as the administration of L-arginine, fermented food, nitrate, nuclear factor erythroid 2-related factor 2 agonists, or prolyl hydroxylase 2 inhibitors, improve clinical outcome in patients treated with HD warrants future research.

Intradialytic Hypotension: Mechanisms and Outcome

Intradialytic hypotension (IDH) occurs in approximately 10-12% of treatments. Whereas several definitions for IDH are available, a nadir systolic blood pressure carries the strongest relation with outcome. Whereas the relation between IDH may partly be based on patient characteristics, it is likely that also impaired organ perfusion leading to permanent damage, plays a role in this relationship. The pathogenesis of IDH is multifactorial and is based on a combination of a decline in blood volume (BV) and impaired vascular resistance at a background of a reduced cardiovascular reserve. Measurements of absolute BV based on an on-line dilution method appear more promising than relative BV measurements in the prediction of IDH. Also, feedback treatments in which ultrafiltration rate is automatically adjusted based on changes in relative BV have not yet resulted in improvement. Frequent assessment of dry weight, attempting to reduce interdialytic weight gain and prescribing more frequent or longer dialysis treatments may aid in preventing IDH. The impaired vascular response can be improved using isothermic or cool dialysis treatment which has also been associated with a reduction in end organ damage, although their effect on mortality has not yet been assessed. For the future, identification of vulnerable patients based on artificial intelligence and on-line assessment of markers of organ perfusion may aid in individualizing treatment prescription, which will always remain dependent on the clinical context of the patient.

Dialysate temperature reduction for intradialytic hypotension for people with chronic kidney disease requiring haemodialysis

BACKGROUND

Intradialytic hypotension (IDH) is a common complication of haemodialysis (HD), and a risk factor of cardiovascular morbidity and death. Several clinical studies suggested that reduction of dialysate temperature, such as fixed reduction of dialysate temperature or isothermal dialysate using a biofeedback system, might improve the IDH rate.

OBJECTIVES

This review aimed to evaluate the benefits and harms of dialysate temperature reduction for IDH among patients with chronic kidney disease requiring HD, compared with standard dialysate temperature.

SEARCH METHODS

We searched Cochrane Kidney and Transplant's Specialised Register up to 14 May 2019 through contact with the Information Specialist using search terms relevant to this review. Studies in the Register are identified through searches of CENTRAL, MEDLINE, EMBASE, conference proceedings, the International Clinical Trials Register (ICTRP) Search Portal, and ClinicalTrials.gov.

SELECTION CRITERIA

All randomised controlled trials (RCTs), cross-over RCTs, cluster RCTs and quasi-RCTs were included in the review.

DATA COLLECTION AND ANALYSIS

Two authors independently extracted information including participants, interventions, outcomes, methods of the study, and risks of bias. We used a random-effects model to perform quantitative synthesis of the evidence. We assessed the risks of bias for each study using the Cochrane 'Risk of bias' tool. We assessed the certainty of evidence using Grades of Recommendation, Assessment, Development and Evaluation (GRADE).

MAIN RESULTS

We included 25 studies (712 participants). Three studies were parallel RCTs and the others were cross-over RCTs. Nineteen studies compared fixed reduction of dialysate temperature (below 36°C) and standard dialysate temperature (37°C to 37.5°C). Most studies were of unclear or high risk of bias. Compared with standard dialysate, it is uncertain whether fixed reduction of dialysate temperature improves IDH rate (8 studies, 153 participants: rate ratio 0.52, 95% CI 0.34 to 0.80; very low certainty evidence); however, it might increase the discomfort rate compared with standard dialysate (4 studies, 161 participants: rate ratio 8.31, 95% CI 1.86 to 37.12; very low certainty evidence). There were no reported dropouts due to adverse events. No study reported death, acute coronary syndrome or stroke.Three studies compared isothermal dialysate and thermoneutral dialysate. Isothermal dialysate might improve the IDH rate compared with thermoneutral dialysate (2 studies, 133 participants: rate ratio 0.68, 95% CI 0.60 to 0.76; I² = 0%; very low certainty evidence). There were no reports of discomfort rate (1 study) or dropouts due to adverse events (2 studies). No study reported death, acute coronary syndrome or stroke.

AUTHORS' CONCLUSIONS

Reduction of dialysate temperature may prevent IDH, but the conclusion is uncertain. Larger studies that measure important outcomes for HD patients are required to assess the effect of reduction of dialysate temperature. Six ongoing studies may provide much-needed high quality evidence in the future.

Effect of isolated ultrafiltration and isovolemic dialysis on myocardial perfusion and left ventricular function assessed with 13N-NH3 positron emission tomography and echocardiography

Hemodialysis is associated with a fall in myocardial perfusion and may induce regional left ventricular (LV) systolic dysfunction. The pathophysiology of this entity is incompletely understood, and the contribution of ultrafiltration and diffusive dialysis has not been studied. We investigated the effect of isolated ultrafiltration and isovolemic dialysis on myocardial perfusion and LV function. Eight patients (7 male, aged 55 ± 18 yr) underwent 60 min of isolated ultrafiltration and 60 min of isovolemic dialysis in randomized order. Myocardial perfusion was assessed by ¹³N-NH₃ positron emission tomography before and at the end of treatment. LV systolic function was assessed by echocardiography. Regional LV systolic dysfunction was defined as an increase in wall motion score in ≥2 segments. Isolated ultrafiltration (ultrafiltration rate 13.6 ± 3.9 ml·kg^-1·h^-1) induced hypovolemia, whereas isovolemic dialysis did not (blood volume change -6.4 ± 2.2 vs. +1.3 ± 3.6%). Courses of blood pressure, heart rate, and tympanic temperature were comparable for both treatments. Global and regional myocardial perfusion did not change significantly during either isolated ultrafiltration or isovolemic dialysis and did not differ between treatments. LV ejection fraction and the wall motion score index did not change significantly during either treatment. Regional LV systolic dysfunction developed in one patient during isolated ultrafiltration and in three patients during isovolemic dialysis. In conclusion, global and regional myocardial perfusion was not compromised by 60 min of isolated ultrafiltration or isovolemic dialysis. Regional LV systolic dysfunction developed during isolated ultrafiltration and isovolemic dialysis, suggesting that, besides hypovolemia, dialysis-associated factors may be involved in the pathogenesis of hemodialysis-induced regional LV dysfunction.

Effect of Lowering the Dialysate Temperature in Chronic Hemodialysis: A Systematic Review and Meta-Analysis

BACKGROUND AND OBJECTIVES

Lowering the dialysate temperature may improve outcomes for patients undergoing chronic hemodialysis. We reviewed the reported benefits and harms of lower temperature dialysis.

DESIGN, SETTING, PARTICIPANTS, & MEASUREMENTS

We searched the Cochrane Central Register, OVID MEDLINE, EMBASE, and Pubmed until April 15, 2015. We reviewed the reference lists of relevant reviews, registered trials, and relevant conference proceedings. We included all randomized, controlled trials that evaluated the effect of reduced temperature dialysis versus standard temperature dialysis in adult patients receiving chronic hemodialysis. We followed the Grading of Recommendations Assessment, Development and Evaluation approach to assess confidence in the estimates of effect (i.e., the quality of evidence). We conducted meta-analyses using random effects models.

RESULTS

Twenty-six trials were included, consisting of a total of 484 patients. Compared with standard temperature dialysis, reduced temperature dialysis significantly reduced the rate of intradialytic hypotension by 70% (95% confidence interval, 49% to 89%) and significantly increased intradialytic mean arterial pressure by 12 mmHg (95% confidence interval, 8 to 16 mmHg). Symptoms of discomfort occurred 2.95 (95% confidence interval, 0.88 to 9.82) times more often with reduced temperature compared with standard temperature dialysis. The effect on dialysis adequacy was not significantly different, with a Kt/V mean difference of -0.05 (95% confidence interval, -0.09 to 0.01). Small sample sizes, loss to follow-up, and a lack of appropriate blinding in some trials reduced confidence in the estimates of effect. None of the trials reported long-term outcomes.

CONCLUSIONS

In patients receiving chronic hemodialysis, reduced temperature dialysis may reduce the rate of intradialytic hypotension and increase intradialytic mean arterial pressure. High-quality, large, multicenter, randomized trials are needed to determine whether reduced temperature dialysis affects patient mortality and major adverse cardiovascular events.

Intradialytic hypotension

Intradialytic hypotension (IDH) is common in children during conventional, 4 hour haemodialysis (HD) sessions. The declining blood pressure (BP) was originally believed to be caused by ultrafiltration (UF) and priming of the HD circuit, however emerging data now supports a multifactorial aetiology. Therefore strategies to improve haemodynamic stability need to be diverse and address specific patient requirements or risks. In the treatment of IDH immediate action is required to stop or reduce the severity of symptoms that may precede or follow. Typically UF is slowed or stopped, a fluid bolus is given and in resistant cases the HD session is prematurely discontinued. Patients complete their treatment under-dialysed and volume expanded. Chronically, repeated episodes of IDH cause devastating, multi-system morbidity with an increased risk of mortality. This had provided the impetus for more haemodynamically friendly dialysis prescriptions that attenuate the risk of IDH. During pediatric HD several preventative strategies have been tested but with variable success. Of these, dialysate sodium profiling, UF guided by relative blood volume (RBV) algorithms, cooling and intradialytic mannitol appear to be the most effective. However in refractory cases one may be left with no option but to switch dialysis modality to haemodiafiltration (HDF) or more frequent or prolonged HD regimens.

Dialysate and blood temperature during hemodialysis: comparing isothermic dialysis with a single-pass batch system

The Genius is a convenient single-pass batch hemodialysis system. Dialysate is heated during the preparation phase. Once the container is filled at one of the three available starting temperatures, dialysate cools spontaneously. As dialysate temperature plays an important role in hemodynamic stability, knowledge of the instantaneous dialysate temperature is important. We documented the evolution of dialysate temperature during Genius dialysis with dialysate prepared at two different temperatures (low and medium) as well as its effect on blood temperature and hemodynamic stability. Genius dialysis was compared to isothermic dialysis obtained by a module programmed to obtain a constant blood temperature. With Genius, dialysate temperature progressively decreased from 36.3 (low) and 37.6°C (medium) in the beginning to 34.4 and 35.3°C at the end of the session, both following first-order kinetics. During isothermic dialysis, dialysate cooled from 37.3 to 35.4°C, being warmer than with Genius low. Blood temperature decreased during dialysis with Genius low, from 36.4 to 36.2°C, whereas with medium temperature, blood temperature initially rose from 36.4°C at 15 min to 36.5 and 36.6°C at 30 and 60 min, respectively, followed by a decrease from 180 min onward, to 36.4°C at the end. During isothermic dialysis, blood temperature remained stable, as expected. Blood pressure decreased during all three schedules, with the highest value at 15 min during isothermic dialysis, and at the end of the session, the lowest value with isothermic dialysis.

Pro/con debate: continuous versus intermittent dialysis for acute kidney injury: a never-ending story yet approaching the finish?

The question of whether renal replacement therapy should be applied in an intermittent or continuous mode to the patient with acute kidney injury has been the topic of several controlled studies and meta-analyses. Although continuous renal replacement therapy (CRRT) has a theoretical advantage due to offering the opportunity to remove excess fluid more gradually, none of the several outcome studies that have been undertaken in the meanwhile was able to demonstrate its superiority over intermittent renal replacement therapy (IRRT). In the present article, therefore, questions are raised regarding which are the specific advantages of each strategy, and which are the specific populations that might benefit from their application. Although several advantages have been attributed to CRRT - especially more hemodynamic stability allowing more adequate fluid removal, better recovery of renal function, and more efficient removal of small and large metabolites - none of these could be adequately proven in controlled trials. CRRT is claimed to be better tolerated in combined acute liver and kidney failure and in acute brain injury. IRRT is more practical, flexible and cost-effective, allows the clinician to discontinue or to minimize anticoagulation with bleeding risks, and removes small solutes such as potassium more efficiently in acute life-threatening conditions. Sustained low-efficiency daily dialysis is a hybrid therapy combining most of the advantages of both options.

Control of core temperature and blood pressure stability during hemodialysis

BACKGROUND AND OBJECTIVES

Cool dialysate may ameliorate intradialytic hypotension (IDH). It is not known whether it is sufficient to prevent an increase in core temperature (CT) during hemodialysis (HD) or whether a mild decline in CT would yield superior results. The aim of this study was to compare both approaches with regard to IDH.

DESIGN, SETTING, PARTICIPANTS, & MEASUREMENTS

Fourteen HD patients with a history of IDH were studied. During three mid-week HD treatments, CT was set to decrease by 0.5 degrees C ("cooling") or to remain unchanged at the baseline level ("isothermic"). "Thermoneutral" HD (no energy is added to or removed from the patient) was used as a control. Central blood volume (CBV), BP, skin temperature, heart rate variability [low and high frequency] were recorded.

RESULTS

CT increased during thermoneutral and remained respectively stable and decreased during isothermic and cooling. Skin temperature decreased significantly during isothermic and cooling, but not during thermoneutral. Nadir systolic BP (SBP) levels were lower during isothermic and thermoneutral compared with cooling. CBV tended to be higher during cooling compared with isothermic and thermoneutral. Three patients complained of shivering during cooling. Change in LF/HF was not different between cooling, isothermic, and thermoneutral.

CONCLUSIONS

IDH may be slightly improved by cooling compared with the isothermic approach, possibly because of improved maintenance of CBV. The hemodynamic effects of mild blood cooling should be balanced against a potentially higher risk of cold discomfort.

Mortality risk for patients receiving hemodiafiltration versus hemodialysis: European results from the DOPPS

Hemodiafiltration (HDF) is used sporadically for renal replacement therapy in Europe but not in the US. Characteristics and outcomes were compared for patients receiving HDF versus hemodialysis (HD) in five European countries in the Dialysis Outcomes and Practice Patterns Study. The study followed 2165 patients from 1998 to 2001, stratified into four groups: low- and high-flux HD, and low- and high-efficiency HDF. Patient characteristics including age, sex, 14 comorbid conditions, and time on dialysis were compared between each group using multivariate logistic regression. Cox proportional hazards regression assessed adjusted differences in mortality risk. Prevalence of HDF ranged from 1.8% in Spain to 20.1% in Italy. Compared to low-flux HD, patients receiving low-efficiency HDF had significantly longer average duration of end-stage renal disease (7.0 versus 4.7 years), more history of cancer (15.4 versus 8.7%), and lower phosphorus (5.3 versus 5.6 mg/dl); patients receiving high-efficiency HDF had significantly more lung disease (15.5 versus 10.2%) and received a higher single-pool Kt/V (1.44 versus 1.35). High-efficiency HDF patients had lower crude mortality rates than low-flux HD patients. After adjustment, high-efficiency HDF patients had a significant 35% lower mortality risk than those receiving low-flux HD (relative risk=0.65, P=0.01). These observational results suggest that HDF may improve patient survival independently of its higher dialysis dose. Owing to possible selection bias, the potential benefits of HDF must be tested by controlled clinical trials before recommendations can be made for clinical practice.

A systematic review of the clinical effects of reducing dialysate fluid temperature

BACKGROUND

Intradialytic hypotension (IDH) is a frequent complication of haemodialysis. Reducing the temperature of the dialysis fluid is a simple therapeutic strategy but is relatively underused. This may be due to concerns regarding its effects on symptoms and dialysis adequacy. We performed a systematic review of the literature to examine the effects of cool dialysis on intradialytic blood pressure, and to assess its safety in terms of thermal symptoms and small solute clearance.

METHODS

We searched the Cochrane Central Register of Controlled Trials, Medline, Embase, Cumulative Index to Nursing and Allied Health Literature, databases of ongoing trials, the contents of four major renal journals as well as hand-searching reference lists. We included all prospective randomized studies that compared any technique of reducing dialysate temperature with standard bicarbonate dialysis. These techniques included an empirical, fixed reduction of dialysate temperature or use of a biofeedback temperature-control device (BTM) to deliver isothermic dialysis or programmed patient cooling.

RESULTS

A total of 22 studies comprising 408 patients were included (16 studies examined a fixed empirical temperature reduction and six examined BTM). All studies were of crossover design and relatively short duration. IDH occurred 7.1 (95% CI, 5.3-8.9) times less frequently with cool dialysis (both fixed reduction and BTM). Post-dialysis mean arterial pressure was higher with cool-temperature dialysis by 11.3 mmHg (95% CI, 7.7-15.0). No studies reported that cool dialysis led to a reduction in dialysis adequacy as assessed by urea clearance. The frequency and severity of thermal-related symptoms were generally reported inadequately.

CONCLUSIONS

Reducing the temperature of the dialysate is an effective intervention to reduce the frequency of IDH and does not adversely affect dialysis adequacy. This applies to the fixed reduction of dialysate temperature and BTM. It remains unclear as to what extent cool-temperature dialysis causes intolerable cold symptoms during dialysis. There are no trials comparing fixed empirical temperature reduction with BTM, and no trials examining the long-term effects of cool dialysis on patient outcomes.

Effect of Ultrafiltration on Thermal Variables, Skin Temperature, Skin Blood Flow, and Energy Expenditure during Ultrapure Hemodialysis

The cause of the increase in core temperature (CT) during hemodialysis (HD) is still under debate. It has been suggested that peripheral vasoconstriction as a result of hypovolemia, leading to a reduced dissipation of heat from the skin, is the main cause of this increase in CT. If so, then it would be expected that extracorporeal heat flow (Jex) needed to maintain a stable CT (isothermic; T-control = 0, no change in CT) is largely different between body temperature control HD combined with ultrafiltration (UF) and body temperature control HD without UF (isovolemic). Consequently, significant differences in DeltaCT would be expected between isovolemic HD and HD combined with UF at zero Jex (thermoneutral; E-control = 0, no supply or removal of thermal energy to and from the extracorporeal circulation). During the latter treatment, the CT is expected to increase. In this study, changes in thermal variables (CT and Jex), skin blood flow, energy expenditure, and cytokines (TNF-alpha, IL-1 receptor antagonist, and IL-6) were compared in 13 patients, each undergoing body temperature control (T-control = 0) HD without and with UF and energy-neutral (E-control = 0) HD without and with UF. CT increased equally during energy-neutral treatments, with (0.32 +/- 0.16 degrees C; P = 0.000) and without (0.27 +/- 0.29 degrees C; P = 0.006) UF. In body temperature control treatments, the relationship between Jex and UF tended to be significant (r = -0.51; P = 0.07); however, there was no significant difference in cooling requirements regardless of whether treatments were done without (-17.9 +/- 9.3W) or with UF (-17.8 +/- 13.27W). Changes in energy expenditure did not differ among the four treatment modes. There were no significant differences in pre- and postdialysis levels of cytokines within or between treatments. Although fluid removal has an effect on thermal variables, no single mechanism seems to be responsible for the increased heat accumulation during HD.

Predilution hemodiafiltration displays no hemodynamic advantage over low-flux hemodialysis under matched conditions

BACKGROUND

It is the prevailing view that convective dialysis techniques stabilize blood pressure. The aim of this study was to compare the intrasession hemodynamics during high-dose predilution hemodiafiltration (HDF) and low-flux hemodialsis, under strict controlled conditions.

METHODS

Twelve stable hemodialysis patients were investigated in a randomized crossover blinded controlled trial. The patients were allocated to one session of predilution HDF (substitution fluid 1.20 +/- 0.10 L/kg body weight) and one session of hemodialysis at 4(1/2) hours. To eliminate confounding factors, dialysis dose, ultrafiltration volume and arterial temperature were matched. At the start of the dialysis the patients' core temperature was "locked" by an automatic feedback system regulating the dialysate temperature; thereby, patients' temperature was kept stable throughout the whole treatment. The calcium-ion concentration in the substitution/dialysis fluid was 1.25 mmol/L. Cardiac output was measured hourly by the ultrasound velocity dilution method.

RESULTS

Mean blood pressure, cardiac output, stroke volume, cardiac work, and relative blood volume was significantly reduced in both treatments. Total peripheral resistance increased significantly in both groups. Ultrafiltration volume, cardiopulmonary recirculation, Kt/V, and total energy transfer were similar for hemodialysis and HDF. The pulse rate showed no significant change throughout both sessions. No significant differences were revealed between hemodialysis and HDF.

CONCLUSION

The hemodynamics of predilution HDF and low-flux hemodialysis displayed a similar profile during matched conditions. An acute circulatory benefit of convective solute removal over diffusive could not be demonstrated.

Optimal composition of the dialysate, with emphasis on its influence on blood pressure

Introduction. From the beginning of the dialysis era, the most appropriate composition of the dialysate has been one of the central topics in the delivery of dialysis treatment.

METHODS

A discussion is employed to achieve a consensus on key points relating to the composition of the dialysate, focusing on the relationships with blood pressure behaviour.

RESULTS

Sodium balance is the cornerstone of intra-dialysis cardiovascular stability and good inter-dialysis blood pressure control. Hypernatric dialysis carries the risk of positive sodium balance, with the consequent possibility of the worsening sense of thirst and hypertension. Conversely, hyponatric dialysis may lead to negative sodium balance, with the possibility of intra-dialysis cardiovascular instability and 'disequilibrium' symptoms including fatigue, muscle cramps and headache. The goal is to remove with dialysis the exact amount of sodium that has accumulated in the inter-dialysis interval. The conductivity kinetic model is applicable on-line at each dialysis session and has been proved to be able to improve intra-dialytic cardiovascular stability in hypotension-prone patients. Therefore, it should be regarded as a promising tool to be implemented in everyday clinical practice. Serum potassium concentration and variations during dialysis treatment certainly play a role in the genesis of cardiac arrhythmia. Potassium profiling, with a constant gradient between plasma and dialysate, should be implemented in clinical practice to minimize the arrhythmogenic potential of dialysis. Calcium plays a role both in myocardial contractility and in peripheral vascular resistance. Therefore, an increase in dialysate calcium concentration may be useful in cardiac compromised hypotension-prone patients. Acid-buffering by means of base supplementation is one of the major roles of dialysis. Bicarbonate concentration in the dialysate should be personalized in order to reach a midweek pre-dialysis serum bicarbonate concentration of 22 mmol/l. The role of convective dialysis techniques in cardiovascular stability is still under debate. It has been demonstrated that dialysate temperature and sodium balance play a role and this should be taken into account. Whether removal of vasoactive, middle-sized compounds by convection plays an independent role in improving cardiovascular stability is still uncertain.

CONCLUSIONS

The prescription of dialysis fluid is moving from a pre-fixed, standard dialysate solution to individualization of electrolyte and buffer composition, not only during the dialysis session, but also within the same session (profiling) in order to provide patients with an optimal blood purification coupled with a high degree of tolerability.

Effects of continuous venovenous haemofiltration-induced cooling on global haemodynamics, splanchnic oxygen and energy balance in critically ill patients

BACKGROUND

A number of haemodialysis studies have demonstrated beneficial effects of cooler dialysates on global haemodynamics in chronic dialysis patients. However, the effects of continuous venovenous haemofiltration (CVVH)-induced cooling on regional perfusion and energy metabolism in critically ill septic patients have not been well defined.

METHODS

Nine septic mechanically ventilated patients (age 40-69 years) were investigated during CVVH (ultrafiltration 30-35 ml/kg/h). Baseline data (=WARM 1) were collected when core temperature (Tc) was >37.5 degrees C; the second data set (=COLD) was obtained after 120 min of 'cooling'; and a third set (=WARM 2) was obtained after 120 min of 'rewarming'. During 'warming' (WARM 1 and 2, respectively), both substitution fluids (SFs) and 'returned' blood (RB) were warmed (37 degrees C), whereas during 'cooling', the SFs were at 20 degrees C and RB was not warmed. We measured hepatic venous (HV) haemoglobin oxygen saturation (ShvO(2)), blood gases, lactate and pyruvate. Gastric mucosal PCO(2) (PgmCO(2)) was measured by air tonometry and the gastric mucosal - arterial PCO(2) difference (PCO(2) gap) was calculated. Haemodynamic monitoring was performed with arterial and pulmonary arterial thermodilution catheters.

RESULTS

Tcs were significantly altered [WARM 1, 37.9 degrees C (37.6, 38.3); COLD, 36.8 degrees C (36.3, 37.1); WARM 2, 37.5 degrees C (37.0, 38.0); P<0.001; data are median, 25th and 75th percentiles, respectively]. Systemic vascular resistance significantly increased during cooling. As a result, mean arterial pressure increased. Cooling was associated with significant decreases in heart rate, cardiac output, systemic oxygen delivery and consumption. ShvO(2) did not change [WARM 1, 51.0% (44.0, 59.5); COLD, 49.0% (42.0, 58.0); WARM 2, 51.0% (46.0, 57.0); P = NS]. The splanchnic oxygen extraction ratio, the HV lactate to pyruvate ratio, HV acid base status and PCO(2) gap remained unchanged.

CONCLUSION

Mild core cooling induced by CVVH may not affect hepatosplanchnic oxygen and energy balance in septic critically ill patients, even though it affects global haemodynamics.

RENAL RESEARCH INSTITUTE SYMPOSIUM: Temperature Control by the Blood Temperature Monitor

The rationale of temperature control during hemodialysis (HD) is to prevent heat accumulation, which increases body temperature and enhances hypotensive susceptibility. Treatments where thermal energy is neither delivered nor removed from the patient through the extracorporeal circulation (so-called extracorporeal thermoneutral treatments) lead to a marked increase in body temperature and to considerable heat accumulation during HD. Since this accumulation of heat cannot be explained by increased heat production, it must be related to reduced heat dissipation through the body surface. Peripheral vasoconstriction, and cutaneous vasoconstriction in particular, compensating for the ultrafiltration-induced decrease in blood volume is considered an important component in this setting. Therefore, to maintain temperature homeostasis, thermal energy has to be cleared from the patient by the extracorporeal system because cutaneous clearance of thermal energy is compromised intradialytically. The focus on dialysate temperature alone does not properly address the problem of controlled extracorporeal heat removal because dialysate temperature is only one of the variables involved in that process. These difficulties can be addressed by changing from the control of dialysate temperature to control of body temperature. Control of body temperature and temperature homeostasis is achievable by the physiologic feedback control system realized in the temperature control mode (T-mode) of the blood temperature monitor (BTM). The delivery of isothermic dialysis, that is, dialysis where body temperature is controlled to remain constant during the treatment, has impressively improved hemodynamic stability in hypotension prone patients.

Heat accumulation with relative blood volume decrease

BACKGROUND

Both hypovolemia and heat accumulation act as powerful perturbations of blood pressure control. In hemodialysis, hypovolemia and heat accumulation often develop simultaneously, and the question arises of whether and to what extent these perturbations are linked.

METHODS

Heat accumulation was measured by the amount of thermal energy (E) removed from a patient during prescribed ultrafiltration under isothermic hemodialysis conditions, ie, constant patient temperature. Measurement and control of temperatures and thermal energies were performed using the blood temperature monitor. Relative blood volume (RBV) was measured using the blood volume monitor.

RESULTS

Thirty-eight treatments were analyzed in 12 patients (3 women). During treatments lasting 189 +/- 28 minutes, 5.1% +/- 1.3% of postdialysis body weight were removed from patients by ultrafiltration at a mean rate of 1.1 +/- 0.3 L/h. Blood volumes decreased to 85% +/- 7% of initial values, and 229 +/- 106 kJ of E were removed from patients at a cooling rate (J) of 20 +/- 8 W, corresponding to 28% +/- 11% of estimated energy expenditure (H%). E, J, and H% significantly increased as RBV decreased (P < 0.05). Linear regression analysis between J and RBV showed that approximately 1 W had to be removed from the patient for each percentage of change in blood volume (J = -102.38 + 0.97* RBV; r2 = 0.63).

CONCLUSION

Results show that the probability for the effect of heat stress during hemodialysis increases with ultrafiltration-induced blood volume changes. Temperature control is an important aspect of hemodialysis treatment.

Temperature and thermal balance in hemodialysis

The analysis of thermal balance and temperature in hemodialysis patients reveals both striking similarities and important differences to urea kinetics. Both urea and thermal energy need to be removed during hemodialysis, however, for different reasons. Urea accumulates between hemodialysis treatments, whereas thermal energy accumulates within hemodialysis treatments. Urea concentration is ideally reduced by approximately 70% during a treatment, whereas temperature is ideally kept constant by removing up to 50% of resting energy expenditure because heat dissipation from the body surface is obstructed as a result of ultrafiltration-induced hypovolemia. Extracorporeal heat removal is controlled by several factors. Dialysate and patient temperatures play the main role. Low body temperatures are not uncommon with hemodialysis patients, so that dialysate temperatures less than 36 degrees C are often required to maintain reasonable temperature gradients. Another important role is played by extracorporeal blood flow. At the same temperature gradient, heat transfer by extracorporeal blood flow used with high-efficiency dialysis is approximately six times more efficient than the dissipation of heat across the body surface. And, last but not least, the venous section of the extracorporeal circulation provides constant cooling of approximately 10 W. Almost all dialysis treatments provide extracorporeal cooling, even those using dialysate at 37 degrees C. Therefore it is probably better to define the thermal aspects of hemodialysis with regard to the physiologic effects on the patient. Since thermoregulation responds to changes in body temperature, treatments should be characterized as isothermic, hypothermic, and hyperthermic.

Thermal effects and blood pressure response during postdilution hemodiafiltration and hemodialysis: the effect of amount of replacement fluid and dialysate temperature

It has been suggested that the incidence of hypotensive episodes is less with hemodiafiltration (HDF) than with hemodialysis (HD). The aim of the present study was to assess the BP response during HD and postdilution HDF in relation to the thermal effects of these different treatment modalities by manipulating the dialysate temperature (Td) during HD and the amount of replacement fluid during HDF. In 12 patients, energy transfer rate (in watts) and maximal decline in mean arterial pressure during HD at Td 37.5 degrees C, HD at Td 35.5 degrees C, and postdilution HDF with amounts of replacement fluids infused at room temperature of 1 L/h and 2.5 L/h, respectively, were assessed. All measurements were done twice in each patient. Energy transfer rate was comparable between HD 35.5 degrees C (-26.61 +/- 5.33) and HDF 2.5 L/h (-25.25 +/- 7.91) and was significantly more negative compared with HD 37.5 degrees C (-3.53 +/- 6.44) and HDF 1 L/h (-15.88 +/- 6.94). The maximum decline in mean arterial pressure was significantly higher during HD 37.5 degrees C (-25.6 +/- 13.5) than during HD 35.5 degrees C (-15.1 +/- 13.8) and HDF 2.5 L/h (-19.2 +/- 17.7), whereas there was no significant difference with HDF 1 L/h (-23.0 +/- 14.0). In conclusion, thermal effects during postdilution HDF are dependent on the amount of replacement fluid. Also during HDF, the BP response is strongly related to thermal effects. The use of postdilution HDF with low or intermediate amounts of replacement fluids infused at room temperature seems to have no advantage in preventing hemodynamic instability, compared with HD 35.5 degrees C.