Logistics of peritoneal dialysis in the obese population

It is widely assumed1 that obese or large-framed patients on peritoneal dialysis (PD) will have difficulty achieving solute clearance targets considered adequate by National Kidney Foundation (NKF)-KDOQI guidelines,2 especially in the absence of residual renal function. A U.S. national survey has shown that U.S. nephrologists are 2.3 times more likely to recommend PD to patients with body weight less than 200 lb than to those weighing more than 200 lb.3

This perceived difficulty arises from the larger total body water (TBW) volume and body surface area (BSA) present in larger patients, which are used to normalize weekly urea (Kt/V) and weekly creatinine clearances (WCC), respectively. Larger TBW and BSA obligate larger and sometimes unobtainable drain volumes to achieve targeted clearances. Mathematical models predict that patients over 80 kg cannot receive adequate dialysis with even four 3-L continuous ambulatory peritoneal dialysis (CAPD) exchanges daily.4, 5 A multicenter study reported a Kt/V >1.7/wk in 54.2% and a WCC >54.4 L/1.73 m²/wk in 70.8% of patients heavier than 100 kg.6 We have reported success rates in reaching the higher solute clearance targets recommended by NKF-KDOQI for large or obese patients.7 Since 2006 KDOQI guidelines suggest lower Kt/V targets (of 1.7/wk) than the initial KDOQI guidelines and since WCC is not taken into consideration, a higher proportion of patients is expected to achieve the current targets.8

There are other potential barriers to successful PD outcomes in obese patients. The larger exchange volumes and frequency and/or the long time required on the cycler for large patients to receive adequate dialysis may significantly impair quality of life, including the ability to work or engage in recreational activities. Finally, obese patients, who may not be able to directly visualize the exit site due to their protruding abdomen, may have difficulty taking proper care of their catheters. This may result in an increased risk of exit-site infection or peritonitis. Piraino et al. showed that patients weighing more than 110% of ideal body weight have similar peritonitis and exit-site infection rates but a greater risk of catheter loss due to infection.9

The increased emphasis in recent years on providing adequate solute clearance to PD patients has led many nephrologists to conclude that obese patients may be too “big” to reach PD adequacy targets, especially after the loss of residual renal function. A national survey on U.S. nephrologists' recommendation of dialysis modality clearly showed that only 28% of patients with weight over 200 lb were recommended for PD compared with 44% of patients with weight less than 200 lb (adjusted odds ratio 0.44, 95% CI 0.35-0.55).3

Nolph et al.4 calculated that patients with standard weights more than 64, 77.6, and 91 kg could not achieve a weekly Kt/V_urea > 1.6 using four 2-, 2.5-, and 3-L exchanges per day, respectively. Standard weights assume that total body water equals (total body weight) × (0.58). Although instructive, the weight limits specified by this model do not apply to the majority of PD patients today who have residual renal function or use other PD modalities including nighttime exchange devices, or continuous cycling peritoneal dialysis (CCPD) with or without daytime exchanges since this model only refers to anuric patients on CAPD. We have reported that most (more than 80%) obese patients achieved both Kt/V and WCC targets set by NKF-KDOQI guidelines.7 Even among anuric obese patients, 90 and 70% were able to achieve Kt/V and WCC targets, respectively. Clearly, “obesity” defined as body mass index (BMI) ≥ 29 does not preclude adequate solute clearance in PD. In addition, reasonable percentages of patients weighing ≥ 100 kg or with BSA ≥ 2.0 achieved both solute clearance targets.

Conceptually, there are two types of “large-sized” patients: “large-obese” patients have increased body fat without necessarily having high TBW while “large-muscular” patients have high TBW but could be at their ideal body weight. The greater impact of high TBW than obesity on dialysis adequacy was clear in our study, which showed that in “large-obese” patients with both BMI ≥ 29 and TBW < 48 L, 94% (15 of 16) of obese patients with small or average frames achieved both of the KDOQI solute clearance targets.

It is important to note that NKF-KDOQI guidelines suggested much higher solute clearance than the Ad Hoc Committee on Peritoneal Dialysis Adequacy and the Canadian Society of Nephrology, which has adopted lower clearance targets10, 11 than those of the NKF-KDOQI.2 The former group considers a weekly Kt/V of ≥ 1.9 acceptable for CAPD and CCPD. Ninety-two percent of our obese patients were able to achieve this. The more recent 2006 KDOQI guidelines recommend a weekly Kt/V of 1.7 without consideration of WCC, and this is still easier to achieve.

The proportion of TBW to actual body weight has generally been thought to decrease progressively as patients get more obese, because the fat has very low water content. Thus, “standard weight” may overestimate TBW and thereby underestimate the actual maximal body weight compatible with adequate dialysis via a given modality. Wong et al.,12 employing the “gold standard” of deuterium oxide dilution to measure TBW in PD patients, found that the Watson volume formula13 generally underestimated TBW in obese patients whereas weight multiplied by 0.58 was more likely to overestimate TBW. However, Woodrow et al.14 found in a small study that Watson volume overestimated the TBW compared with that by the deuterium oxide method.

In view of these contradictory studies, it is reasonable to use the Watson volume to calculate TBW in obese patients, since it does take obesity into consideration by including both height and weight in the equation. However, it is important to note that the Watson equation was based on data from the cohort of healthy patients with body weight of 72.2 ± 14.2 kg (mean ± SD) in males and 69.1 ± 23.2 kg (mean ± SD) in females. It was not validated in patients with renal failure, and there was only small proportion of obese patients in the cohort. (Mean BMIs were 23.7 in males and 26.1 in females.) Adequest, the Baxter software for computing, uses the Watson formula to calculate the TBW. KDOQI guidelines recommend the Humes formula as another way to calculate TBW, and this formula is used by Pack PD, the Fresenius software to compute the adequacy.

There are other lines of evidence suggesting that the dangers of underdialysis to obese patients have been overestimated. Afthentopoulos and Oreopoulos15 reported in a retrospective study that patients weighing more than 80 kg had a survival rate similar to those weighing 60-80 kg. Fried et al.16 also reported that neither weight nor body surface area affected survival in PD patients. Moreover, Kopple et al.17 have reported that hemodialysis patients whose BMI was lower than the fiftieth percentile had a poorer survival than those with higher BMI. “The bigger, the better” was also shown in a recent article by Wolfe et al.18 They confirmed that higher body weight to height ratio is correlated with lower mortality in maintenance hemodialysis patients.

One consequence of the favorable effect of higher body weight on mortality is that mortality as a function of Kt/V shows a reversed J-shaped curve in hemodialysis patients.19 This has led some to suggest that Kt might be a more accurate prognostic factor than Kt/V or urea reduction ratio (URR). In our study, although Kt/V tends to be lower in obese patients, Kt tends to be higher. This may offset some of the potential negative impact of lower dialysis dose in obese patients.

There also is an interesting theoretical argument which may partially explain the good outcomes reported in large or obese PD patients. Singer and Morton20 explored the problems of normalizing Kt by TBW. Studies of comparative physiology have shown that glomerular filtration rate (GFR) is proportional to body weight to the power of 0.77 (BW^0.77), not to BW itself. Thus, GFR or Kt/V should be corrected by BW^0.77 or possibly V^0.77 instead. As a corollary of this mathematical relationship, normal solute clearance, expressed as GFR per BW or Kt/V, decreases significantly with increasing body weight. If the same concept is applied to peritoneal dialysis, the delivery of a weekly Kt/V of 2 to a 100-g individual should result in approximately 1.2 times as much equivalent dialysis as the same Kt/V in a 50-kg man. This implies that heavy patients could require less Kt/V for adequate dialysis than smaller patients.

There is a concern that obese patients might have a higher infection rate. This fear comes from the fact that many obese patients have difficulty in visualizing their exit site directly, which may compromise their care and lead to infection. When questioned, 35% of our obese patients could not directly visualize their exit sites because of a protruding abdomen or large breast (unpublished experience). Most of these patients either used a mirror or received help from a partner for their exit-site care. Some of the patients admitted that they checked the exit sites by touching with fingers without directly visualizing them.

Piraino et al.9 found similar peritonitis and exit-site infection rates in obese (defined as greater than 110% of ideal body weight) and non-obese patients, but a greater risk of catheter loss due to infection in obese patients. In our study we found a non-significant tendency for higher exit-site infection rates in obese patients. In addition to interfering with proper exit-site care, a thick fat pad in the abdominal wall may prevent adequate healing of the tunnel, which may allow retraction of the skin and subcutaneous tissues around the catheter during exit-site care with contamination of the tunnel. Surgeons need to be very creative in choosing the exit site so as to have as thin a subcutaneous fat pad as possible around the catheter. To enable the patient to see the exit site directly, choosing a more lateral exit site may be reasonable. Other logical options are “tummy-tuck” surgery to remove the fat pad prior to the PD catheter placement or the use of a pre-sternal catheter.

Finally, in our experience, obese PD patients were able to maintain an employment rate of 43.5% despite their relatively high exchange volumes and number of exchanges. This percentage compares favorably with that for non-obese patients in this study and with dialysis patients in general, suggesting that the dialysis prescription did not interfere unacceptably with quality of life. (A. Shetty, unpublished data).

To summarize, it is undeniable that extremely large size presents challenges to PD patients, especially after loss of their residual renal function. Compared with their slimmer counterparts, obese or very large patients can expect to require more frequent daytime exchanges, spend longer time on the cycler at night, or both. However, the vast majority of even very large patients can achieve adequate dialysis with clinically feasible dialysis prescriptions. These prescriptions may be potentially demanding but do not appear to restrict the ability to work or impair quality of life significantly more than other dialysis regimens. We feel there is no sound basis for denying these patients the opportunity to elect peritoneal dialysis if they are so inclined. Prior to electing a dialysis modality, large patients should be given a realistic picture of how much dialysis they are likely to eventually require. The option of switching to hemodialysis should also be reviewed with the patient when residual function is lost if the dialysis prescription becomes more burdensome. Clearly some patients will decide in this setting to voluntarily change to hemodialysis, although many others remain committed to the PD modality.

We have been frequently struck by the difficult situation of some unfortunate large patients who were initially directed away from peritoneal dialysis when their residual renal function would have made PD relatively easy. Ironically, the patients were referred later to PD after multiple access failures at a time when they were functionally anephric and had to struggle with their PD prescription. Another problem with the use of “salvage” PD is that in case of severe peritonitis or other PD complications it can be difficult or impossible to reestablish an access for hemodialysis. Initial use of PD may preserve residual renal function for a longer duration, and hemodialysis options may be reserved until they are more critically needed. PD may also prevent premature access failure by minimizing the need for central venous catherization and allowing more time for creation and maturation of arteriovenous fistulas. The increased potential for exit-site infections does not appear to be marked but should also not be ignored. Careful consideration of surgical options including optimal catheter location or pre-sternal catheter use may reduce this risk.