Question 28 from the second paper of 2021 was the first time PD was examined in any detail in the CICM Part Ii exam. The UpToDate article on PD in AKI by Golper & Ponce is comprehensive and free to read at the time of writing. In case their corporate lords drop the portcullis in the future, Ponce et al (2009) is just as good. If the reader insists on paying for stuff because to them the exchange of money is representative of value, they are invited to buy Critical Care Nephrology, where Chapter 179 by Ash & Crepaldi (2019) covers this territory very well.
Principles of peritoneal dialysis in the ICU
For dialysis and ultrafiltration to occur, you need three main components:
- Blood flow
- A diffusion membrane
In PD, two of these are supplied by the patient's peritoneal cavity.
- Access in dialysis is usually established via an indwelling catheter of some sort. In CVVHDF, you insert an intravascular device to access the circulating blood, and in PD you access the peritoneal cavity to access it for the instillation of PD fluid. The PD catheter is usually a long-term tunnelled tube with a fenestrated pigtailed end, usually made of some sort of inert silicone rubber with some acron cuffs to promote a fibrous seal. The catheter tip is usually left in some dependent portion of the cavity, so as to make it easier for the user to drain the fluid completely when semirecumbent. Here's an excellent diagram from Crabtree et al (2019) which illustrates the different positions for catheter exit (eg. Position D is for patients with stomas or multiple skin folds).
Obviously these tunneled versions are not easy to insert in a hurry, but non-tunnelled percutaneous options are also available, and the technique of inserting them can be similar to the technique of inserting a pigtail chest drain (modified Seldinger). Zou et al (2021) describes a comparison of complications from both techniques and found them reasonably similar. Unfortunately, the entire process typically belongs outside the comfort zone of most conventionally trained intensivists in a way in which vas cath insertion does not.
- Blood flow is required for clearance of solutes, just as in CVVDF. This blood flow is supplied by the mesenteric circulation. This circulatory territory can enjoy rather vigorous blood flow, which is much greater than the blood flow through the CVVHDF circuit, but the specific regional circulation of the peritoneal membrane is generally much lower. According to Solass et al (2016), the peritoneum gets 60-100ml/min, of 1-2% of the total cardiac output, mainly via the circumflex, iliac, lumbar, intercostal, and epigastric arteries. From there, blood from the parietal peritoneum drains back to the inferior vena cava, whereas the visceral peritoneum drains back to the portal vein.
- A large surface area is required for diffusion to be efficient. In the CVVHDF filter, this is a thin biocompatible polymer membrane with a surface area of 1.0-2.0 m2. In PD, this requirement is satisfied by the peritoneum, a thin (30-40 μm) membrane of mainly mesothelial cells which has approximately the same area in the adult as their skin surface- i.e. 1.7-2.0 m2 (Gotloib & Shostak, 1989). That thickness is obviously not the diffusion distance for substances: peritoneal capillaries can be at the surface, or they can be as deep as they like. To add an extra level of complexity, the capillaries exchange their content not directly with the peritoneal cavity but with the peritoneal interstitial fluid, which then exchanges its solutes with the contents of the peritoneal cavity. Lastly, only about 25-30% of the cavity surface actually ever comes into contact with the PD fluid, if rat studies are to be believed (Flessner et al, 1997)
- Dialysate is required. For CVVHDF, one can afford to throw vast quantities of corrosive chemicals like bicarbonate and potassium chloride into their dialysate, but in PD the fluid is in close contact with a delicate cell layer, which means it needs to play nice with cells. This limits the possible composition of PD dialysate. Moreover, conventional ambulatory PD does not involve dialysate flow, per se. Whereas a CVVHDF circuit may have a dialysate flow of 2L/hr, the same volume of PD dialysate fluid is instilled for a period of 8 hours. The total volume tends to range from 1.5L/volume in small adults to 2.5L/volume in larger ones, i.e. about 30-40ml/kg. One would not wish to use more than that for intermittent PD, as the pressure in the abdominal compartment would increase, and you want to keep it low so it does not interfere with ultrafiltration.
Essential elements of a PD prescription
- Dialysate concentration: The dialysate is usually a bicarbonate-buffered dextrose solution, available as sterile bags of different concentrations. It should be warmed to body temperature before it is instilled. The starting concentration is usually 1.5-2.5% dextrose, when one is aiming for a negative daily fluid balance of about 1L. The concentration of dextrose can be increased to achieve greater fluid removal (eg. up to 4.25%)
- Exchange volume is the administered volume, and this is usually 2L (30-40ml/kg)
- Inflow time is the time it takes to fill the peritoneal cavity, and this is usually 15-30 minutes (200ml/min). It is usually limited by catheter factors and patient tolerance.
- Dwell time is the period for which the exchange volume remains in the peritoneal cavity. A good broad rule of thumb is that anything less than 30 minutes is not enough, and anything more than 90 minutes is excessive.
- Outflow time depends on the volume and the catheter; it is a gravity-dependent drainage of dialysate fluid.
- Number of exchanges: this is determined by the effluent rate, much like CVVHDF. The more effluent per day you want to remove, i.e. the greater the urea clearance you want to achieve, the more exchanges you will need. On average, a stable ESRD patient on PD will have 4-6 exchanges per day, achieving a total daily effluent output of 8-10 L. For patients with a raised urea or those who require more rapid solute control (eg. hyperkalemia, acidosis), the number of daily exchanges can be increased.
Potential indications for PD in acute renal failure
- Patients with severe cardiovascular instability
- Severely coagulopathic patients
- Those with difficult vascular access
- Patients with existing ascites
- Patients with some residual renal function
- Where conventional dialysis modalities are not available
Advantages of peritoneal dialysis in the ICU
- Anticoagulation is not required
- There is no immunogenic extracorporeal circuit
- Haemodynamically better tolerated
- Markedly decreased risk of dialysis disequilibrium syndrome
- Does not require an expensive machine
- Less nursing staff workload, as compared to CVVHDF
- The patient may remain ambulant while intermittent PD is in progress
- An unexpected source of glucose, for patients who need extra calories
Disadvantages of peritoneal dialysis
- Catheter placement may not be available urgently (this skill set is not common among critical care staff)
- Shocked patients will have poorer peritoneal blood flow, and therefore lower efficiency of dialysis via the peritoneal membrane.
- An intact peritoneal cavity is required, i.e. this modality is not available for patients who have recently had abdominal surgery
- Slow solute and fluid removal means this is not a good choice where immediate management of either issue is required (eg. life-threatening hyperkalemia or pulmonary oedema)
- Large volume PD can result in respiratory compromise by pushing on the diaphragm from below
Contraindications to peritoneal dialysis
- Recent abdominal surgery
- Peritoneal adhesions or previous major abdominal surgery is only a relative contraindication, as it only makes laparoscopic catheter placement more challenging.
- Patients who have diaphragmatic pleuroperitoneal connections (i.e. all the fluid will end up in the chest)
- Patients in severe respiratory failure, who are at the border of requiring mechanical ventilation
- Patients with intraabdominal sepsis or abdominal wall infections
- Pregnant patients
- Patients with poorly controlled diabetes
Potential complications of peritoneal dialysis
- Perforation of a viscus (emergency catheter placement)
- Respiratory failure
- Pleural effusion
- Protein loss- up to 20g/day, or perhaps even higher if you have peritonitis
- Hypernatremia due to water removal
- Gastro-oesophageal reflux
- Poor solute clearance
- Catheter insertion site leakage
- Abdominal hernias