Renal Replacement Therapy (RRT)

Definition

• Current Surgical Therapy, 14e, p. 1604

Indications

• Miller's Anesthesia, 10e, p. 5702; Current Surgical Therapy, 14e, p. 1606

Mechanisms of Solute Clearance

• Diffusion: Movement of solutes down a concentration gradient across a semipermeable membrane. Best for small molecules (<1 kDa). Used in hemodialysis.
• Convection (Ultrafiltration): Solute drag driven by hydrostatic pressure gradients. Effective for larger molecules up to 50 kDa, and for correcting fluid overload or clearing drugs/toxins.

• Current Surgical Therapy, 14e, p. 1604

Modalities

1. Peritoneal Dialysis (PD)

• A catheter is placed in the peritoneal cavity; dextrose-containing dialysate drives solute and fluid removal via osmosis.
• Best suited for outpatient/chronic use.
• Limitations in acute/ICU setting: risk of peritonitis, contraindicated after recent abdominal surgery, suboptimal clearance in hypercatabolic critical illness.
• Important in low- and middle-income countries (LMICs) where other modalities are unavailable.

2. Intermittent Hemodialysis (IHD)

• Sessions of 3-6 hours, 3-6 days per week.
• Blood and dialysate flow in opposite directions across the membrane (countercurrent), maximizing the concentration gradient.
• Solute clearance is increased by raising blood or dialysate flow rates.
• Limitation: rapid fluid/solute shifts can cause intradialytic hypotension - less suitable for hemodynamically unstable patients.

3. Prolonged Intermittent RRT (PIRRT) / Slow Low-Efficiency Dialysis (SLED)

• A "hybrid" modality: slower rates over 6-18 hours per session (daily or alternate days).
• Creates more gradual volume/solute shifts than standard IHD - better hemodynamic tolerance.
• When prescribed daily, its clearance profile approaches that of continuous therapies.

4. Continuous Renal Replacement Therapy (CRRT)

• Performed 24 hours/day, making it the best-tolerated modality for hemodynamically unstable patients requiring vasopressors or inotropes.
• Main venovenous CRRT subtypes:
  • CVVH (Continuous Venovenous Hemofiltration) - convective clearance
  • CVVHD (Continuous Venovenous Hemodialysis) - diffusive clearance
  • CVVHDF (Continuous Venovenous Hemodiafiltration) - both mechanisms
• Arteriovenous approaches (e.g., CAVH) are largely abandoned due to arterial cannulation bleeding risks.
• Anticoagulation is generally required to prevent filter clotting - options include systemic heparin, regional heparin, or regional citrate anticoagulation (citrate preferred when systemic anticoagulation is contraindicated).
• Current Surgical Therapy, 14e, p. 1604-1606; Miller's Anesthesia, 10e, p. 5703

Modality Selection

• Current Surgical Therapy, 14e, p. 1606; Miller's Anesthesia, 10e, p. 5703

Timing of Initiation

• Early initiation (at AKI stage 2) showed benefit in some single-center cardiac surgery studies (lower mortality: 39% vs. 55%).
• Three large RCTs (including IDEAL-ICU) in critically ill patients found no mortality difference between early vs. delayed initiation, and up to 29% of patients in the delayed arm recovered without ever needing dialysis.
• The consensus is that unnecessary early initiation should be avoided, but dialysis should not be delayed once hard indications (acidosis, hyperkalemia, oliguria, fluid overload) are met.
• Miller's Anesthesia, 10e, p. 5702-5703

Long-term RRT: Chronic Dialysis and Transplantation

• Chronic hemodialysis (in-center or home): most common; ~5x more patients than PD in diabetic ESRD
• Chronic peritoneal dialysis: home-based; suitable for select patients
• Kidney transplantation: the preferred long-term RRT modality, offering superior survival, quality of life, neurodevelopment (in children), and cost-effectiveness compared to chronic dialysis

• Brenner and Rector's The Kidney, p. 3184; Goldman-Cecil Medicine, p. 1278

Complications

• Access-related: infection (catheter/tunnel), thrombosis, arterial injury
• Dialysis disequilibrium syndrome: rapid osmotic shifts causing cerebral edema (mainly with aggressive first-session IHD)
• Intradialytic hypotension: especially with IHD in hemodynamically fragile patients
• Filter clotting: managed with anticoagulation in CRRT
• Electrolyte disturbances: hypokalemia, hypophosphatemia
• Infection: peritonitis (PD), bacteremia (vascular access)
• AKI-to-CKD progression: AKI survivors have elevated risk of CKD, cardiovascular disease, hypertension, stroke, and renal malignancy

Summary

MD Anaesthesiology Exam Notes

PART 1: CENTRAL VENOUS CATHETER (CVC) BUNDLE

1. Definition and Rationale

• Current Surgical Therapy, 14e, p. 1612; Fuster & Hurst's The Heart, 15e, p. CICU chapter

2. CVC INSERTION BUNDLE

• Current Surgical Therapy, 14e, p. 1612-1613

Pre-procedure Time Out:

• Identify patient
• Obtain informed consent
• Verify side/site

During the Procedure:

• Maintain sterile field at all times
• Stop if contamination occurs

After the Procedure:

• Attach caps/IV tubing to lumens before removing sterile drape
• Apply sterile dressing; label with date and time
• Dispose sharps immediately; wash hands

3. CVC MAINTENANCE BUNDLE

• Current Surgical Therapy, 14e, p. 1613

4. Site Selection Summary

5. Special Considerations (Exam High-Yield)

• Catheters placed in non-sterile emergent situations (field, cardiac arrest) must be removed and replaced as soon as clinically feasible.
• Antimicrobial locks are considered for patients with prior CLABSI.
• Chlorhexidine-impregnated dressings reduce infection risk.
• Daily chlorhexidine baths for ICU patients are part of a broader CLABSI prevention strategy.
• Patients on ECMO, Impella, or IABP are at especially high infection risk due to emergency insertion, hemodynamic instability, and femoral access.

PART 2: PULMONARY ARTERY WEDGE PRESSURE (PAWP)

1. Definition

• Miller's Anesthesia, 10e, p. 4843

2. PAC Structure (Relevant to PAWP)

• 7.0-9.0 Fr, 110 cm in length, marked at 10-cm intervals
• 4 standard lumens:
  1. Distal port (tip) - PAP monitoring and PAWP measurement
  2. Proximal port (30 cm from tip) - CVP monitoring and injectate for thermodilution CO
  3. Balloon lumen - inflation with air for flotation and wedging
  4. Thermistor lumen - temperature sensing for cardiac output measurement
• Modern PACs may also include: continuous SvO2 monitoring (mixed venous oximetry) and a heating coil for continuous CO measurement

3. PAC Insertion and Flotation

• Preferred route: Right internal jugular vein - most direct path to right heart
• Balloon inflated with air; catheter advanced through:
  • Right Atrium (RA) → Right Ventricle (RV) → Pulmonary Artery (PA) → Wedge position
• Characteristic pressure waveforms confirm passage through each chamber (Fig. 32.36):

• Tip position confirmation: Chest X-ray; catheter tip should be within 2 cm of cardiac silhouette on AP film

Insertion Troubleshooting:

• RV waveform not seen by 40 cm → coiling in RA → deflate, withdraw to 20 cm, repeat
• PA waveform not seen by 50 cm → coiling in RV → deflate, withdraw, repeat
• Head-down position aids passage across tricuspid valve
• Right lateral + head-up position aids exit from RV (also reduces arrhythmias)
• Deep inspiration (spontaneous ventilation) increases venous return - facilitates flotation in low CO states
• Ice-cold solution (10-20 mL) injected through distal lumen can stiffen catheter for difficult insertions
• Counterclockwise rotation aids passage from RA through tricuspid valve

4. Physiologic Basis of PAWP

PAWP ≈ Pulmonary Venous Pressure ≈ Left Atrial Pressure (LAP) ≈ LVEDP

PAWP vs. Pulmonary Artery Diastolic Pressure (PADP):

• PADP is a continuous surrogate for PAWP and is preferred for ongoing monitoring
• PADP ≈ PAWP when pulmonary venous resistance is low (normal conditions)
• PADP > PAWP when pulmonary vascular resistance is elevated (e.g., pulmonary hypertension, PE, ARDS)

5. Clinical Uses of PAWP

• Miller's Anesthesia, 10e, p. 4843-4848

6. Abnormal PAWP Waveforms (High-Yield for Exam)

A. Mitral Regurgitation (MR)

• Tall, prominent V wave beginning in early systole (retrograde ejection into LA)
• Fusion of c and v waves; obliteration of x descent
• Mean PAWP overestimates LVEDP (use pre-V wave pressure for LVEDP estimate)
• Good approximation for mean LAP and risk of hydrostatic pulmonary edema

B. Mitral Stenosis (MS)

• Mean PAWP markedly elevated
• Attenuated diastolic y descent (due to obstruction to LA-LV flow)
• Absent a waves if patient is in atrial fibrillation

C. Pericardial Constriction

• "Dip-and-plateau" pattern ("square root sign") in RV and LV pressure traces
• Steep y descent (rapid early diastolic filling) followed by mid-diastolic plateau (h wave)
• PAWP elevated and equalized with other filling pressures

D. Cardiac Tamponade

• Elevated PAWP/CVP
• Attenuated or absent y descent (early diastolic filling impaired by compressive pericardial fluid)
• Waveform dominated by systolic x descent (monophasic appearance)
• Pulsus paradoxus present

E. PAWP During Positive Pressure Ventilation

• Inspiratory positive pressure artificially raises measured PAWP
• Always measure PAWP at end-expiration to eliminate this artifact
• Manual waveform analysis on calibrated monitor is more reliable than digital monitor algorithms

7. Limitations of PAWP as a Preload Marker

• Mitral regurgitation (large V waves inflate mean PAWP)
• Catheter not in Zone 3 (alveolar pressure transmitted)
• Positive pressure ventilation (unless corrected for end-expiration)
• Decreased LV compliance (ischemia, hypertrophy, cardiomyopathy)
• Increased juxtacardiac pressure (tamponade, constriction)

• Aortic regurgitation (premature mitral valve closure)
• High PEEP (may raise PAWP falsely or the relationship breaks down)

• Ventricular compliance is variable
• Juxtacardiac pressure affects the transmural distending pressure
• Ventricular interdependence (septal shift from acute RV pressure overload raises LVEDP while LVEDV falls)
• Multiple studies have shown PAWP (and CVP) poorly predicts fluid responsiveness in most clinical situations - though in systolic ventricular dysfunction, filling pressures are more accurate than volume indices
• Miller's Anesthesia, 10e, p. 4844-4845

8. Complications of PAC (PAWP Monitoring)

Insertion-related:

• Arrhythmias (most common; occur in >50% during insertion - usually self-limited PVCs/RBBB)
• RBBB (2-5%); complete heart block if pre-existing LBBB
• Pneumothorax, arterial puncture, air embolism (same as CVP)
• Coiling/knotting of catheter

In-vivo/continuous:

• Pulmonary artery rupture (rare but fatal; risk increased with pulmonary hypertension, elderly, anticoagulation, distal migration)
• Pulmonary infarction (due to persistent wedging or distal migration)
• Over-wedging (balloon over-inflation forces tip against vessel wall → gradually rising non-pulsatile trace; correct by withdrawing catheter)
• Catheter migration distally (wedges on partial balloon inflation - withdraw before pulmonary infarction occurs)
• Infection/sepsis
• Thrombosis

Misuse/misinterpretation:

• In studies, 30-50% of clinicians incorrectly measured PAWP - the single most important "complication" of PAC use
• Major morbidity specifically attributable to PAC is uncommon (serious complications: 0.1-0.5% of monitored patients)
• Overall minor complications (including self-limited arrhythmias) in >50% of patients
• Miller's Anesthesia, 10e, p. 4824-4825

9. Key Pressure Artifact: "Over-Wedging"

• Caused by distal catheter migration + eccentric balloon inflation forcing tip against vessel wall
• Trace shows: gradually rising, non-pulsatile pressure (flush system builds up against obstructed distal lumen)
• Management: Immediate gentle catheter withdrawal to a more proximal PA location

10. Normal Hemodynamic Values (Exam Quick Reference)

11. Exam Key Formulas

• PVR = (MPAP - PAWP) / CO × 80
• SVR = (MAP - CVP) / CO × 80
• CO (Fick) = VO₂ / (CaO₂ - CvO₂)
• Transmural PAWP = Measured PAWP - Juxtacardiac pressure (estimated as ~half of PEEP)

Summary Table for Rapid Revision