Overview
Definition:
Acute Normovolemic Hemodilution (ANH) is an intraoperative technique where a portion of the patient's blood is intentionally removed and simultaneously replaced with cell-free colloid and/or crystalloid solutions
This reduces the red blood cell (RBC) mass in circulation prior to anticipated surgical bleeding, thereby decreasing blood loss during the procedure
The removed blood is typically stored and reinfused later, often after surgical hemostasis is achieved.
Epidemiology:
The use of ANH is not universally standardized and its prevalence varies significantly across institutions and surgical specialties
It is most commonly employed in elective surgical procedures with expected significant blood loss, such as major orthopedic surgeries (hip and knee replacements), cardiac surgery, and complex abdominal resections
Data on its precise incidence is limited due to these variations.
Clinical Significance:
ANH is a vital component of modern blood conservation strategies, aimed at minimizing allogeneic blood transfusions, reducing associated risks (e.g., transfusion reactions, immunomodulation, infection transmission), and potentially lowering healthcare costs
For surgical residents preparing for DNB and NEET SS examinations, understanding the principles, indications, contraindications, and management of ANH is crucial for safe and effective patient care in the operating room.
Indications
Surgical Indications:
Elective surgical procedures with anticipated moderate to massive blood loss (estimated >1000-1500 mL)
Procedures where allogeneic blood transfusion is undesirable due to religious beliefs (e.g., Jehovah's Witnesses) or patient preference
Patients with rare blood types or difficult-to-match blood
Patients at high risk of transfusion-related complications.
Patient Selection Criteria:
Adequate baseline hemoglobin (e.g., >12 g/dL for males, >11 g/dL for females)
Adequate cardiovascular and renal reserve to tolerate acute fluid shifts and potential transient anemia
Absence of significant coagulopathy or platelet dysfunction
Absence of severe sepsis or significant hypovolemia prior to the procedure.
Contraindications:
Emergent surgery
Patients with severe cardiovascular disease (e.g., unstable angina, recent MI, severe heart failure)
Patients with significant renal or hepatic insufficiency
Patients with known coagulopathies or severe anemia preoperatively
Sepsis or active infection
Hypovolemia
Severe pulmonary hypertension
Patients unable to tolerate fluid administration.
Preoperative Preparation
Patient Assessment:
Thorough preoperative evaluation of cardiovascular, renal, and respiratory function
Assessment of coagulation status with PT, aPTT, and platelet count
Review of patient's medications, particularly anticoagulants and antiplatelets.
Informed Consent:
Detailed discussion with the patient regarding the ANH procedure, its benefits, risks, and alternatives
Ensuring understanding and obtaining informed consent, especially crucial in cases of religious objections to transfusion.
Equipment And Personnel:
Availability of sterile collection bags (e.g., double-bag systems for blood storage), anticoagulant (e.g., citrate-phosphate-dextrose solution - CPD), infusion pumps, and appropriate IV access
Trained personnel (anesthesiologist, surgical team) to manage the process.
Procedure Steps
Timing Of Removal:
ANH is typically initiated after the induction of anesthesia and before the start of the surgical incision or critical dissection
This allows for replenishment of circulating volume while the patient is hemodynamically stable.
Blood Withdrawal And Replacement:
A calculated volume of blood (e.g., 1-2 units or 10-20 mL/kg) is withdrawn through a large-bore IV line
Simultaneously, an equivalent volume of cell-free fluid (e.g., balanced crystalloid like Ringer's Lactate or Normal Saline, or colloid like 5% albumin or hydroxyethyl starch - HES) is infused intravenously to maintain normovolemia and adequate circulatory filling pressures
Blood is collected in sterile bags with an anticoagulant.
Monitoring During AnH:
Continuous monitoring of vital signs (heart rate, blood pressure, oxygen saturation), central venous pressure (CVP), and urine output is essential
Hematocrit and hemoglobin levels are monitored to guide the extent of hemodilution
Target hematocrit levels are typically around 25-30%.
Blood Storage And Reinfusion:
The withdrawn autologous blood is stored at room temperature
It is typically reinfused later in the procedure, usually after surgical hemostasis is achieved and before closure, or postoperatively in the recovery room if significant bleeding persists
The decision for reinfusion timing depends on the surgical field, patient's hemodynamic status, and estimated blood loss.
Postoperative Care
Monitoring And Recovery:
Close monitoring of vital signs, hemodynamic parameters, and urine output in the postoperative period
Assessment for signs of anemia or hypovolemia
Continuation of fluid management and electrolyte balance correction.
Anemia Management:
If significant anemia persists postoperatively, conservative management with oxygen therapy and close observation may be sufficient
In some cases, iron supplementation or even allogeneic blood transfusion might be considered if the patient is symptomatic or hemodynamically unstable despite adequate volume resuscitation.
Potential Complications Postop:
Delayed wound healing due to poor oxygen delivery to tissues, ongoing anemia, fluid overload, or electrolyte imbalances
Careful management is required to mitigate these risks.
Complications
Early Complications:
Transient hypotension during withdrawal if fluid replacement is inadequate
Hypocoagulability due to hemodilution (dilutional coagulopathy)
Risk of air embolism if air enters the IV line during rapid infusion
Fluid overload leading to pulmonary edema.
Late Complications:
Prolonged anemia, leading to impaired wound healing and delayed recovery
Increased risk of infection in severely anemic patients
Potential for organ ischemia if oxygen delivery is compromised due to severe hemodilution and inadequate compensatory mechanisms.
Prevention Strategies:
Careful patient selection
Adequate IV access for simultaneous fluid replacement
Meticulous technique during blood withdrawal and infusion to prevent air embolism
Close hemodynamic monitoring
Avoiding excessive hemodilution (maintaining target hematocrit >25%)
Timely reinfusion of autologous blood
Prophylactic measures against coagulopathy if indicated.
Key Points
Exam Focus:
Understand the core principle of ANH: reducing RBC mass pre-bleeding to minimize blood loss, replaced by cell-free solutions
Key indications: elective surgeries with high anticipated blood loss, religious objections to transfusion
Contraindications: emergent surgery, compromised cardiopulmonary function
Target Hct: 25-30%.
Clinical Pearls:
ANH is a tool to manage expected blood loss, not an emergency measure
Always ensure adequate fluid replacement to maintain hemodynamic stability
Be vigilant for dilutional coagulopathy and manage it with judicious use of blood products if necessary
The decision to reinfuse depends on surgical hemostasis and patient status.
Common Mistakes:
Performing ANH in emergent situations or on patients with significant comorbidities
Inadequate fluid resuscitation leading to hypotension
Over-diluting to a very low hematocrit (<25%) without adequate physiological reserve
Forgetting to collect anticoagulant in the collection bags
Failure to monitor vital signs and fluid balance diligently.