What are the normal venous and arterial pressures in a dialysis patient with an Arteriovenous (AV) Fistula?

Normal Venous and Arterial Pressures in AV Fistulas for Dialysis Patients

For dialysis patients with AV fistulas, normal arterial pressure ratios range from 0.13-0.43, while normal venous pressure ratios range from 0.08-0.34. 1

Understanding Normal Pressure Values

Normal Pressure Ranges by Access Type

Native AV Fistulas:

• Arterial pressure ratio: 0.13-0.43
• Venous pressure ratio: 0.08-0.34

AV Grafts:

• Arterial pressure ratio: 0.35-0.74
• Venous pressure ratio: 0.15-0.49

These values represent normalized static intra-access pressure (IAP) ratios, which are calculated by dividing the measured pressure by the patient's mean arterial pressure (MAP) after accounting for offset pressures.

Pressure Measurement Methodology

The proper measurement of these pressures requires a standardized approach:

1. Static Intra-Access Pressure Measurement:

   • Stop the blood pump
   • Cross-clamp the venous line proximal to the venous drip chamber
   • Wait 30 seconds until pressure stabilizes
   • Record arterial and venous IAP values
   • Calculate normalized pressure ratios using the formula:
     • Arterial PIA = (arterial IAP - arterial Poffset - arterial P0)/MAP
     • Venous PIA = (venous IAP - venous Poffset - venous P0)/MAP

2. Dynamic Venous Pressure Measurement:

   • Measure at blood flow of 200 mL/min during first 2-5 minutes of dialysis
   • Use 15-gauge needles (or establish protocol for different needle sizes)
   • Ensure proper venous needle placement in vessel lumen
   • Using 15-gauge needles, threshold pressures indicating stenosis are:
     • 125 mmHg for Cobe Centry 3 machines
     • 150 mmHg for Gambro AK 10 machines

Clinical Significance of Pressure Values

Indicators of Access Dysfunction

Pressure values outside the normal range may indicate:

1. Venous outlet stenosis:

   • Arterial ratio ≥0.75 in grafts or ≥0.43 in native fistulas
   • Venous ratio >0.5 in grafts or >0.35 in native fistulas

2. Intra-access stenosis:

   • Arterial ratio >0.75 and venous ratio <0.5 in grafts
   • Arterial ratio >0.43 and venous ratio <0.35 in native fistulas

3. Arterial inflow problems:

   • Arterial ratio <0.3 in grafts
   • Arterial ratio <0.13 in native fistulas

Importance of Trend Analysis

Single measurements are less valuable than tracking trends over time. A progressive increase in venous or arterial segment PIA of ≥0.25 above previous baseline suggests a hemodynamically significant lesion, regardless of access type 1.

Practical Considerations

Pressure Differences Between Access Types

Native AV fistulas typically have lower intra-access pressures than grafts. In one study, only 29.2% of fistulas exhibited intra-access pressure above 40 mmHg, compared to 94.1% of grafts 2. This has implications for monitoring strategies, as venous pressure monitoring may be less effective for detecting venous needle dislodgement in fistulas.

Access Flow Considerations

Adequate access flow (Qa) is crucial for proper dialysis:

• KDOQI guidelines recommend intervention when Qa is <450-500 mL/min in AVFs 1
• Grafts with access blood flows <600 mL/min have higher thrombosis rates 1
• A ratio of venous access pressure to MAP >0.55 reliably predicts outflow stenosis in AVGs 1

Common Pitfalls in Pressure Measurement

1. Needle position issues:

   • Improper needle placement can cause falsely elevated pressures
   • Ensure venous needle is properly positioned in vessel lumen
   • Three consecutive elevated measurements are required to confirm significance

2. Equipment variations:

   • Different dialysis machines have different pressure thresholds
   • Needle gauge significantly affects pressure readings
   • Each unit should establish its own threshold values

3. Central stenoses with collateral circulation:

   • May present with "normal pressures" despite significant stenosis
   • Usually accompanied by ipsilateral edema

4. Failure to normalize pressures:

   • Raw pressure values must be normalized to the patient's MAP
   • Failure to account for offset pressures leads to inaccurate assessment

Regular monitoring of these pressure values, along with physical examination and access flow measurements, is essential for early detection of access dysfunction and prevention of complications such as thrombosis, which is the leading cause of access loss.