Methods for Measuring Tissue Pressure
Tissue pressure can be measured using various techniques including needle manometry, pressure transducers, and specialized monitoring systems, with the choice of method depending on the clinical context and anatomical location.
Direct Measurement Techniques
Needle Manometry: Traditional method using a needle inserted into the tissue compartment connected to a pressure transducer system. This technique should be zeroed and calibrated to account for differences between the measurement site and the right atrium 1.
Microcatheter Measurement: Used particularly for vascular measurements, where a microcatheter is passed through a guiding catheter. Simultaneous pressures can be recorded with the tip of the microcatheter visualized approximately 1 cm past the orifice of the guiding catheter 1.
Transducer-Tipped Intracompartmental Monitors: Dedicated devices that directly measure pressure within tissue compartments, providing more accurate readings than traditional needle techniques 1.
Slit Catheter System: Provides more accurate measurements than standard 18-G needles, which may overestimate compartment pressure by up to 18 mmHg 1, 2.
Measurement Parameters and Calibration
Calibration Process: The pressure transducer system should be zeroed and calibrated taking into account any difference between the height of the measurement site and the right atrium as the zero level for atmospheric pressure 1.
Pressure Units: Tissue pressures are recorded in millimeters of mercury (mmHg), relative to the right atrium as the zero level 1.
Normal Values: Normal tissue pressure in compartments is approximately zero mmHg, with significant increases indicating potential compartment syndrome 3.
Clinical Applications
Compartment Syndrome Assessment
Absolute Pressure Threshold: Traditionally, an absolute compartment pressure of ≥30 mmHg has been regarded as a diagnostic cut-off for acute compartment syndrome requiring fasciotomy 1.
Differential Pressure Threshold: More accurate approach comparing tissue pressure to diastolic blood pressure. Fasciotomy is usually indicated when tissue pressure rises to within 10-30 mmHg of the patient's diastolic blood pressure 1, 3.
Continuous vs. Single Measurements: Continuous compartment pressure monitoring may be used in high-risk, obtunded patients, though there is little evidence it reduces the risk of missed acute compartment syndrome compared with serial examination in alert patients 1.
Vascular Measurements
Feeding Artery Pressure: Measured at a point distal to which there are no nutritive vessels. Free flow of contrast should indicate that the catheter tip is not wedged 1.
Pulse Wave Velocity: Can be measured using ultrasound techniques to assess arterial stiffness, with pressure and flow velocity measurements used to calculate local pulse wave velocity 1.
Non-Invasive Alternatives
Segmental Pressure Measurements (SPMs): Compare systolic pressures at sequential levels in extremities to evaluate for significant pressure drops (20 mmHg between adjacent measurements) suggesting hemodynamically significant stenoses 1.
Pulse Volume Recordings (PVRs): Provide qualitative measurement of limb perfusion using pneumoplethysmography cuffs inflated to specified pressures at predetermined levels 1.
Tissue Perfusion Pressure (TPP): Defined as the difference between mean arterial pressure and critical closing pressure, providing information about tissue perfusion that may predict clinical outcomes 4.
Transcutaneous Oxygen Pressure (TcPO2): Allows determination of oxygen tension within tissue, with values >40 mmHg considered predictive of successful healing in areas surrounding ulcers or amputation sites 1.
Potential Pitfalls and Considerations
Equipment Selection: The obtained compartment pressure may be affected by the technique and equipment used. Standard needles may overestimate pressure compared to slit catheters 1.
Measurement Location: Pressure should be measured in the relevant compartments in the affected limb, as values can vary significantly between different anatomical sites 1.
Interpretation Context: Pressure measurements should be interpreted in the clinical context, as no single sign or investigation is guaranteed to diagnose or exclude conditions like compartment syndrome 1.
Pressure Gradients: High pressure gradients through the skin can be caused by tissue rigidity (fibrosis) and dissipation of applied compression force to proximal noncompressed regions 5.