Plethysmography measures blood volume changes in body parts. It's crucial for assessing circulation and diagnosing vascular issues. This section covers different types, like strain gauge and photoplethysmography, and their applications in clinical settings.

Understanding plethysmography is key to grasping blood pressure measurement techniques. It allows for non-invasive monitoring of blood flow, oxygen levels, and vascular health, making it an essential tool in cardiovascular assessment and patient care.

Types of Plethysmography

Strain Gauge and Impedance Plethysmography

• Strain gauge plethysmography uses a thin, stretchable band (mercury-in-rubber or indium-gallium) around a limb to measure changes in circumference
  • As blood volume increases, the band stretches, changing its electrical resistance
  • Allows for measurement of blood flow and venous capacity in the limb (forearm or calf)
• Impedance plethysmography measures changes in electrical impedance of a body segment
  • Electrodes are placed on the skin, and a small alternating current is applied
  • Changes in blood volume alter the electrical impedance between the electrodes
  • Can be used to assess blood flow in the limbs or thorax (impedance cardiography)

Photoplethysmography and Air Plethysmography

• Photoplethysmography (PPG) uses light to detect blood volume changes in the microvascular bed of tissue
  • A light source (LED) illuminates the skin, and a photodetector measures the amount of light absorbed or reflected
  • As blood volume increases, more light is absorbed, producing a pulsatile signal (AC component) superimposed on a slowly varying baseline (DC component)
  • Commonly used in pulse oximetry to measure oxygen saturation and heart rate
• Air plethysmography involves enclosing a body part (limb) in an airtight chamber
  • Changes in volume displace the air within the chamber, which can be measured using a pressure transducer
  • Allows for assessment of venous function and detection of venous obstruction or reflux

Respiratory Plethysmography

• Respiratory plethysmography measures changes in volume associated with breathing
  • Whole-body plethysmography: The subject sits inside a sealed chamber, and volume changes are measured as pressure fluctuations within the chamber
  • Inductance plethysmography: Elastic bands with embedded wires are placed around the chest and abdomen, and changes in cross-sectional area are measured as changes in inductance
  • These techniques allow for measurement of lung volumes, airway resistance, and respiratory patterns

Principles and Measurements

Volume Changes and Venous Occlusion Plethysmography

• Plethysmography is based on the principle that changes in blood volume within a body segment lead to proportional changes in the segment's volume or circumference
• Venous occlusion plethysmography is a common technique used to measure blood flow in the extremities
  • A cuff is inflated around the proximal part of the limb (above venous pressure but below arterial pressure) to temporarily occlude venous outflow
  • As arterial inflow continues, the limb swells, and the rate of volume change is proportional to the arterial blood flow
  • The cuff is then deflated, allowing the accumulated blood to drain, and the cycle is repeated

Pulse Oximetry

• Pulse oximetry is a non-invasive method that uses photoplethysmography to measure arterial oxygen saturation (SpO2) and heart rate
• It relies on the different light absorption properties of oxygenated (HbO2) and deoxygenated hemoglobin (Hb) at red (660 nm) and infrared (940 nm) wavelengths
  • HbO2 absorbs more infrared light, while Hb absorbs more red light
• By measuring the ratio of light absorbed at these two wavelengths, the device calculates the proportion of HbO2 to total hemoglobin, which represents the SpO2
• The pulsatile nature of the PPG signal allows for the isolation of arterial blood from the non-pulsatile background (venous blood, tissue, and bone)

Clinical Applications

Peripheral Arterial Disease Diagnosis

• Plethysmography can be used to diagnose peripheral arterial disease (PAD), which is characterized by narrowing or blockage of the arteries in the legs
• Ankle-brachial index (ABI) is a common screening test for PAD
  • Blood pressure cuffs are placed on the arms and ankles, and the systolic pressures are measured using a Doppler probe
  • The ABI is calculated as the ratio of the ankle systolic pressure to the higher of the two arm systolic pressures
  • An ABI < 0.9 indicates the presence of PAD
• Segmental pressure measurements and pulse volume recordings (PVRs) using air plethysmography can help localize the level of arterial obstruction
  • Cuffs are placed at various levels on the leg (thigh, calf, ankle, and foot), and the pressures and PVRs are compared
  • A significant drop in pressure or change in PVR waveform between segments suggests the presence of an obstruction

Venous Insufficiency Assessment

• Plethysmography is valuable in assessing venous function and diagnosing venous insufficiency, which occurs when the veins have difficulty sending blood back to the heart
• Venous filling time (VFT) can be measured using air plethysmography
  • The patient is asked to elevate the leg, allowing the veins to empty
  • The leg is then lowered, and the time taken for the veins to refill (VFT) is measured
  • A short VFT (< 20 seconds) suggests the presence of venous insufficiency
• Venous outflow obstruction can be detected using impedance plethysmography
  • The patient is asked to perform a series of tiptoe movements, which activate the calf muscle pump and promote venous outflow
  • If there is an obstruction, the impedance change will be reduced compared to the expected response
• These techniques help guide treatment decisions, such as the need for compression stockings or surgical intervention (venous ablation or stripping)