Finger Cuff Method (Volume Clamp / Vascular Unloading Technique)

Core Principle

• Uses an inflatable finger-cuff + an infrared plethysmograph to measure blood volume in the finger arteries, which changes during the cardiac cycle as arterial pressure (AP) changes.
• An automated feedback system inflates and deflates the cuff rapidly to keep finger artery volume constant (unloaded) by applying a counter pressure.
• From this counter pressure, the AP waveform is reconstructed indirectly, then analyzed using Pulse Wave Analysis (PWA) to estimate Cardiac Output (CO).

Systems Using This Method

First described by Peňáz and colleagues.

ClearSight System

• Uses one or two finger-cuffs + a heart reference sensor to compensate for hydrostatic pressure differences between heart level and the finger-cuff.
• Internally calibrates the AP waveform continuously to compensate for vasodilation and vasoconstriction.
• The displayed AP waveform is scaled and adapted to resemble brachial AP.
• AP waveform is analyzed using a three-element Windkessel model via PWA to estimate CO.

CNAP System

• Uses an alternating double finger-cuff.
• Scales the obtained AP waveform to intermittent oscillometric measurements (text cut off in image).

Key Concepts Involved

• PWA (Pulse Wave Analysis) - used to derive CO from the reconstructed AP waveform.
• Windkessel model - a mathematical model of arterial compliance used to estimate CO from the pressure waveform.
• Hydrostatic correction - necessary because the finger is typically not at heart level.
• Continuous calibration - needed to account for peripheral vascular tone changes.

Non-Invasive Cardiac Output Monitoring Methods

1. Finger Cuff Method (Volume Clamp / Vascular Unloading Technique)

The Core Idea

How It Works - Step by Step

• An inflatable air bladder (to apply counter pressure)
• An infrared plethysmograph (a light-based sensor that detects how much blood is in the artery by measuring light absorption)

• Normally, when your heart beats, blood pressure rises → finger arteries swell → more blood → less light passes through.
• The system detects this change and instantly inflates the cuff to squeeze the artery back to its original volume.
• When pressure falls between beats, the cuff instantly deflates.
• This happens many times per second - an active, real-time servo feedback loop.

• The pressure the cuff must apply to keep the artery volume constant = the actual arterial blood pressure at that moment.
• This gives a continuous beat-to-beat arterial pressure (AP) waveform - without any needle in an artery.

• The reconstructed AP waveform is analyzed using Pulse Wave Analysis (PWA).
• PWA uses the shape, area under the curve, and characteristics of each pulse to calculate Stroke Volume (SV).
• CO = SV × Heart Rate.

ClearSight System (Edwards Lifesciences)

• Uses 1 or 2 finger cuffs.
• Has a heart reference sensor worn on the chest - this corrects for the height difference between the finger and the heart (hydrostatic pressure correction). If your hand is below your heart, there is extra pressure from the column of blood, which would falsely elevate the reading.
• Continuously internally recalibrates for vasodilation (blood vessels widening) and vasoconstriction (blood vessels narrowing) which would otherwise alter the finger pressure.
• The final displayed waveform is scaled to resemble brachial artery pressure (the standard reference).
• Uses the three-element Windkessel model for PWA - this models the aorta as a compliant chamber (capacitor) with resistance, mimicking how blood flows out of the heart.

CNAP System (CNSystems Medizintechnik)

• Uses an alternating double finger-cuff - alternates between two cuffs to allow each finger to rest and maintain circulation.
• Scales the waveform to intermittent oscillometric (standard cuff) BP measurements for calibration.

2. Partial Gas Re-breathing (NICO System)

The Core Idea

Fick's Principle Refresher

"The amount of a substance taken up or released by an organ equals the blood flow through that organ multiplied by the difference in concentration of that substance between arterial and venous blood."

CO2 produced = Blood flow (CO) × (CvCO2 - CaCO2)

Therefore: CO = VCO2 / (CvCO2 - CaCO2)

How Partial Re-breathing Works

• CO2 is being continuously eliminated via the lungs.
• VCO2 (CO2 elimination), PaCO2, and PETCO2 are all at steady baseline.

• A re-breathing valve is opened, adding dead space to the circuit.
• The patient now re-breathes some of their own exhaled CO2.
• CO2 elimination (VCO2) decreases because some CO2 is being re-inhaled.
• End-tidal CO2 (PETCO2) and PaCO2 rise proportionally.
• The change in VCO2 and change in PETCO2 between baseline and re-breathing states allows calculation of CO.
• Since CvCO2 does not change in 30 seconds (CO2 diffuses so fast it equilibrates rapidly), the venous CO2 term cancels out.

• Parameters return to baseline.
• This cycle repeats every 3 minutes.

Equipment

• Connected to ventilator circuit via:
  • Infrared CO2 sensor
  • Pneumotachometer (measures airflow/volume)
  • Re-breathing valve

Limitations

1. Requires tracheal intubation and mechanical ventilation - not usable in spontaneously breathing patients.
2. Not accurate in:
   • Severe chest trauma
   • Significant intrapulmonary shunt (blood bypasses ventilated alveoli)
   • High CO states
   • Low minute ventilation
3. Validation studies have not confirmed accuracy compared to Pulmonary Artery Catheter (PAC).
4. Underestimates CO preoperatively and overestimates postoperatively after cardiac surgery.

3. Transthoracic Electrical Bioimpedance (TEB)

The Core Idea

How It Works

• Electrodes are placed on the neck and lower thorax (typically 4 pairs - outer electrodes transmit current, inner electrodes measure voltage).
• A high-frequency, low-amplitude current of known amplitude is passed through the thorax.
• The ratio of voltage measured to current applied = impedance (Zo).

• Zo varies with the amount of fluid in the thorax.
• During systole (heart ejection), blood rushes into the aorta → Zo decreases (more conductor present).
• The rate of change of Zo over time (dZ/dt) reflects the speed of this blood movement.
• SV = function of (maximum dZ/dt) × Ventricular Ejection Time (VET)
• CO = SV × HR

The Diagram Explained

4. Thoracic Bioreactance (NICOM Device, Cheetah Medical)

What is it?

Key Difference from Bioimpedance

How It Works

• A 75 kHz sine-wave current is transmitted across the thorax via one electrode.
• Another electrode receives the resulting voltage signal.
• The received voltage is compared to the original current using a phase comparator.

• When blood is ejected into the aorta (a pulsatile event), the inductive and capacitive electrical properties of the thorax change.
• This causes a phase shift - the received voltage signal is slightly delayed or advanced compared to the transmitted current.
• This phase shift specifically reflects pulsatile blood flow, making it much less contaminated by static fluid (lung water, pleural effusion, etc.).

• Bioimpedance signal is affected by ALL thoracic fluid (including lung water, edema).
• Bioreactance phase shift is primarily driven by pulsatile aortic flow only.
• Result: bioreactance is less affected by intra- and extravascular lung water, making it more accurate in critically ill patients with pulmonary edema.

• Two dual electrode pairs placed on either side of the thorax (4 electrodes total).
• Mean of two side measurements gives the final CO value.
• No need for intubation - works in spontaneously breathing patients.

Quick Comparison Summary