What is Peripheral Arterial Tonometry?
Peripheral arterial tonometry (PAT) is a non-invasive plethysmographic technique that measures pulsatile volume changes in the digital arteries of the fingertip using a probe that applies uniform pressure around the entire digit, primarily used to assess sympathetic nervous system activity, vascular function, and arterial stiffness.
Technical Mechanism
PAT technology works by applying a uniform pressure field (near diastolic blood pressure levels) that completely envelopes the distal finger, including its tip 1. This pressure optimization:
- Maximizes the dynamic range of the pulse signal
- Prevents confounding veno-arteriolar reflex vasoconstriction at the measurement site
- Reduces respiratory and movement artifacts
- Facilitates accurate long-term measurement 1
The fingertip vascular bed is a major site of sympathetic nervous system-mediated vasoconstrictor activity, making it an ideal location for detecting autonomic changes 1.
Clinical Applications
Arterial Stiffness Assessment
PAT is one of several tonometry-based techniques used to measure pulse wave velocity (PWV) for assessing arterial stiffness 2. Tonometry-based devices use piezoelectric transducers placed at pulse-detectable sites to record pressure waveforms, with measurements typically ECG-gated to calculate transit times between arterial segments 2.
Common tonometry devices include SphygmoCor, Complior, and PulsePen, which have demonstrated good reproducibility in measuring carotid-femoral PWV—the gold standard for arterial stiffness assessment 2, 3.
Sleep Apnea Detection
PAT has been extensively validated for ambulatory sleep apnea diagnosis through devices like Watch-PAT 4, 5, 1. The technology detects:
- Apneas and hypopneas with high correlation to polysomnography (r = 0.89) 4, 5
- Autonomic arousals with strong correlation (r = 0.77) 4
- Sympathetic activation associated with respiratory events 6
PAT identifies 92% of respiratory events compared to only 35% detected by visual EEG arousals, demonstrating its high sensitivity to autonomic changes 7. However, because arousals are not specific to sleep apnea, PAT's specificity is limited for diagnostic purposes in children 7.
Vascular Function Assessment
PAT measures two distinct vascular parameters:
- Baseline pulse amplitude: Reflects basal blood flow and arterial pressure pulsatility 8, 9
- PAT ratio (Reactive Hyperemia Index): Calculated as the post-to-pre occlusion pulse signal ratio during reactive hyperemia, corrected for baseline vascular tone 9
The baseline digital pulse amplitude correlates strongly with brachial artery blood flow velocity (r = 0.63-0.64) but does NOT correlate with flow-mediated dilation or reactive hyperemia-induced changes 8. This indicates PAT primarily reflects basal hemodynamics rather than endothelial-dependent vasodilation.
Clinical Outcomes Data
In the Framingham Heart Study (n=3,865), higher PAT ratio during reactive hyperemia was associated with:
- 24% lower risk of incident stroke (HR 0.76, p=0.013) 9
- 32% lower risk of ischemic stroke specifically (HR 0.68, p=0.001) 9
- No significant association with composite CVD events or myocardial infarction 9
In critically ill patients, PAT measurements correlate with disease severity and mortality:
- Lower baseline PAT values predict early mortality (each 0.1 decrease in ln(RHI) increases mortality odds by 13%) 10
- PAT improvement over 7 days predicts survival (survivors: +63.9% vs. non-survivors: -24.2%, p=0.026) 10
- Lower PAT correlates with renal and cardiovascular dysfunction 10
Important Caveats
- PAT cannot substitute for direct blood pressure measurement but provides complementary information on sympathetic and parasympathetic tone 6
- The reactive hyperemia PAT ratio does not reflect traditional flow-mediated dilation measured by brachial ultrasound 8
- In pediatric populations, PAT cannot replace EEG-based arousal scoring due to limited specificity, though it detects autonomic arousals not visible on EEG 7
- PAT is most useful for therapy monitoring in established sleep apnea patients rather than initial diagnosis 4, 5