Measuring Microvascular Flow through Photoplethysmography in an Acute Compartment Syndrome Model
Yuki E. Iizuka, Brandon R. Macias, Robert M. Healey, Alan R. Hargens
UCSD Department of Orthopaedic Surgery, San Diego, CA
DISCLOSURES: Yuki E. Iizuka (N), Brandon R. Macias (N), Robert M. Healey (N), Alan R. Hargens (N)
INTRODUCTION: Acute compartment syndrome (ACS) is a serious complication in which elevated intramuscular pressure (IMP) within a closed fascial
space leads to microvascular occlusion, leading to ischemia of muscle compartments. Elevated IMP is caused by an increase in compartment content or
decrease in compartment volume. Unfortunately, the diagnosis of ACS continues to be challenging. Previous studies have shown that the current method of
slit catheter has been unreliable with low specificity, significant risk of pain or infection and poor diagnostic cutoff [1]. Furthermore, while it is known that
early fasciotomies lead to better outcomes, some cases self-resolve and do not require such treatment [2]. This uncertainty in diagnosis has led to the
exploration of new, non-invasive measurement techniques to diagnose ACS. One such method is photoplethysmography (PPG) which generates peak-topeak (PTP) amplitude, an indicator of pulsatile change in microvascular flow [3]. Previous PPG studies have investigated short-term (<10 minutes) in
microcirculation under elevated IMP conditions [4]. In this study, we were interested in observing changes over a longer period of time and wider pressure
ranges to better understand the altered microvascular flow in ACS. It is hypothesized that prolonged exposure to ACS-like conditions will fatigue the
myogenic response and reduce microvascular blood flow.
METHOD: Each subject was placed in a supine position and the PPG was secured onto the subject’s tibialis anterior. The PPG system consists of an LED
and a photoreceptor. An infrared light is emitted from the PPG system, which is attenuated differentially depending upon the microvascular flow of blood.
The attenuated light is captured by the receiver. The subject’s leg was placed inside a sealed 12”x36”x12” chamber with a neoprene sleeve. The chamber
was covered to reduce ambient interference. Positive air pressure was then applied by vacuum pump. PPG signals were captured and analyzed on LabVIEW
System Design Software. Following a 5 minute baseline acclimatization period at standard room pressure and temperature, the chamber pressure was raised
to either 20mmHG for 45 minutes, 40 mmHg for 45 minutes or 60mmHg for 30 minutes. Each experimental condition was followed by a 5 minute recovery.
Time points 30 minutes and later were not performed at the 60mmHg pressure to minimize risk of vascular compromise in subjects. All subjects were
consented and approved by the UC San Diego Institutional Review Board/Human Research Protections Program. Significant differences were determined at
p<0.05.
RESULTS: Eight healthy subjects were recruited for this study. PTP amplitude was significantly elevated with the application of all three pressures
compared with baseline (Figure 1). Microvascular flow was slightly obstructed at the 60 mmHg level. At 20, 40, and 60mmHg, the average baseline flow
was observed to be 0.59 +/- 0.20 V, 0.72+/-39 V, and 0.55 +/- 0.19 V respectively. The pulsatile waveform of the PPG signal demonstrated a dicrotic notch
at baseline and 20mmHg, which was lost at higher applied pressures (figure 2).
DISCUSSION: The results confirm that external compression and time of exposure are both significant factors affecting tibialis anterior microvascular
flow. Elevation of chamber pressure led to significantly higher PTP amplitudes from baseline at all chamber pressures, although not equally. Over a longer
length of time, the change in PTP amplitude is varied with 20, 40, and 60mmHg averages having a positive, neutral and negative trend in slope respectively.
This is consistent with the idea that there is a myogenic response that fatigues over time and pressure. In addition, while the 20mmHg and 40mmHg graphs
are similar in PTP amplitude values, preliminary analyses of the waveform of the PPG suggest otherwise. At 40mmHg and higher, the dicrotic notch is lost,
showing a simpler wave. This indicates a loss of compliance in the arteries and potentially a valuable indicator of the current state of vascular perfusion.
Further studies are needed to evaluate this new finding as a potential marker for progression in acute compartment syndrome, along with clinical signs of
acute compartment syndrome.
SIGNIFICANCE: This research explores a possible alternative method (PPG) for diagnosing compartment syndrome. Though significant changes were
observed over time for most signals with elevated pressure, loss of the dicrotic notch may be an indicator of vascular stability.
REFERENCE [1] Nelson JA. J Emergency Medicine 44(5) 1039-44, 2013. [2] Ivatury RR. J Trauma Acute Care Surg. 76(6):1341-8, 2014. [3] Sandberg
M. Acta Physiologica 183(4):335-43, 2005. [4] Elgendi M. Curr Cardiol Rev. 8(1):14-25, 2012. [5] Zhang Q, Styf J. Scand J. Med Sci Sports. 14(4):215-20,
2004; [6] Zhang Q, Macias BR, Neuschwander T, and Hargens AR. Journal of Sports Science and Technology. No 2S:232-238, 2010
IMAGES AND TABLES
Sample baseline PPG
20mmHg
40mmHg
60mmHg
2
1.5
1
20mmHg
0.5
40mmHg
0
60mmHg
Baseline
0
5
10
15
20
25
30
35
40
Recovery
Post 5…
PPG pulse amplitude
Cumulative Average Bloodflow Change
Figure 1. Average changes in PTP amplitude for 20, 40 and
60 mmHg
Figure 2. Typical waveform of baseline, 20, 40 and 60mmHg. Note loss of dicrotic
notch starting at 40mmHg
ORS 2017 Annual Meeting Poster No.0661