Morteza Naghavi MD,

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Functional Evaluation of Atherosclerotic Plaque,
Temperature, pH and Lactate

• Morteza Naghavi MD,
• Tania Khan, Babs Soller PhD, Peter Melling PhD,
• Mohammad Madjid MD, Ward Casscells MD,
• James T. Willerson MD

The University of Texas-Houston
Texas Heart Institute
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Introduction

• Casscells et al have previously discovered that
  plaque temperature correlates with other markers
  of vulnerability and that hot plaque may be a
  vulnerable plaque
• (The Lancet 1996)

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Plaque temperature correlates with number of
macrophages but not with smooth muscle cells
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• Our ex-vivo findings have been recently
  corroborated by others in-vivo, suggesting that
  increased plaque temperature compared to adjacent
  arterial wall correlates with patients clinical
  presentation.

Stefanadis et al 1999
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Why should vulnerable plaque be hot?

• Vulnerable plaque is inflamed and inflammation
  generates heat

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What else can be found in a site of inflammation?

• Increased metabolic rate namely increased oxygen
• and glucose consumption
• Free radical formation (ROS and RNS)
• Hypoxia
• ?

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pH

• Extra-cellular low pH or acidic environment is
  characteristic of an inflamed site
• This is mainly because of massive lactate
  formation resulted from increased metabolic
  activity of inflammatory cells. These cells
  normally do not find enough oxygen to burn
  glucose therefore, go through anaerobic pathway
  and generate a large a mount of lactate

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Back to old literature

• Atherosclerotic plaques are known to be hypoxic
  due to
• - Increased oxygen consumption by macrophages
• - Decreased oxygen diffusion through plaque
  fibrous cap
• Both result in anaerobic metabolism.
• (Bjornheden et al 1987)
• Macrophages are also known to be metabolically
  very active. For example the ATP consumption of
  every 2 activated macrophages equals to one
  isolated maximally loaded cardiomyocyte.
  (Newsholme et al 1989)

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why plaque pH or lactate?

• When incubated with Ox-LDL, macrophages are seen
  to reduce their environmental pH to as low as
  5.5. (De Vries et al 1998)
• Macrophages oxidize LDL in both neutral and
  acidic pH, but the LDL oxidation is more
  pronounced in an acidic environment.
  (Leake et al 1995)

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What could be the significance of an acidic
plaque?

• Low pH can activate or increase activity of
  acidic matrix digesting enzymes resulting in
  plaque softening, weakening and rupture.
• Others recently reported presence of acidic
  enzymes in human atherosclerotic plaques
• (Libby et all, 1998)

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pH Distribution in 48 Human Carotid Plaques
This histogram demonstrates distribution of pH
measured in 858 points in 48 carotid plaques of
48 patients. A marked variation ranging from 6.5
to 8.9 is seen.
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pH Heterogeneity Within and Between Plaques in 48
Human Carotid Specimens
Red box shows the middle quartiles, solid lines
shows median, cross-bars show pH range and
circles represents extreme values. This graph
shows a significant pH variation within and
between plaques. The variation within the plaque
holds the majority of the total variation.
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pH Heterogeneity in 9 Watanabe Rabbit Aortas
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pH Heterogeneity in 11 Human Umbilical Arteries
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pH in Lipid Rich Yellow Areas vs. Calcified areas
p lt 0.01
Areas with large lipid core exhibit higher
temperature and lower pH
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Lipid Rich Yellow Areas (A and B) vs. Calcified
Areas (C and D)
Movat
HE
Lipid core
Calcification
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Correlation of pH and Temperature in Human
Carotid Plaques Varied by the Areas
p lt 0.01
We see a marked inverse correlation between
temperature and pH of plaques that varies by
macroscopic characteristics of plaques.
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Correlation of ph and Temperature in 9 Watanabe
Rabbit Aorta
p lt 0.01
A significant inverse correlation between
temperature and pH is seen these rabbits similar
to human carotid plaques
A significant inverse correlation between
temperature and pH is seen these rabbits similar
to human carotid plaques
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Correlation of pH and Temperature in 11 Human
Umbilical Arteries
p 0.2
In contrast to human and rabbit plaques in human
umbilical there is no relationship seen between
temperature and pH
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Fluorescence Ratio (dual emission) Imaging
Microscopy of A Predominantly Calcified Plaque
Shows Significant Microscopic pH Heterogeneity
with Higher pH in Calcified Areas
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Fluorescence Single-Emission Imaging Microscopy
of a Predominantly Lipid Rich Plaque Shows
Significant Microscopic pH Heterogeneity Mostly
Acidic

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What else, besides Temp and pH?

• Oxidized lipid and proteins
• Ox-LDL
• Ox-collagen
• Nitrated proteins
• Nitrosyl hemoglobin
• Nitrosyl tyrosine
• ?

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How to measure Temperature, pH and the other
factors in plaque invivo?