Cardiovascular Ischemic Heart Disease

Group E

Presentation ScenarioMrs TZ, aged 60 years, takes the following medications on a regular basis. Moduretic 50/5 1 M (commenced 3 weeks ago)Norvasc 10mg 1 DPlavix 75mg 1 D

Recent clinical chemistry data: Recent BP readings:Total Cholesterol 7.6mmol/L (<5.5) 180/95, 185/90, 185/95Triglycerides 3.3mmol/L (<2.0) HDL 0.7mmol/L (0.9-2.0)

 Estimate her LDL. Some medications are known to have an adverse effect on lipid levels. Which medications in Mrs TZ regimen effect lipid levels? Is the effect of thiazide diuretics on the lipid profile of clinical significance? When do thiazide diuretics impact on lipid levels occur with respect to drug therapy and is it a sustained effect? What is the mechanism of thiazide induced effect on lipid levels?

 Disorders in lipoprotein metabolism (dyslipidemia) can result in premature atherosclerosis or pancreatitis. The accurate classification of dyslipidemias is critical to the appropriate treatment of dyslipidemia. Define what specific tests are performed to classify dislipidemias and the specific results expected for each Fredrickson class of dislipidemia.

LDL-C Estimations• Laboratory measurements of total cholesterol

(TC), high-density lipoprotein cholesterol (HDL-C) and fasting triglyceride (TG) allow low-density lipoprotein cholesterol (LDL-C) to be calculated.

• LDL-C = TC – TG/2.2 – HDL-C

LDL-C = 7.6 – 3.3/2.2 – 0.7

LDL-C = 5.4mmol/L (<3.5mmol/L)

Medication Related Lipid Changes• Plavix/clopidogrel 75mg d

- Alterations to the lipid profile not documented

• Norvasc/amlodipine 10mg d- Alterations to the lipid profile not documented- Exerts an artheroprotective effect by reducing LDL-C accumulation and aggregation

• Moduretic/hydrochlorothiazide 50mg & amiloride 5mg- Alterations to lipid profile are a well established and common ADR of thiazide diuretics.- Alterations to the lipid profile not documented for amiloride a K-sparing diuretic.

HCTZ Induced Lipid Changes• There are differences in the nature and extent of lipid-level

changes reported

• A paper that surveyed the literature (31 studies) found

1) TC increases of 3.8% (ldTZ) and 6.3% (hdTZ)

2) TG increases of 10.8% (ldTZ) and 19.5% (hdTZ)

• A separate study found no statistically significant differences in cholesterol levels from baseline to 36 months during ldTZ therapy when adjusted for age, weight, baseline dietary percentage fat and saturated fat.

HCTZ Induced Lipid Changed Cont’

• The magnitude of the reported change in lipid levels differs significantly and is dependent on - 1) study population variables (age, gender,

disease state) 2) the nature of the study (duration,

placebo-controlled)3) the dose of drug used (ldTZ vs hdTZ)

Clinical Significance of Thiazide Induced Lipid Changes

• To be clinically significant a relationship between the measured change and a clinical outcome (ie mortality/morbidity) should be shown

• Establishing clinical significance is difficult- the artherosclerotic process is complex and dynamic - the follow-up period must be long enough to detect an eventual impact on morbidity

• There is no consensus on the clinical significance of thiazide induced lipid changes

Clinical Significance of Thiazide Induced Lipid Changes Cont’

• Study #1 (middle-aged, treated hypertensive men)

- High entry-level TC and TG levels +ive predictors of CHD

- TZ induced TC elevation independent +ive predictor of CHD

- TZ induced TG elevation not a independent +ive predictor of CHD

• Study #2 (middle-aged, normotensive, healthy men and women)

- No statistically significant TC, LDL-C, TG, HDL-C increases

- No increase in adverse cardiovascular events

- Potential sex differences

Clinical Significance of Thiazide Induced Lipid Changes Cont’

• Early studies showed lower than expected reductions in CAD for TZ treated patients and commentators hypothesized that TZ induced metabolic changes might be the cause.

• More recent studies have demonstrated that for hypertensive patients, any -ive effect of ldTZs on the lipid profile are more than balanced by the overall decrease in the risk of stroke, CVD and death.

• Data from new studies that are comparing ‘metabolically neutral’ anti-hypertensive agents with traditional 1st line agents will provide us with more information.

Thiazide Induced Lipid Changes are Dose-dependent

• Meta-analysis have shown that the effects of thiazides are related to dose. Older studies that used high doses (>50mg/d) consistently found increased cholesterol levels. More recent studies have had varying results. Some have found that even low doses of thiazides (12.5-25mg/d) can increase serum cholesterol levels, but others have not found any change in cholesterol levels with low doses. 

When do Thiazide Induced Lipid Changes Occur and are they Sustained?

• Plasma cholesterol levels rises during the first 3 months of treatment but at 12 months the cholesterol level falls to below baseline.

• The lowering of HDL-C is an unfavourable alteration, findings indicates that the lipid-lipoprotein disturbance of diuretic therapy does indeed persist but may manifest itself differently in long term study.

Thiazide Induced Lipid Changes

• In summary, elevation of serum cholesterol concentrations with thiazide appears to be a short term phenomenon, since most studies indicate the elevation reverts to baseline during long term treatment.

The Mechanism Of Thiazide Induced Lipid Changes

• Thiazide diuretics have been shown to increase total cholesterol and serum LDL levels. In clinical studies no effects on VLDL or HDL were reported.

• The cause of metabolic lipid changes in uncertain.

Mechanism Cont’

• One explanation is the stimulation of catecholamine release in response to the diuretic induced volume depletion. Catecholamines stimulate endogenous hepatic cholesterol synthesis. 

Mechanism Cont’• Another possibility is that it is not the diuretic itself that is

causing the metabolic changes but the resultant hypokalemia.

• It has been proposed that hypokalemia causes a diminished pancreatic insulin release and insulin resistance with impaired muscle uptake of glucose in response to insulin. This reduction in insulin sensitivity causes glucose intolerance and may result in increased hepatic reduction of cholesterol.

 • Studies have shown that estrogens are likely to be an effective

protective mechanism as premenopausal women experienced no lipid changes as opposed to postmenopausal.

Mechanism Cont’

• Studies have indicated that changes in lipid levels are primarily associated with higher dosages of thiazide diuretics. Back in the 1960’s doses of hydrochlorothiazide were as high as 200mg daily. Nowadays it is understood doses above 25mg daily do not further increase hypertension control due to the flat dose response curve.

 

Mechanism Cont’

• It is important to monitor and consider any changes to the lipid profile as each % point reduction in total cholesterol may reduce risk of cardiovascular mortality by 2%.

Classification of DyslipidemiaPhenotype Average of

overnight serum

Lipoprotein(s) Elevated

Blood Cholesterol

Levels

Blood Triglyceride

Levels

Atherogenicity

I Creamy top layer

Chylomicrons normal to ↑ ↑ ↑ ↑ ↑ none seen

IIa Clear LDL ↑ ↑ normal +++

IIb Clear LDL & VLDL ↑ ↑ ↑ ↑ +++

III Turbid IDL ↑ ↑ ↑ ↑ ↑ +++

IV Turbid VLDL normal to ↑ ↑ ↑ +

V Creamy top, turbid bottom

VLDL & Chylomicrons

normal to ↑ ↑ ↑ ↑ ↑ +

Laboratory Test Sample Collection

1. Collect a fasting blood sample. Patient must have been fasting for at least 12hrs before the blood is taken to ensure

that all chylomicrons from the last source of food have been cleared.

2. Place the separated plasma in a refrigerator and let stand overnight. Then check for the presence of a creamy layer,

and note whether the plasma is clear or turbid.

Observation of Plasma

TURBID OR CLOUDY: increased VLDL

CLEAR: elevated TC that must be caused by elevated LDL or HDL

CREAMY TOP LAYER: increased chylomicrons

Laboratory Test Sample Collection cont…

3. Mix the sample, take aliquots, and measure the concentration of plasma total cholesterol and

triglycerides. If an estimate of the risk of coronary artery disease is desired then the HDL cholesterol

concentration is also measured.

4. Sometimes a lipoprotein electrophoretic pattern may aid in making finer distinctions

Determination of Total Cholesterol

• Enzymatic methods for the hydrolysis of cholesterol esters and oxidation of cholesterol are simpler, faster to perform and use less corrosive chemicals than the more tedious chemical analysis.

• The serum can be used directly, with little interference from other serum constituents.

Enzymatic Determination of Total Cholesterol cont…

• PRINCIPLE: Cholesteryl esters in the serum are hydrolysed by cholesterol ester hydrolase.

• All the cholesterol is then oxidised by cholesterol oxidase to the corresponding ketone, with a shift in the location of a double bond.

• The H₂O generated by the oxidation is decomposed by horseradish peroxidase in the presence of 4-aminoantipyrine and phenol to yield a quinoneimine dye

• The absorbance of the dye, measured at 500nm is proportional to the cholesterol concentration.

Determination of Plasma Triglycerides

• All laboratories today quantify triglycerides by measuring the glycerol moiety of the molecule after hydrolysis.

• They perform the analysis by utilising enzymes in every step from hydrolysis to liberate glycerol to the formation of the end products that are measured spectrophotometrically, fluorometrically, or colourmetrically dependind on the series of coupled enzymatic reactions and the end products.

• For example the disappearance of NADH is measured spectrophotometrically in one method and fluorometrically in another method.

Lipoprotein Electrophoresis• The principle reason for requesting a lipoprotein electrophoresis is

to ascertain the type of hyperlipoproteinemia in patients with hyperlipidemia.

• Lipoproteins carry an electric charge at pH 8.6 and can be separated by electrophoresis on ararose gel.

• Chylomicrons are uncharged and remain at the origin.

• After electrophoresis separation, the lipoproteins are stained with a fat stain; the support medium is destained and then dried.

• The lipoprotein bands and chylomicrons are visible because of the fat stain dissolved in them and abnormal bands can be seen upon visual inspection or quantified from a densitometric scan of the membrane

Other Tests• Other tests are needed to rule out

secondary causes of hyperlipoproteinemia such as: hypothyroidism, alcoholism and kidney disease.

• Therefore, when hyperlipoproteinemia is secondary to another disorder, treatment of the latter usually will correct the hyperlipoproteinemia.