Diagnostic Testing: Difference between revisions

Several examples of the beneficial uses in diagnosis and treatment of cardiac patients are mentioned. However, keep in mind that the current list is only a brief summary and in real life the use of the ECG is broader. First, the ECG is an important tool in diagnosing and managing acute myocardial infarction. In patients with chest pain that is suspect for myocardial ischemia, the characteristic ST-segment changes (elevations or depressions) are one of the important corner stones of diagnosis and subsequent treatment.  In addition, rapid resolution of the ECG changes of myocardial infarction after reperfusion therapy has prognostic value and identifies patients with reperfused coronary arteries.  

In the diagnosis of the cause for severe rhythm disturbances, cardiac shock or after cardiac arrest the ECG is also of great importance for rapidly assessing possible underlying (cardiac) causes. Metabolic disturbances or medication induced arrhythmias can induce characteristic changes of the QT time, QRS and ST morphology. The diagnosis based on the ECG observations can be life-saving in emergency situations with patients in shock or after a cardiac arrest.

Echocardiography is based on the use of ultrasound directed at the heart to create images of cardiac anatomy and display them in real time on a digital screen. The transthoracic echocardiography is obtained by placing a transducer in various positions on the anterior chest The processing of the ultrasound waves creates cross-sectional images of the heart and great vessels in a variety of standard planes. In general, echocardiography is a sensitive and non-invasive tool for detecting anatomic abnormalities of the heart and great vessels.

The two-dimensional echocardiographic imaging technique is used to investigate the heart in multiple planes in order to asses the existence of (dys)function and structural abnormalities of cardiac chambers and valves throughout the cardiac cycle. Both the cross sectional and longitudinal views are used to look for the presence of any anatomical or functional abnormalities with most of the structures of the heart. [''Figure 4 & 5'']

In addition, in the cross sectional planes ventricular wall motion and left ventricular wall thickening during systole (an important measure of myocardial viability) can be investigated. The systematically assessment of cross sectional segment can also be used to estimate left ventricular volumes and ejection fraction. [''Figure 6'']

   

Although the two-dimensional imaging technique gives superior view of the important structures of the heart, the analog echocardiographic display referred to as M-mode, motion-mode, or time-motion mode, is still in use for the high resolution axial and temporal imaging. The analog technique is preferred to measure the size of structures in its axial direction, and its high sampling rate allows for the resolution of complex cardiac motion patterns. [''Figure 7'']

Doppler ultrasound is a technique of combined with the traditional ultrasound technique. The Doppler technique assesses changes in frequency of the reflected ultrasound compared with the transmitted ultrasound. The difference is used to be translated in a picture of the flow velocity. The continuous-wave Doppler mode is used to quantitate the exact velocity of the flow and estimate the pressure gradient when high velocities are suspected. The technique creates a graphic representation of the flow velocity in echotransducers’ beam in a time continuous wave. The technique is hampered by the fact that anatomical structures can make disturb the beam and subsequently the flow velocity measurement. When there is ambiguity about the source of the high velocity, pulsed-wave Doppler could be a more useful tool. This technique is range-gated in order to make it possible to investigate specific areas along the beam (sample volumes).  Another technique widely integrated in echocardiography is the colour Doppler. The colour Doppler technique projects in coloured images informative for the direction of flow, the velocity, and the presence or absence of turbulent flow. The flow velocity colour images are in real-time combined with the two-dimensional structural imaging to investigate blood flow in the heart and great vessels. The colour Doppler image technique is in particular of use in detecting regurgitant blood flows across cardiac valves or to visualise any abnormal communications in the heart. [''Figure 8'']

   

The non-invasive echocardiography has now largely replaced cardiac catheterization for calculation of the hemodynamics changes caused by valvular disease. Several examples of methods to examine hemodynamics of the heart and valves by echocardiopgraphy are:

Unfortunately, it is impossible to obtain high-quality images or Doppler signals in as many a small percent of patients. Underlying conditions such as obesity, emphysema or chest wall deformities can limit the use of transthoracic echocardiography. A technique to partly cope with these limitations is transoesophageal echocardiography (TEE) [''Figure 9''].  

   

With TEE a smaller ultrasound probe is placed on a gastroscopic device for introduction in the oesophagus behind the heart. Besides overcoming structural problems, in general TEE produces much higher resolution images of posterior cardiac structures. With TEE left atrial thrombi, small mitral valve vegetations, and thoracic aortic dissection can be diagnosed a high degree of accuracy. The downside of the techniques is the invasiveness of the procedure; the introduction of a probe into the oesophagus is very often experienced as rather uncomfortable by patients.

==Cardiac stress test==

The level of mechanical stress is progressively increased by adjusting the difficulty (steepness of the slope) and speed. The test administrator or attending physician examines the symptoms and blood pressure response. With use of ECG, the test is most commonly called a cardiac stress test, but is known by other names, such as exercise testing, stress testing treadmills, exercise tolerance test, stress test or stress test ECG.

[[Image:StressECG STDepression.jpg|thumb|right|300px|Stress-ECG of a patient with coronary heart disease: ST-segment depression (arrow) at 100 watts of exercise. A: at rest, B: at 75 watts, C: at 100 watts, D: at 125 watts.]]

The American Heart Association recommends ECG treadmill testing as the first choice for patients with medium risk of coronary heart disease according to risk factors of smoking, family history of coronary artery stenosis, hypertension, diabetes and high cholesterol.

The common approach for stress testing by American College of Cardiology and American Heart Association indicates the following:<cite>Two</cite>

(Sensitivity is the percentage of sick people who are correctly identified as having the condition. Specificity indicates the percentage of healthy people who are correctly identified as not having the condition.)

Stress cardiac imaging is not recommended for asymptomatic, low-risk patients as part of their routine care.<cite>Three</cite> Some estimates show that such screening accounts for 45% of cardiac stress imaging, and evidence does not show that this results in better outcomes for patients.<cite>Three</cite> Unless high-risk markers are present, such as diabetes in patients aged over 40, peripheral arterial disease; or a risk of coronary heart disease greater than 2 percent yearly, most health societies do not recommend the test as a routine procedure.<cite>Three</cite><cite>Four</cite><cite>Five</cite><cite>Six</cite>

*Acutely ill for any reason<cite>Seven</cite>

Side effects from cardiac stress testing may include

*As the tracers used for this test are carcinogenic, frequent use of these tests carries a small risk of cancer.

The stress test does not detect:

*Atheroma

The test has relatively high rates of false positives and false negatives compared with other clinical tests.

Once the stress test is completed, the patient generally is advised to not suddenly stop activity, but to slowly decrease the intensity of the exercise over the course of several minutes.

*Unstable atheroma produces ''vulnerable plaques'' hidden within the walls of coronary arteries which go undetected by this test.

*Limitation in blood flow to the left ventricle can lead to recurrent angina pectoris.

==References==