Advances In Hemodynamic Monitoring

Clinical cardiac ultrasound or echocardiography is a technology that was pioneered in the post-world war II era evolving out of naval sonar technology. Initial images consist of M-mode imaging. Subsequently, over 60 years technology advanced to introduce 2D (1971) and 3D (1990) imaging into clinical practice and have led to greater accessibility and easy interpretation by clinicians.

Transesophageal echocardiography (TEE) was introduced in 1977 and was applied to an intraoperative environment in 1981 for continuous cardiac evaluation and monitoring during surgery.

Recent technological advances have significantly improved the perioperative care of patients with complex cardiac pathophysiology.

There is no monitor more powerful to diagnose and assess cardiac function perioperatively than transthoracic echocardiography (TTE) and TEE. Intra-operatively limited access to chest makes TEE as an ideal option to visualize the heart, using disposable TEE probes. Echocardiography employs ultrasound from 2 to 10 MHz and a probe transducer containing piezoelectrode which converts electrical energy to ultrasound waves. TEE attempts to determine if the heart is adequately filled, contracting appropriately, not externally compressed, and devoid of any grossly obvious structural defects. It estimates hemodynamic parameters like stroke volume, cardiac output, and intracavitary pressures.

Epiaortic and epicardiac ultrasound imaging is done by thoracic surgeons intraoperatively with a sterile echo probe to view aorta and heart. Air filled trachea prevents TEE imaging of ascending aorta and epicardium hence TEE is contraindicated in esophageal and gastric pathology. Recent advances of TEE include 3D imaging, continuously monitoring patients, strain imaging and assessment of diastolic ventricular function in patients in ICU. 3D echocardiography makes a diagnosis of complex pathologies more accurate and is essential for guiding procedural interventions in real time. Improvements in real time 3D resolution facilitates high-quality imaging without artifacts caused by arrhythmias and cardiac translation and is particularly helpful in MVR (mitral valve repair) and percutaneous transcatheter aortic valve replacement. An emerging technology of continuous TEE using disposable TEE probe which remains inpatient for up to 72 hrs in ICU, is showing some early promise and in coming years may replace pulmonary artery catheter for hemodynamic evaluation of unstable tracheostomized patients.

Myocardial strain analysis directly tracks myocardial compression and expansion throughout the cardiac cycle by measuring changes in sarcomere length as opposed to percentage volume change in the left ventricular cavity. Two modalities have developed to assess ventricular strain i.e. tissue Doppler imaging and speckle tracking echocardiography. Doppler effect is routinely used in the echocardiographic examination to determine the direction and velocity of blood flow and tissue movement of the heart. Color flow Doppler identifies areas of abnormal flow by creating a visual picture of the heart’s blood flow by assigning a color code to the velocities in the heart. Blood flowing away from the echo transducer is blue and blood moving towards probe is red.

Esophageal aortic Doppler ultrasound assess descending aortic flow which determines cardiac output by measuring aortic blood flow and aortic cross-sectional area by assuming a constant partition between caudal and cephalic blood supply areas. Cross-sectional area are obtained either from nomograms or by M-mode ultrasound. Doppler probe is smaller than that for TEE and is disposable probes with a life span of 24 to 48 hrs. It correlates well with cardiac output measured by thermodilution. High inter-observer variability, probe displacement, cost, and poor tolerance in awake patients are some of the problems.

Electrical bioimpedance/bioreactance uses constant electrical current stimulation for identification of thoracic or body impedance variations induced by vascular blood flow. Impedance to the current flow produces a waveform which is electronically evaluated along with the timing of aortic opening and closing which can be used to calculate left ventricular ejection time and stroke volume.

Fick’s principle for cardiac output measurement is based on the assumption that blood flow through the pulmonary circulation is kept constant and there is no shunt. The Fick’s principle relies on the observation that the total uptake (or release) of a substance by the peripheral tissue is equal to the product of the blood flow to the peripheral tissues and the arterio-venous concentration gradient of the substance.