Minimally invasive and non-invasive methods of estimation of cardiac output (CO) were developed to overcome the limitations of invasive nature of pulmonary artery catheterization (PAC) and direct Fick method used for the measurement of stroke volume (SV). The important minimally invasive techniques available are: oesophageal Doppler monitoring (ODM), the derivative Fick method (using partial carbon dioxide (CO2 ) breathing), transpulmonary thermodilution, lithium indicator dilution, pulse contour and pulse power analysis. Impedance cardiography is probably the only non-invasive technique in true sense. It provides information about haemodynamic status without the risk, cost and skill associated with the other invasive or minimally invasive techniques. It is important to understand what is really being measured and what assumptions and calculations have been incorporated with respect to a monitoring device. Understanding the basic principles of the above techniques as well as their advantages and limitations may be useful. In addition, the clinical validation of new techniques is necessary to convince that these new tools provide reliable measurements. In this review the physics behind the working of ODM, partial CO2 breathing, transpulmonary thermodilution and lithium dilution techniques are dealt with. The physical and the physiological aspects underlying the pulse contour and pulse power analyses, various pulse contour techniques, their development, advantages and limitations are also covered. The principle of thoracic bioimpedance along with computation of CO from changes in thoracic impedance is explained. The purpose of the review is to help us minimize the dogmatic nature of practice favouring one technique or the other.
A forty nine year old male who had sustained acute myocardial infarction two days earlier, experienced respiratory arrest and complete heart block and was admitted with a diagnosis of left cerebellar infarct and obstructive hydrocephalus. On examination he was responding to deep painful stimuli with right hemiparesis. ECG indicated acute inferolateral myocardial infarction. Echocardiogram showed an ejection fraction of 63%. Arterial blood gases revealed severe respiratory alkalosis and arterial desaturation. The lungs were ventilated and inotropic support was started to ensure a stable hemodynamic status. Evacuation of the cerebellar infarct was planned as an emergency procedure. Anesthesia was maintained using N 2 O -O 2 -narcoticneuromuscular blocking drugs sequence. Continuous monitoring of ECG, ST segment, invasive arterial blood pressure, CVP, ETCO 2 , SaO 2 and temperature were instituted. The intraoperative period was uneventful. Elective postoperative ventilation was continued for three days. He was then transferred to the step down intermediate care unit for definitive therapy.
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