Authors

1. Tartavoulle, Todd DNS(c), CNS-BC
2. Manning, Jennifer DNS(c), CNS-BC
3. Fowler, Leanne H. MSN, MBA, RN, CCRN

Abstract

Review question/objective: The objective is to systematically review the best available evidence to determine the reliability and validity of cardiac index (CI) in postoperative cardiothoracic surgery (CTS) adult patients in bed position versus chair position. The research question used for the systematic review will be: "What is the effect of bed position in comparison with chair position on reliability and validity of cardiac index in the adult patient post cardiothoracic surgery?"

Background: Invasive hemodynamic monitoring has evolved over the past 30 years into an everyday practice of the measurement of cardiac specific vital signs in critical care environments. The pulmonary artery catheter, introduced in 1970, measures the hemodynamic parameter of CI. Measuring CI is a common nursing practice in hemodynamic monitoring of postoperative CTS patients.1 CI is an extremely important measurement in the immediate postoperative period for the maintenance of oxygenation and hemodynamic stability. The parameter is a body surface area specific measure of the amount of blood ejected from the left ventricle in one minute. CI is a product of heart rate and stroke volume (the amount of blood ejected from the left ventricle with each contraction). Stroke volume is influenced by preload (volume of blood in ventricle at end of diastole), afterload (ventricular wall tension during systolic ejection), and contractility (the ability of heart muscle to contract and distend).2 The normal range of CI is 2.2-4.0 L/min/2 and is variable over a 24 hour period peaking during activity and decreasing during sleep. CI is a primary determinant of end organ perfusion and oxygen delivery to the tissues.3 One of the key indices of adequate CI is the ability of the cardiovascular system to meet the metabolic demands of the tissues. Venous oxygen saturation, which provides an assessment of the amount of oxygen extracted by the tissues, is the percentage of hemoglobin saturation in the central venous circulation. The oxygen saturation of mixed venous blood provides a global picture of oxygen delivery and consumption of the metabolic needs of the various organs and tissues. CI, hemoglobin, and arterial oxygen saturation affect venous oxygen saturation.2 Morbidity and mortality is directly related to the delivery of oxygen to the tissues. Oxygen deprivation to the tissues results in higher morbidity and mortality rates in postoperative CTS adult patients.1

To ensure accuracy of CI readings, three major components are necessary: (1) patient positioning, (2) leveling the transducer (air-fluid interface) to the phlebostatic axis (external anatomical reference point), and (3) zeroing the transducer. Traditionally CI has been measured in patients while in supine position as most patients report more comfort with head of bed (HOB) elevated; and because HOB elevation is a key factor in the prevention of complications such as ventilator associated pneumonia. Research demonstrates accurate measurement of CI can be performed up to 45 degree HOB elevation. The practice has evolved from supine measurement of CI to 45 degree HOB elevation measurement of CI and must include a properly leveled and calibrated (zeroed) transducer.4 The accuracy of measurement is unknown when patients are in positions other than flat and supine.5

The phlebostatic axis is used as a baseline for consistent transducer height placement. The phlebostatic axis is located by identifying the fourth intercostal space at the midaxillary line. The axis approximates the level of the atria. Maintaining the transducer at the level of the axis allows for accurate changes in hydrostatic pressure in blood vessels above and below the level of the heart. Variations in the height of the transducer system by a little as one centimeter above or below the phlebostatic axis results in false readings of CI. Thus when the position of the patient changes, the nurse must re-level the transducer to maintain accurate measurements of CI.6

Calibrating the equipment to atmospheric pressure, referred to as zeroing the transducer, is the third component for accurate CI measurement. Atmospheric pressure exerts a force of 760 mmHg on any object on the earth's surface. A three-way stopcock nearest to the transducer is opened to air (atmospheric pressure) to eliminate the effects of atmospheric pressure on CI. The scale on the monitor is then zeroed to equal atmospheric pressure. Zero referencing is done upon insertion of pulmonary artery catheter, at beginning of each shift, when patient is moved, or when significant changes in hemodynamic status occur.4

Current standards of care on postoperative day one for CTS adult patients include transferring the patient out of bed to a chair to prevent post-surgical complications. When patients are placed in an upright position, there is a reflexive increase in total vascular pressure due to a decrease in pressure on the carotid baroreceptor.7 Carotid baroreceptors are sensitive to stretch and pressure and are located at the bifurcation of the common carotid artery and aortic arch. Impulses originating in baroreceptors project impulses to the nucleus of tractus solitaries (vasomotor center in the medulla). When a patient goes from a bed to a chair position the distribution of blood volume changes. Consequently, a reduction in CI and blood pressure triggers the baroreceptors to increase the heart rate to maintain blood pressure and CI. Additionally throughout patient position changes, baroreceptors maintain a constant mean arterial pressure (MAP) to maintain perfusion to vital organs. The ability of such compensatory mechanisms to maintain MAP is dependent upon the capacity of the heart and vascular system to function properly.2 Sitting in a bedside position will cause increased extremity dependency and knee and hip flexion possibly creating differences in blood flow, venous return and/or CI. When patients can tolerate the hemodynamic changes of chair sitting, postoperative CTS guidelines recommend implementing the activity early (postoperative day one). However, reliability and validity of measuring CI versus chair sitting position remains unclear in the practice setting. Moving from an early out-of-bed to a chair sitting position is important in the postoperative CTS adult population: however, reliability and validity of measuring CI in a bed position versus a chair sitting position remains unclear in the practice setting.8

The significance of measuring CI for the critical care clinician is to facilitate the physical exam and decision-making process for a treatment plan. Therefore, accuracy of the measure is vital for hemodynamic management of a critically ill or postoperative CTS adult patient.2 Postoperative CTS adult patients are generally in need of postoperative cardiovascular support due to a low CI resulting from preexisting heart disease, prolonged intraoperative cardiopulmonary bypass pump use, inadequate myocardial protection, or some combination of these factors. CI can be increased by correcting heart rate, preload, and afterload. Failure to raise CI greater than 2.2 L/min/2 results in a decrease in delivery of oxygen to the tissues and organs thereby increasing morbidity and mortality.6 The purpose of the systematic review is to identify the best available evidence to determine the reliability and validity of CI in postoperative CTS surgery adult patients in bed versus chair position. An initial search of the Cochrane Library, CINAHL, PubMed, Prospero, and the Joanna Briggs Institute Library did not locate any systematic reviews on this topic.