Non Invasive Monitoring of Cardiac Output

Improving technology in the field of sonography has definitely contributed to better application of non-invasive techniques of assessment of haemodynamics

"The search for the most ideal method of assessing CO continues. No method known so far gives perfect information" - Dr Srinivas Samavadam Chief Intensivist Care Hospital, Hyderabad

Monitoring of haemodynamics is one of the most common reasons for admission to Intensive Care Units (ICU). The term haemodynamics implies the pattern, adequacy and consequences of output from and venous return to the heart. Several conditions could result in a state of compromised haemodynamics. Any condition that results in such a derangement is said to result in 'shock.' Although shock has been defined in several ways, the bottom line remains an inadequacy of blood supply to and utilisation by the various tissues.

Conditions which could result in a 'shock' state include severe blood loss ( hemorrhagic shock), loss of fluids
(hypovolemic shock), poor cardiac function ( cardiogenic shock) and alterations in autonomic nervous system (neurogenic shock). Early recognition and appropriate reversal of this state is crucial if compromise in organ function (multi - organ failure) is to be minimised.

Monitoring of the pattern of haemodynamic compromise response to therapy and the consequences of interventions therefore plays a crucial role in the management of shock. Traditionally, such monitoring techniques involved catheterisation of the pulmonary artery and using principles of thermo-dilution to determine the pattern of haemodynamic abnormality. This technique has remained the 'gold standard' for haemodynamic assessment and management, for close to three decades. Although, complication rates with this technique are not excessively high, it is considered to be a highly invasive method. Moreover, this technique, surprisingly, has not shown to improve the outcome of patients, except among young victims of trauma and those who have undergone cardiac surgery.

In addition, in a resource limited environment like ours, any technique which involves catheters, monitors and laboratory tests is bound to be expensive. Therefore, in the last decade or so, the emphasis has been on techniques which are less invasive, equally reliable and relatively inexpensive. Improving technology in the field of sonography has definitely contributed to better application of non invasive techniques of assessment of haemodynamics.

Markers of haemodynamic adequacy: The adequacy of blood supply is reflected in the extent of tissue dysfunction. Several markers have been identified which reflect the integrity of the haemodynamic status. Markers which assess flow and pressure in the heart, vena cava, pulmonary artery and aorta are designated as upstream markers. This group includes Systemic Blood Pressure (SBP), heart rate, Central Venous Pressure (CVP), Pulmonary Capillary Wedge Pressure (PCWP) and Cardiac Output (CO). Upstream markers have been traditionally used for haemodynamic assessment of critically ill patients. However, as knowledge about the microcirculation improves, it is increasingly being recognised that markers of tissue under-perfusion are more sensitive markers of the haemodynamic status. These are the 'downstream' markers. These markers include urine output, blood lactate, base excess, tissue carbon dioxide ( CO2) levels and mixed venous oxygen and CO2 levels.

Monitoring of Upstream Markers: Cardiac output is the most important upstream marker of the haemodynamic status. All newer modalities of haemodynamic assessment have been evaluated against the performance of the PA catheter.

Echocardiography

This technique utilises ultrasound waves to generate real time images of the heart. Size of the chambers, contractility of ventricles and function of the valves can be assessed by this technique. Application of Doppler facilitates assessment of flow. Assessment of global ventricular function helps in the assessment of the critically ill patient. Hyper-contractility of the left ventricle implies a hyperdynamic state (septic shock), while poor left ventricular function implies a cardiogenic origin of haemodynamic instability. This would help in prioritising therapy in terms of fluid challenge, vasopressors and inotropes.

Cardiac output and pulmonary artery pressures can be measured, if flow can be measured and assessed accurately. However, initial expenditure on th CO2 partial re-breathing can be used to calculate cardiac output using the modified 'Fick principle.' This technique compares end tidal CO2 partial pressure between a non re-breathing and a subsequent re-breathing period. An estimate of CO2 can be obtained from the ratio of change in Et CO2 and CO2 elimination after a brief period of re-breathing. However, this technique has not been validated in general ICU patients and is not known to be reliable in a spontaneously breathing patient.

Oesophageal Doppler

This technique measures blood flow velocity in the descending aorta using Doppler signals. The diameter of the aorta, distribution of cardiac output to the descending aorta and the measured aortic blood flow velocity are utilised to estimate the cardiac output. This technique is also user dependent, associated with a long learning curve and is logistically difficult in a critically-ill patient. However, effect of therapeutic interventions on cardiac output can be reliably identified with this technique.

Pulse Contour Analysis

This method utilises the variations in the pulse pressure wave form to estimate CO. This technique utilises the direct relationship between pulse pressure and stroke volume and the inverse relationship between pulse pressure and vascular compliance. Compliance, which is difficult to measure, is calculated based on age, sex, ethnicity and body-mass- index. The CO is calculated based on the contour of the pulse pressure wave form. The stroke volume thus calculated, is compared to the SV obtained by thermo dilution technique. This gives a continuous estimate of CO. However, the use of vasoactive agents (dilators and constrictors) can cause false changes in the estimated CO. This principle can also be applied by using lithium dilution for external calibration.

The wash out curve of lithium is generated over time which substitutes for thermo dilution. This technique is reported to have good co-relation with data obtained using the PAC.

Plethysmography

Thoracic electrical bioimpedance has been used to estimate cardiac output. Electrical impedance or resistance to low voltage is measured across the chest. The higher the fluid content, lower the impedance, since fluid conducts electricity. But the co-relation with conventional techniques, this technique has not been validated in critically ill patients.

Heart Lung Interactions

In critically-ill patients who are being ventilated mechanically, the effect of the respiratory cycle on stroke volume of the left ventricle can be used as an index of fluid responsiveness. Positive pressure ventilation reduces the venous return to the right heart during inspiration. This reduces the left ventricular filling which results in a fall in left ventricular SV, CO and systemic blood pressure. This response is exaggerated in hypovolemic patients. Using this principle, pulse pressure variation and Stroke Volume Variation (SVV) have been combined with the pulse contour analysis to predict volume responsiveness.

Monitoring of Downstream Markers

Lactate: In situations where CO is suboptimal, the inadequate blood supply creates an anaerobic environment. In such a milieu, a partial metabolic pathway is followed which generates lactate. Lactate, therefore, is commonly used as a global downstream marker of adequacy of resuscitation and tissue perfusion. However, owing to the multiple sources of lactate in the body, it might be a marker of severity of disease rather than an isolated marker for poor perfusion.

Gastric tonometry and sublingual capnography: A derangement of the haemodynamic balance triggers a compensatory response whereby blood flow to vital organs ( brain, heart and kidney) is preserved at the expense of other organs. One of the earliest organs from where blood flow is diverted is the gastro intestinal tract. Some authors have in fact suggested that gastro intestinal dysoxia might be an 'early warning of impending trouble.' Changes in gastrointestinal mucosal PCO2 have been shown to reflect gastrointestinal oxygen uptake during periods of 'no-flow.' This response is reflected in the sublingual mucosa. The CO2 in the stomach wall as well as sub mucosa increases predictably during hemorrhagic and septic shock. Sublingual capnometry is technically simple, non-invasive, inexpensive and provides instantaneous information. Clinical experience is, however, limited.

In Conclusion

The search for the most ideal method of assessing CO continues. No method known so far gives perfect information. Critically-ill patients cannot be managed by data provided by these methods alone, unless this data is analysed rationally by thoughtful intensivists.

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