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Functional Echocardiography in Assessment of the Cardiovascular System in Asphyxiated Neonates

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Perinatal asphyxia commonly results in multi-organ damage, and cardiovascular dysfunction is a frequent association. Myocardial damage, right ventricular dysfunction, abnormal circulatory transition, and impaired autoregulation may all contribute to postnatal neurological damage. Adequate monitoring and appropriate targeted treatment therefore are essential after an asphyxial insult. Standard methods of cardiovascular monitoring in the neonate have limitations. Point of care ultrasound scanning or functional echocardiography offers extra information to assist the clinician in identifying when there is significant cardiovascular impairment, classifying the underlying abnormal physiology and potentially targeting appropriate therapy, thereby optimizing the post-insult cerebral blood flow and oxygen delivery.

Section snippets

Assessment of the Cardiovascular System

Transient myocardial ischemia is a recognized association of perinatal asphyxia, with an incidence from 30% to 82% of severely asphyxiated neonates.2, 9, 10 The incidence of myocardial dysfunction may be higher in preterm neonates who already have risk factors for myocardial impairment because of immaturity of the myocardium.11 Transient myocardial ischemia is often associated with evidence of myocardial damage, such as a rise in cardiac troponin levels.12 Diagnosis of myocardial involvement

Clinical Assessment

The mainstays of clinical assessment of asphyxiated neonates have been capillary refill time, acidosis, and blood pressure, which are all limited in both accuracy of assessment and the information obtained. Capillary refill time is used in adult and pediatric intensive care settings as an indicator of poor perfusion with some correlation;14, 15 however, accuracy in predicting low cardiac output is limited. Similarly, acidosis, as indicated by lactate level or base deficit, probably reflects

Qualitative Assessment of Cardiac Function

A simple visual assessment with echocardiography in either the parasternal long axis or a short-axis parasternal view can provide significant information about the function of the myocardium. Visual assessment of contractility is assessed with the movement of the septum and posterior myocardial wall and is reasonably accurate for clinicians experienced in echocardiography.19 Similarly, the filling of the heart can be assessed by observing the residual volume in diastole—the LV end diastolic

M Mode Measurements

M Mode imaging allows measurement of fractional shortening (FS) and ejection fraction (EF), which are more quantitative measures of the contractility of the ventricles. The reference range for FS in term neonates is 25% to 40%. EF is similar to FS, but each of the measures is cubed to allow assessment of volume changes; this also multiples any measurement error, particularly because in the neonate the RV dominance alters the shape of the ventricles. Other measures of EF, such as the modified

Systolic/Diastolic Time Intervals

Other measures of cardiac function can be determined by measuring peak velocity, mean acceleration, acceleration time, and LV ejection time. The systolic time interval ratio (acceleration time/LV ejection time) was not predictive of asphyxia.20 Use of more sophisticated measures of contractility that are independent of preload/afterload, such as the velocity of circumferential shortening or the ratio of velocity of circumferential shortening to end systolic wall stress, an afterload adjusted

Myocardial Performance Index

The myocardial performance index (MPI) combines a measure of both the systolic and diastolic intervals to characterize global myocardial performance.24 The MPI is a Doppler ultrasound scanning-derived index of myocardial performance, combining the isovolumetric contraction and relaxation time intervals, used to assess ventricular function. The index is independent of heart rate and blood pressure and does not rely on geometric assumptions. Preterm neonates with evidence of mild perinatal

Cardiac Output with Doppler Ultrasound Scanning

A more global assessment of cardiac function can be achieved by measuring cardiac output non-invasively by using Doppler ultrasound scanning techniques.3, 26, 27, 28, 29 Ventricular outputs are measured with two-dimensional echocardiography to measure outflow tract diameter and the velocity time integral.3 The cardiac index is derived by correcting for birth weight. This Doppler ultrasound scanning-derived non-invasive measure of cardiac output has been well validated against invasive measures

Superior Venal Caval Flow

Measurement of superior venal cava (SVC) flow has been used primarily in premature neonates to assess cardiac output independent of the transitional circulation shunts that result in the actual measured LV and RV output being higher than the true systemic blood flow. The LV output is increased by the left-to-right shunt through a patent ductus arteriosus, and the RV output is increased by the shunt through the patent foramen ovale.11, 31 SVC flow is potentially a proxy measure for cerebral

Right Ventricular Function

Assessment of the RV function can be achieved with a visual qualitative assessment of the contractility and filling in a long-axis view. Paradoxical movement of the septum or bowing to the left with increased RV pressure can be assessed in a short-axis view. The pattern of the velocity time ratio in the RV outflow tract (systolic interval) may also provide information about RV function and increased pulmonary pressure, with a reduction in the time to peak velocity as a proportion of total

Assessment of Volume Status/Fluid Responsiveness

Both hypotension and impaired cardiac output are common outcomes of asphyxial damage to the myocardium, and use of volume resuscitation in the setting of asphyxia is common. Volume is critical when there is evidence of hypovolemia or an acute change in the systemic vascular resistance resulting in vasodilation of the peripheral vasculature. Volume is also an important adjunct to the use of inotropes. Volume increases cardiac output in sick term neonates by increasing stroke volume rather than

Pulmonary Hypertension

Raised pulmonary pressures and PPHN are common complications of perinatal asphyxia, but the role of functional echocardiography in this area is also critically important. PPHN is a multifactorial dynamic process, and the treatment of asphyxiated neonates is assisted greatly by understanding the underlying physiology, particularly as the clinical situation changes.34, 42 The components of this include measures of myocardial and ventricular output (particularly right sided), various

Therapeutic Hypothermia and Cardiovascular Function

With the use of therapeutic hypothermia for the management of perinatal asphyxia, there is a lack of information on the effect of cooling on cardiac function and hemodynamics. This is likely to be particularly relevant during whole body cooling. Hypotension and sinus bradycardia are adverse effects of cooling that have been consistently noted.44 More recently, significantly decreased LV cardiac output (67% of post-hypothermic range) was demonstrated during therapeutic cooling, with a consistent

Training in Functional Echocardiography

The usefulness of bedside functional echocardiography is high in the setting of perinatal asphyxia. Bedside functional echocardiography has become an important tool in the treatment of critically ill intensive care patients in both adult and pediatric intensive care units,46 permitting rapid and precise diagnosis and longitudinal assessment of hemodynamic function. Increasingly the role of functional echocardiography in the neonatal unit has also been recognized.8, 47, 48 The expertise of a

Conclusion

Perinatal asphyxia commonly results in multiorgan damage and cardiovascular dysfunction. Myocardial damage, RV dysfunction, abnormal circulatory transition, and impaired autoregulation may all contribute to postnatal neurological damage. Adequate monitoring and appropriate targeted treatment are essential components of the intensive care of an asphyxial insult. Because standard methods of monitoring have limitations in assessing cardiovascular adequacy, functional echocardiography offers extra

Author Disclosures

Martin Kluckow, MBBS, FRACP, PhD, has no financial arrangement or affiliation with a corporate organization or a manufacturer of a product discussed in this supplement.

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