Best Practice Guideline articleClinical monitoring of systemic hemodynamics in critically ill newborns
Introduction
The hemodynamic status of newborns is usually estimated by the interpretation of indirect parameters of systemic blood flow, like blood pressure, heart rate and urine output. A survey questionnaire, sent to all directors of Dutch neonatal intensive care units, revealed that blood pressure, heart rate, urine output, capillary refill time and serum lactate concentration were the most used hemodynamic variables to diagnose circulatory failure [1], [2]. Remarkably cardiac output measurement was not even mentioned, which is not different from other countries [3], [4].
An overview is given about the predictive value of various indicators of circulatory failure in newborn infants with focus on the most used variables, which are blood pressure (BP), heart rate (HR), urine output, capillary refill time (CRT), serum lactate concentration, central–peripheral temperature difference (dCPT), blood gas analysis (BGA), central venous oxygen saturation and colour.
Section snippets
Blood pressure
Measurement of blood pressure is the most frequently used method for assessment of the hemodynamic status in a neonatal intensive care unit [4], [5]. Three different definitions of neonatal hypotension are in widespread use. The first definition is a blood pressure below the tenth (or fifth) percentile of normative blood pressure values derived from a reference population with regard to gestational age, birth weight and postnatal age. The second and probably most used definition of neonatal
Heart rate
Ventricular output is determined by stroke volume and heart rate. Stroke volume is considered to be at a fixed level in neonates without much variation. Supposing that cardiac output is almost entirely dependent on heart rate, the circulatory status is often evaluated by interpretation of the heart rate. Low systemic blood flow is considered when tachycardia is present, whereas a stable heart rate is regarded as a sign of stable hemodynamics. An increase in heart rate for compensation of low
Urine output
Under normal conditions neonatal urine output changes during the first days of life with an initial phase of low urine production (first 24 h), followed by a period of transient polyuria (second and third days of life), after which diuresis stabilizes and depends on fluid intake [10]. These physiologic changes in urine output are difficult to differentiate from oliguria due to impaired renal perfusion. Urine output is a poor marker of circulatory failure in the absence of a direct relationship
Capillary refill time (CRT)
CRT is a clinical test that is frequently used in paediatrics to diagnose circulatory failure, because it is easily applicable, non-invasive, quick, and inexpensive. This issue will be discussed by Weindling and Paize in this issue of Early Human Development (ref. Weindling M, Paize Peripheral haemodynamics in newborns: best practice guidelines. Early Human Dev 2010;xx:xxx-xxx.)
Lactate
There is an important difference between lactate and lactic acid. Lactic acid is a so-called strong ion, which dissociates into lactate and hydrogen ions at pH levels in the physiological range. Lactate itself does not act as an acid. Moreover, the conversion of pyruvate to lactate is not coupled with an increase in hydrogen ions. This means that an elevated blood concentration of lactate is not synonymous to lactic acidosis. Only when hydrogen ions, which are released with ATP hydrolysis,
Central–peripheral temperature difference (CPTd)
Measurement of CPTd in children was first described in 1974 [13], [14]. The assumption is that in a thermoneutral environment peripheral vasoconstriction, secondary to poor peripheral perfusion, will decrease skin temperature and thus lead to an elevated CPTd. Under normal conditions CPTd will be < 1 °C during the first postnatal days in ELBW infants. However, CPTd depends largely on body temperature, environmental temperature and the use of vasoactive drugs. No relation was observed between CPTd
Acid–base balance
Traditionally, blood gas parameters, like pH and standard base excess (SBE), are used as indirect indicators of tissue acidosis in critically ill patients. This is based on the assumption that metabolic acidosis assessed by pH and SBE reflects tissue hypoxia secondary to inadequate perfusion and/or oxygenation. However, pH and SBE are poor indicators of circulatory failure [16], [17], [18]. Moreover, hypoalbuminaemia, which is common in ELBW infants, will mask an existing tissue acidosis when
Venous oxygen saturation (svO2)
Mixed svO2 reflects the adequacy of global tissue oxygenation, as it represents the oxygen reserve after tissue oxygen extraction. The level of mixed svO2 depends on arterial oxygen concentration, oxygen consumption, cardiac output and haemoglobin concentration. The common pulmonary artery is in theory the most optimal site for mixed venous blood sampling, but this is not feasible in newborn infants. However, it is fairly easy to sample central venous blood from the right atrium at the junction
Colour
The adequacy of peripheral perfusion and/or oxygenation is often evaluated by clinical assessment of the patient's colour. One should realize that an infant's colour is influenced by many factors, such as oxygenation, haemoglobin concentration, skin temperature, skin thickness, peripheral perfusion, race, gestational age, ambient temperature and light. The determination of the colour of newborn infants has been proven to be very subjective with large interobserver variability [23], [24].
Combination of different clinical hemodynamic variables
Because an instantaneous interpretation of a single clinical hemodynamic parameter has its shortcomings in the prediction of actual systemic blood flow, it will be plausible that a combination of different hemodynamic variables can improve the predictive value for the detection of neonatal circulatory failure.
Objective cardiac output measurement by transpulmonary thermodilution and clinical estimation of systemic blood flow based on the interpretation of several routine clinical and biochemical
Summary and conclusions
Most clinical signs and symptoms of circulatory failure, like CRT, urine output, CPTd and colour, are rather subjective with a questionable reproducibility. The predictive values of individual indicators of compromised perfusion are limited, as can be observed in Table 1, in which available and published data are summarized for blood pressure, renal function, CRT, blood lactate concentration and CPTd. The disappointing predictive values of individual clinical markers mean that only a small
Research directions
Scientific effort should be directed towards the development, validation and implementation of methods of systemic blood flow measurement, because this will provide the clinician with more objective information about oxygen delivery. Knowledge of the actual cardiac output likely improves the indication and choice of subsequent cardiovascular support. Moreover, the response to the therapeutic intervention can be monitored and evaluated. Cardiac output measurement will only provide information on
Key guidelines
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The clinical assessment of cardiac output by the interpretation of indirect parameters of systemic blood flow is inaccurate, irrespective of the level of experience of the clinician
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Using blood pressure to diagnose low systemic blood flow will consequently mean that too many patients will potentially be undertreated or overtreated, both with substantial risk of adverse effects and iatrogenic damage.
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Combining different clinical hemodynamic parameters enhances the predictive value in the detection
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