While hypotension - low blood pressure (BP) - is commonly diagnosed and treated in the very preterm infant there is enormous variation in clinical practice.Hypotension is statistically associated with adverse short-term and long-term outcomes however a systematic review of the literature was unable to find clear criteria to define hypotension. In addition the evidence to support current management strategies is minimal and mostly dependent on small studies that have measured short-term physiologic endpoints. Preterm infants who are diagnosed with and treated for low BP often have no biochemical or clinical signs of shock, they may have normal systemic blood flow, low systemic vascular resistance, and adequate tissue oxygen delivery and probably do not require treatment. Careful observation of such infants without intervention approach previously coined "permissive hypotension" may well be appropriate.

Excessive intervention in preterm infants may be unnecessary or even harmful. Analysis of a large neonatal database (Canadian Neonatal Network, CNN) demonstrated that treatment of hypotension was associated with an increase in serious brain injury. This remained true even after mean BP was included in the regression model suggesting that it may be the treatment of hypotension rather than the presence of hypotension which is harmful. The most common approach to treatment is to give one or more fluid boluses followed by dopamine. However, observational data have shown an association of fluid bolus administration with intracranial bleeding and in animal models correction of hypotension by rapid volume infusion can result in intraventricular haemorrhage; a complication which is associated with increased rates of death and neurosensory impairment in preterm human infants. Fluctuations in BP following commencement of inotropes are well recognised and could also trigger intraventricular haemorrhage. Furthermore dopamine the most commonly used inotrope has effects on many physiologic functions including pituitary effects which lead to secondary hypothyroidism a known risk factor for poor long-term neurodevelopmental outcome in the preterm infant. In addition dopamine elevates BP in the newborn predominantly due to vasoconstriction, which may be associated with a reduction in systemic perfusion.

There is no consensus on definitions of hypotension in the preterm infant. Many clinicians rely on absolute BP values alone to guide intervention. BP reference ranges are often based on birth weight, gestational age and postnatal age criteria. These statistically determined values vary considerably being based on observations of BP made in small cohorts of infants the majority of whom were born before the widespread implementation of important perinatal interventions (e.g antenatal glucocorticoid therapy) which are known to improve outcome and reduce the incidence of intraventricular haemorrhage in preterm infants. The Joint Working Group of the British Association of Perinatal Medicine has recommended that the mean arterial BP in mmHg should be maintained above the gestational age in weeks (e.g. an infant born at 25 weeks gestation should have a mean BP \> 25mmHg). Despite little published evidence to support this 'rule', it remains the most common criterion used to define hypotension and it has been used in a number of recent randomised therapeutic intervention trials where it was the sole entry criteria. However, Cunningham et al have shown a poor relationship between this criterion and the incidence of intraventricular haemorrhage in preterm infants. In a separate study, the CNN report that 52% of preterm infants with birth weight \< 1500g have a mean arterial BP less than their gestational age on the first day of life and thus may be diagnosed with and treated for hypotension.

It is uncertain whether hypotension (however defined) results in adverse clinical outcomes including adverse short-term outcomes (increased incidence of intraventricular haemorrhage) and adverse long-term neurodevelopmental outcome. Furthermore it is unclear whether intervention to treat hypotension results in improved outcomes. Dopamine is the most commonly used agent, an endogenous catecholamine that causes vasoconstriction and elevates BP, but has not been shown to improve clinical outcomes. Epinephrine is another endogenous catecholamine, which at low to moderate doses causes vasodilatation and stimulates cardiac function. It may increase perfusion when used in hypotensive neonates but the data are limited.

Current standard approaches to evaluation and treatment of transitional circulatory problems in the preterm infant are not evidence based. It is essential that these approaches be adequately investigated in this at risk group of infants.