Blood pressure disorders

To minimise errors when using in-dwelling arterial lines, the following factors should be considered:

• Narrow catheters will underestimate systolic BP.
• Occlusion of the tip of the catheter (vessel wall or clot) may dampen wave and underestimate BP.
• Even small air bubbles may have an effect on measurement.
• Peripheral lines read higher than umbilical lines.
• Complications may occur and include:
  • thrombus formation
  • haemorrhage
  • infection.

Points to note:

• Blood pressure increases with:
  • gestation
  • birthweight
  • postnatal age (especially in the first 72 hours of life).
• Blood pressure  may be affected by gender especially on the first day of life.
• There is no significant difference between arm and calf blood pressure in normal infants.
• It is difficult to define 'normal' BP values in ELBW infants.
• In clinical practice, the infant's blood pressure is generally considered to be adequate as long as urine output (> 1 mL/kg/hr) and capillary refill (< 3 seconds) are within normal limits and there is no metabolic acidosis.
• In general all preterm infants, after 72 hours of age, should have a mean blood pressure above 30 mmHg.

However, these are not reliable indicators of tissue perfusion.

Arbitrary definitions of hypertension are as follows:

• term infant: systolic > 90 mmHg, diastolic > 60 mmHg
• preterm infant: systolic > 80 mmHg, diastolic > 50 mmHg.

Low birthweight infants

Preterm infants by gestation

Preterm infants by age

Term infants

Normal blood neonatal blood pressure values 

Clinical features of hypotension may include:

• tachycardia
• bradycardia
• tachypnoea
• mottling of the skin
• prolonged capillary refill time
• cool extremities
• reduced urine output.

The major causes of hypotension include:

• hypovolaemia
  • blood loss
  • disseminated intravascular coagulation (DIC)
  • third space losses with necrotising enterocolitis (NEC)
• cardiogenic shock
  • asphyxia
  • arrhythmias
  • congenital heart disease
  • cardiomyopathy or myocarditis
  • air leak (for example, pneumothorax)
• sepsis
• adrenal haemorrhage or acute adrenal crisis (congenital adrenal hypoplasia)
• drug effects.

Hypertension may occur in up to 3 per cent of NICU admissions.

It is usually due to renal or cardiovascular abnormalities.

Clinical features of hypertension are often non-specific but may include:

• haematuria
• feeding difficulties
• unexplained tachypnoea
• apnoea
• lethargy
• seizures (rare)
• mottling with signs of congestive heart failure.

The major causes of hypertension include:

• Congenital
  • Renal artery stenosis/hypoplasia
  • Coarctation of the aorta
  • Abdominal aortic atresia
  • Polycystic kidney disease
  • Renal obstruction and hydronephrosis
• Acquired renal parenchymal causes
  • Severe acute tubular necrosis
  • Cortical necrosis secondary to asphyxia
  • Thromboembolic renal artery or vein complications secondary to umbilical catheterisation
  • Polycythaemia
• Endocrinological disorders
  • Congenital adrenal hyperplasia
  • Hyperaldosteronism
  • Hyperthyroidism
  • Salt and water overload
  • Hypercalcaemia
• Other causes include
  • Pain
  • Seizures
  • Neonatal abstinence
  • Some medications such as aminophylline
• Tumours
  • Neuroblastoma
  • Wilms
  • Metablastic nephroma

Investigation of neonatal hypertension might include:

• urea, electrolytes and creatinine
• urinalysis
• plasma renin activity
• cortisol
• 17-hydoxyprogesterone
• aldosterone
• thyroid function tests
• chest x-ray
• renal ultrasound
• four limb blood pressure measurements (a differential pressure between the upper and lower limbs is suggestive of coarctation of the aorta) – an echocardiogram would then be warranted.

Treatment of hypertension

• Relates to
  • severity of hypertension
  • underlying cause.

• Medications might include:
  • beta-blockers
  • angiotensin-converting enzyme inhibitors
  • calcium channel blockers
  • diuretics.
• Other approaches:
  • relief of pain
  • discontinuation of medications potentially causing hypertension.

Care must be taken in treating neonatal hypertension to avoid precipitous falls in blood pressure, which may be associated with further complications particularly in preterm infants.

Definitions of 'normal' blood pressure in low birthweight and preterm infants are based on small numbers. Although these are 'healthy' infants, a variety of devices have been used to produce the measurements.

There is very good evidence to suggest that blood pressure cannot necessarily be equated with normal systemic flow or a normal circulating blood volume.

As a general rule, the lower limit of mean BP in mmHg on the day of birth is approximately equal to gestational age in weeks.

References

• Fanaroff JM, Fanaroff AA. Blood pressure disorders in the neonate: Hypotension and hypertension. Sem Fetal Neonatal Med 2006;11:174-81
• Noori S, Seri I.  Pathophysiology of newborn hypotension outside the transitional period. Early Hum Dev 2005;81:399-404
• Kent AL, Chaudhari T. Determinants of Neonatal Blood Pressure. Curr Hypertens Rep 2013;15:426-32
• Lee J, Rajadurai VS,Tan KW. Blood pressure standards for very low birthweight infants during the first day of life. Arch Dis Child Fetal Neonatal Ed 1999;81: F168-F170
• Nuntnarumit P, Yang W, Bada-Ellzey HS. Blood pressure measurements in the newborn. Clin Perinatol 1999;26:981-996
• Puchalski Mary L. Should Blood Pressure be measured in Newborn Infants. May 04,2011
• Rennie, JM. (2012). Appendix 4. In: Rennie, JM and Robertson Rennie & Robertsons Textbook in Neonatology. 5th ed. London: Elsevier Limited. 1301-1305.
• Taeusch HW, Ballard RA. Avery's Diseases of the Newborn 7th Ed. W.B. Saunders Company, Philadelphia. 1998
• Flynn, J.T. Etiology, clinical features, and diagnosis of neonatal hypertension. Wolters Kluwer Health.  UpToDate 2014

Other reading/web links

• Bauer K, Linderkamp O, Versmold. Systolic blood pressure and blood volume in preterm infants. Arch Dis Child 1993;69:521-2
• Kluckow M, Evans, N. Relationship between blood pressure and cardiac output in preterm infants requiring mechanical ventilation. J Pediatr 1996;129:506-12