Acid base balance & Perinatal Implications S Arulkumaran Professor Emeritus Obstetrics & Gynaecology St George’s University of London

Perinatal Implications of asphyxia Intrapartum or immediate NN death Cerebral palsy / Neurological deficits Admission to NNU with HIE (Gr 1-3) Low Apgar scores & metabolic acidosis Concepts of hypoxaemia, hypoxia, asphyxia

Definition of a fetus in jeopardy 1. A normal state. 2. Hypoxaemia (relative lack of O2 in the blood). 3. Hypoxia (decreased oxygenation of tissues). 4. Asphyxia (Hypoxia + Metabolic acidosis in tissues due to anaerobic metabolism).

Fetal response to hypoxia Generates 38 units of ATP Generates 2 units of ATP

Acid base balance Respiratory or metabolic or mixed acidosis (lactic acid + carbonic acid) pH and base excess/ base deficit pH and hydrogen ions (negative logarithmic relationship) Base excess and buffers – bicarbonate, Hb, plasma proteins

Progression of anaerobic metabolism 1.Accumulation of organic acids that slowly cross the placenta in contrast to CO 2 which rapidly crosses the placenta. 2.Asphyxia results from the combination of metabolic acidosis and hypoxia. 3.Injury and death of the cell, tissues, organs and finally to the organism due to the stoppage of cellular enzymes at certain pH level.

Examples of Cell Dysfunction 1.Heart failure. 2.Pneumocytes type 2 injury lead to less surfactant factor. 3.GI system “necrotizing enterocolitis” 4.Renal failure. 5.Endothelial damage leads to DIC. 6.CNS cerebral oedema, seizures and cell death leads to cerebral palsy.

Fetal response to hypoxemia Time Oxygen saturation Days and weeks HoursMinutes Hypoxemia Hypoxia Asphyxia more effective uptake of oxygen Reduced activity decrease in growth rate maintained energy balance

The fetal response to hypoxia Time Oxygen saturation Days and weeks Hours Minutes Hypoxemia Hypoxia Asphyxia surge of stress hormones redistribution of blood flow anaerobic metabolism in the peripheral tissues maintained energy balance

Fetal response to asphyxia Time Oxygen saturation Days and weeks Hours Minutes Hypoxemia Hypoxia Asphyxia Alarm reaction anaerobic metabolism in peripheral tissues brain and heart organ failure

Fetal monitoring goals : 1. To identify a fetus in jeopardy in timely fashion so that we can intervene and prevent permanent damage or death. 2. To identify a normal fetus so that we can safely continue expectant management to avoid unnecessary interventions (CS or IVD).

Umbilical artery A/B Sequential Changes in Tests of Fetal well being Growth Fetal size less than 5th centile Aortic blood blood flow flow Cerebral blood flow Abnormalvenousflow Abnormal FHR Trace AFIOligohydramnios Moderatesevereredistribn

4’th December 2014

GREEN – Normal & Reassuring

Non reassuring Need for Conservative Measures

Abnormal & needs Comservative measures + Additional Testing

Abnormal and Needs URGENT Intervention ???????????

Fetal blood sample Fetal blood sample (FBS) result (pH) => 7.25 Subsequent action FBS should be repeated if the FHR abnormality persists 7.21 – 7.24 Repeat FBS within 30 mins or consider delivery if rapid fall since last sample < 7.20Delivery indicated All scalp pH estimations should be interpreted taking into account the initial pH measurement, the rate of progress in labour and the clinical features of the mother and baby

NICE – Recommendations for FBS Units employing EFM should have ready access to FBS facilities - A Where delivery is contemplated because of an abnormal FHR pattern in cases of suspected fetal acidosis, FBS should be undertaken in the absence of technical difficulties or any contraindications - A

NICE – Contraindications to FBS Maternal infection (e.g. HIV, Hepatitis, Herpes) Fetal bleeding disorders (haemophilia) Prematurity (34 weeks) Where there is clear evidence of acute fetal compromise (e.g. prolonged deceleration > 3 minutes

Umbilical cord A/V sampling Paired samples B Artery pH &BE as a minimum after © Em CS, IVD, FBS was done in labour, poor condition of the baby at birth

Cord blood samples Normal values Artery Vein pH pCO2 kPa 4.9 – – 7.9 BD ecf m.mol/l – 9.0 pH should be atleast 0.03 units less in artery pCO2 should be atleast 1.0 kPa higher in the artery

Short or long lasting hypoxia High BD in artery and normal BD in vein – short lasting hypoxia E.g Art pH 7.01 Vein pH 7.27 CO BD High BD in artery and vein – long lasting hypoxia E.g. Art pH 7.01 Vein pH 7.12 CO BD

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