Acid base balance & Perinatal Implications

S ArulkumaranProfessor Emeritus

Obstetrics & GynaecologySt 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).

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 CO2 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

satu

rati

on

Days and weeks Hours Minutes

Hypoxemia

Hypoxia

Asphyxia

more effective uptake of oxygen

Reduced activity

decrease in growth rate

maintained energy balance

The fetal response to hypoxia

Time

Oxygen

satu

rati

on

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

satu

rati

on

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 sizeless than5th centile

Aortic Aortic blood blood flowflow

Cerebral blood flow

AbnormalAbnormalvenousvenousflowflow

AbnormalFHRTrace

AFI Oligohydramnios

ModerateModerateseveresevereredistribnredistribn

4’th December 2014

GREEN – Normal & Reassuring

Non reassuringNeed for Conservative Measures

Abnormal & needs Comservative measures+ Additional Testing

Abnormal and Needs URGENTIntervention

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Fetal blood sampleFetal 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

< 7.20 Delivery 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 7.05-7.38 7.17-7.48• pCO2 kPa 4.9 – 10.7 3.5 – 7.9• BD ecf m.mol/l -2.5 -10.0 -1.0 – 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

CO2 8.82 5.14

BD 12.8 8.0

• High BD in artery and vein – long lasting hypoxia E.g. Art pH 7.01 Vein pH 7.12

CO2 8.82 6.65

BD 12.8 11.5

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