Acid-base and Blood Gases in the Fetus

Transcription

1 Acid-base and Blood Gases in the Fetus J T Parer, MD, PhD Maternal Fatal Medicine University of California San Francisco 40 th Meeting Antepartum and Intrapartum Management San Francisco, California June, 2016 No disclosures My perpetual AIM with our erudite & wonderful UCSF OB/GYN residents is after 4 years to have at least 50% of them be able to explain and understand the clinical significance of BASE EXCESS Teenage G1P0 at 40 2/7 wks, uneventful PNC 1

2 SVD at 08:52 What is ph? ph is the negative log, to the base 10, of the concentration of [H+] ions Concentration of acid, or hydrogen ions, or [H + ] in blood: mm/lit or 4 X 10-7 or The Dreaded Henderson- Hasselbalch Equation The ph is therefore 7.4 2

3 Henderson-Hasselbalch Equation Henderson-Hasselbalch Equation For convenience this is changed to negative logarithm notation: -log [H+]= -log K + log [base]/[acid] For the H 2 CO 3 and HCO 3- acid base pair: ph= pk + [HCO 3- ]/H 2 CO 3 ] [H+] = K.[acid]/[base] ph= log 24/1.2 = log 20 = = 7.4 Normal Values for Arterial Blood in Adults I can easily and comfortably define Base Excess Nonpreg Pregnant ph PCO 2 mmhg A. True B. False 86% HCO 3 - mm/lit % BE mm/lit 0-4 3

4 Summary of normal values in utero Nicholaides et al, 1986 Umbilical Artery Mean ± 2 SD Umbilical Vein Mean ± 2 SD ph 7.33 ± ± 0.06 Carbon dioxide pressure (mmhg) 45 ± ± 8 Bicarbonate (meq L 1 ) 23 ± 5 23 ± 5 Base excess (meq L 1 ) 3 ± ± 4 Oxygen content (mm L 1 ) 6.7 ± 0.6 Oxygen pressure (mmhg) 35 ± ± 20 Hemoglobin (g dl 1 ) The Tyranny of 3 Variables: ph, HCO 3- and H 2 CO 3 4

5 Before & after labour Rooth et al, 1972 Before Labor ph Carbon dioxide pressure (mmhg) Bicarbonate (meq L 1 ) Base excess (meq L 1 ) 3 5 Second Stage of Labor Respiratory acidosis is due to CO 2 Metabolic acidosis is caused (mainly) by lactic acid Blood Gases Umb artery Umb vein ph PCO 2 mmhg HCO 3 - mm/lit Base Excess mm/lit Normal Values for Arterial Blood in Adults Nonpreg Pregnant ph PCO 2 mmhg HCO 3 - mm/lit BE mm/lit 0-4 5

6 Base Excess Bicarbonate, HCO 3- can change in 2 ways: By reacting with fixed acids, eg lactic, beta hydroxybutyric, acetoacetic acids By changes in CO 2 ; if more CO 2, higher HCO 3 - Because of this [HCO 3- ] is only useful as a measure of metabolic acid derangement when PCO 2 is 40 mmhg Base Excess/Base Deficit was introduced as a measure of metabolic change from normal, even in the presence of CO 2 changes from normal Umbilical blood gases from clamped segment of cord at birth n=15000 normal births, Helwig Mean Standard Deviation Median 2.5 %tile 97.5th %tile UA ph UA pco 2 (mmhg) UA po 2 (mmhg) UA base excess (meq L 1 ) UV ph UV pco 2 (mmhg) UV po 2 (mmhg) UV base excess (meq L 1 ) Threshold of Acceptable Acidemia in Umbilical Arterial Blood at Birth ph > 7.1 Base excess >-12 meq/lit Helwig et al, AJOG 6

7 FHR Management- King s Kascade Category I Category II Category III IIa Recurrent decelerations and IIb Decelerations getting deeper +/- tachycardia and IIb Variability diminishing Expectant management consider Intermittent auscultation 26 Conservative measures and reevaluate in short period of time Conservative measures Consult and make a plan for reevaluation in short period of time Bedside evaluation by clinicians Consider delivery in short period of time Deliver as soon as possible Pathologic fetal acidemia ph < 7.0 BE < -12, or -16, or -20 Goldaber Gilstrap Goodwin Low 7

8 Newborn Cooling Protocol Blood Gas Criteria SVD at 08:52 ph < 7.0 BE< -12 Blood Gases Umb artery Umb vein The most likely outcome for this baby is: ph PCO2 mmhg A. Brain damage B. Impossible to say 43% 48% HCO3- mm/lit C. Normal outcome 9% Base Excess mm/lit

9 Rate of change after acute cessation of oxygen delivery in a monkey fetus- Myers ph fell 0.04 units/min CO2 increased 6 mmhg/min Base Excess fell ca 1 meq/lit/min SVD at 08:52 SVD at 08: gm girl Apgars 1, 7, 10 I am now much more comfortable defining and understanding the clinical significance of fetal Base Excess 60% 30 min-14 2 hrs-66 Home age 18 hrs A. True B. False 40% 9

10 THANK YOU! Fick equation Cardiac output = (O2 consumption) / (arteriovenous O2 concentration difference) or O2 consumption = (cardiac output) x (A-V O2 difference) 10