ESICM LIVES 2020

Introduction: ABGs are performed in acute conditions as the reference method for assessing the acid-base status of blood. Hyperventilation and breath-holding are common ventilatory changes that can
occur around the time of sampling, rapidly altering the ‘true’ status of
the blood.
Objectives: To determine whether mathematically calculated “arterialised” venous values (1) represent a more stable measure of arterial
blood during rapid, transient, changes in ventilation.
Methods: 14 patients without cardiovascular and respiratory illnesses
scheduled for elective surgery were studied. Following anaesthesia
and before the start of the surgery, ventilator settings were altered
to achieve + 100% or − 60% change in alveolar ventilation (‘over-’ or
‘under-ventilation’), thus simulating a transient change in ventilation.
Blood samples were drawn simultaneously from indwelling arterial
and peripheral venous catheters at baseline and every 15s for 1min
following the ventilatory change. Venous values were used to calculate arterial equivalents of acid-base status, using the mathematical
arterialisation method (1).
Results: Figure 1 illustrates measured arterial and calculated arterialised values, shown as changes from baseline for pH and PCO2, for
over- and underventilation. Arterial blood changes rapidly within the
frst 15 - 30s with arterialised values remaining relatively constant until
45s.
Conclusion: In situations prompting transient changes in ventilation,
viz. anxiety in the emergency department or spontaneous breathing
with assisted ventilation, mathematically arterialised venous values
may provide a more stable value of arterial acid-base status closer to
baseline values, prior to the transient change in ventilation.

Reference(s) and grant ackowledgment(s)
1. 1. Toftegaard M, Rees SE, Andreassen S. Evaluation of a method for convert‑
ing venous values of acid-base and oxygenation status to arterial values.
Emerg Med J. 2009;26:268–72