Can physiology based mathematical models better represent arterial blood in the event of transient ventilatory changes?

Abstract

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 canoccur around the time of sampling, rapidly altering the ‘true’ status ofthe blood.Objectives: To determine whether mathematically calculated “arterialised” venous values (1) represent a more stable measure of arterialblood during rapid, transient, changes in ventilation.Methods: 14 patients without cardiovascular and respiratory illnessesscheduled for elective surgery were studied. Following anaesthesiaand before the start of the surgery, ventilator settings were alteredto 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 arterialand peripheral venous catheters at baseline and every 15s for 1minfollowing the ventilatory change. Venous values were used to calculate arterial equivalents of acid-base status, using the mathematicalarterialisation method (1).Results: Figure 1 illustrates measured arterial and calculated arterialised values, shown as changes from baseline for pH and PCO2, forover- and underventilation. Arterial blood changes rapidly within thefrst 15 - 30s with arterialised values remaining relatively constant until45s.Conclusion: In situations prompting transient changes in ventilation,viz. anxiety in the emergency department or spontaneous breathingwith assisted ventilation, mathematically arterialised venous valuesmay provide a more stable value of arterial acid-base status closer tobaseline 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;26:268–7