Mathematically arterialised venous blood is a stable representation of patient acid-base status at steady state following acute transient changes in ventilation

Abstract

Hyper- or hypoventilation are commonly occurring stress responses to arterial puncture around the time of blood sampling and have been shown to rapidly alter arterial blood acid-base parameters. This study aimed to evaluate a physiology-based mathematical method to transform peripheral venous blood acid-base values into mathematically arterialised equivalents following acute, transient changes in ventilation. Data from thirty patients scheduled for elective surgery were analysed using the physiology-based method. These data described ventilator changes simulating 'hyper-' or 'hypoventilation' at arterial puncture and included acid-base status from simultaneously drawn blood samples from arterial and peripheral venous catheters at baseline and following ventilatory change. Venous blood was used to calculate mathematically arterialised equivalents using the physiology-based method; baseline values were analysed using Bland-Altman plots. When compared to baseline, measured arterial and calculated arterialised values at each time point within limits of pH: ± 0.03 and PCO₂: ± 0.5 kPa, were considered 'not different from baseline'. Percentage of values considered not different from baseline were calculated at each sampling timepoint following hyper- and hypoventilation. For the physiological method, bias and limits of agreement for pH and PCO₂ were -0.001 (-0.022 to 0.020) and -0.02 (-0.37 to 0.33) kPa at baseline, respectively. 60 s following a change in ventilation, 100% of the mathematically arterialised values of pH and PCO₂ were not different from baseline, compared to less than 40% of the measured arterial values at the same timepoint. In clinical situations where transient breath-holding or hyperventilation may compromise the accuracy of arterial blood samples, arterialised venous blood is a stable representative of steady state arterial blood.

Keywords: Acid–base; Arterial; Arterialisation; Blood gas analysis; Hyperventilation; Hypoventilation; Mathematical model.

Fig. 1

Graphical illustrations of the concept of a transient ventilatory change A: Illustrates the clinical setting where guidelines recommend waiting for at least 10 min for equilibration of blood gases before drawing an arterial sample. Here the arterial blood most accurately reflects the blood gases intending to be measured. B: Illustrates the changes that can be seen in response to a transient change in ventilation. The figure shows how the arterial blood (red) responds rapidly to the changes in ventilation, while the venous (blue) and hence arterialised blood (green) remains stable for the duration of the blood sampling, thereby better reflecting the steady-state values that were intended to be measured. (Color figure online)

Fig. 2

Physiology based mathematical method from Rees et al. [7]. Figure depicting the principles of the physiology based mathematical method, used to transform venous blood values to arterial equivalents, in 5 steps ('a' to 'e'). Subtexts: a: arterial, v: venous, p: plasma; PCO₂: CO₂ tension (kPa); PO₂: O₂ tension (kPa); Hb: haemoglobin (mmol/l); SO₂: O₂ saturation (%); MetHb: Methaemoglobin (%); COHb: carboxyhaemoglobin (%); tCO₂: total CO₂ content (mmol/l); tO₂: total O₂ content (mmol/l); RQ: respiratory quotient; BE: base excess (mmol/l); SpO₂: peripheral saturation of O₂ measured via pulse oximetry (%); ∆O₂: a-v O₂ concentration difference. Figure reproduced with permissions [36]

Fig. 3

Bland–Altman comparison of the mathematically arterialised blood against the measured arterial blood at baseline. Bland–Altman plot comparing calculated arterialised (‘ca’) blood to the measured arterial (‘a’) blood at baseline, plotted against the measured arterial on the x-axis. N = 30. Bias (solid line) and LoAs (dotted lines) are displayed

Fig. 4

Percentage of pH and PCO₂ in measured arterial and calculated arterialised blood within pre-defined limits, at each sampling timepoint following acute transient changes in ventilation. Proportion of blood gas samples within the limits of ± 0.03 for pH and ± 0.5 kPa for PCO₂ when measured arterial (‘a’; solid lines) and calculated arterialised (‘ca’; dashed lines) values at each sampling timepoint are compared to their respective baseline values, in the 2 min following a simulated acute transient change in ventilation (hyperventilation (black) or hypoventilation (grey), N = 15 each)

Fig. 5

Example of a single patient’s blood acid–base response to acute simulated hyperventilation.Figure illustrating changes (Δ) in pH and PCO₂ from baseline in measured arterial (‘a’; red) and calculated arterialised (‘ca’; black) blood following acute simulated hyperventilation. (Color figure online)