Is venous blood a more reliable description of acid-base state following simulated hypo- and hyperventilation?

Abstract

Background 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 occur around the time of sampling, rapidly altering the ‘true’ status of the blood. This is particularly relevant in emergency medicine patients without permanent arterial catheters, where the pain and anxiety of arterial punctures can cause ventilatory changes. This study aimed to determine whether peripheral venous values could be a more reliable measure of blood gases following acute changes in ventilation. Methods To allow for characterisation of ventilatory changes typical of acutely ill patients, but without the confounding influence of perfusion or metabolic disturbances, 30 patients scheduled for elective surgery were studied in a prospective observational study. Following anaesthesia, and before the start of the surgery, ventilator settings were altered to achieve a + 100% or − 60% change in alveolar ventilation (‘hyper-’ or ‘hypoventilation’), changes consistent with the anticipation of a painful arterial puncture commonly encountered in the emergency room. Blood samples were drawn simultaneously from indwelling arterial and peripheral venous catheters at baseline, and at 15, 30, 45, 60, 90 and 120 s following the ventilatory change. Comparisons between the timed arterial (or venous) samples were done using repeated-measures ANOVA, with post-hoc analysis using Bonferroni’s correction. Results Arterial blood pH and PCO2 changed rapidly within the first 15–30s after both hyper- and hypoventilation, plateauing at around 60s (∆pH = ±0.036 and ∆PCO2 = ±0.64 kPa (4.7 mmHg), respectively), with peripheral venous values remaining relatively constant until 60s, and changing minimally thereafter. Mean arterial changes were significantly different at 30s (P < 0.001) when compared to baseline, in response to both hyper- and hypoventilation. Conclusion This study has shown that substantial differences in arterial and peripheral venous acid-base status can be due to acute changes in ventilation, commonly seen in the ER over the 30s necessary to sample arterial blood. If changes are transient, peripheral venous blood may provide a more reliable description of acid-base status.

Trending peripheral venous PCO2in patients with respiratory failure using mathematically arterialised venous blood gas samples

“…Transient changes in ventilation as might occur with sampling anxiety-related hyperventilation appear to be reflected within seconds in pCO 2 a, while this effect is mitigated in venous blood. 24 As a method where this mitigating effect is maintained and which also corrects for variability in tissue oxygen metabolism, pCO 2 ca via vTAC holds theoretical advantages.…”

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

“…Data for this study were collected as part of a protocol, detailed in a previous publication [ 15 ]. Patients without cardiovascular or respiratory disease, scheduled for elective robotic assisted laparoscopic prostatectomy or hysterectomy, were studied, thereby allowing for the isolation of ventilatory changes without the presence of metabolic and perfusion disturbances.…”

“…When comparing arterialised values to measured arterial values using this physiology-based method, it is often overlooked that inherent variability may exist in arterial measurements. Arterial blood gases (ABGs) are known to exhibit breath-by-breath fluctuations, even in physiological steady state [ 14 ], due to the rapid response of arterial blood to changes in ventilation [ 15 ]. For monitoring changes in mechanical ventilation, this rapid response can be seen as beneficial in assessing patient state (Fig.…”

“…1 B). Fear and anxiety due to the sight of the needle [ 16 ], or a response to the pain of the arterial puncture can cause these rapid, transient changes in ventilation, and can be seen as changes in arterial gases within a minute [ 15 , 17 – 19 ]. It may therefore be difficult to rely upon measured arterial values under such conditions.…”

“…Venous blood however, has been shown to respond more slowly to these changes [ 15 , 18 ]. This would make it slower, and hence less useful, than arterial blood in identifying non-transient changes in ventilation such as changes in ventilator settings, seen in Fig.…”