Editorial

Prevention of hypoxemia: The simple, logical, but incorrect solution

While supplemental O₂ inhalation corrects hypoxemia, its effect on post-anesthesia ventilation remains unknown. This pilot trial tested the hypothesis that hyperoxia increases the time spent with a transcutaneous PCO₂ (TcPCO₂) > 45 mmHg, compared with standard O₂ supplementation.

Single-blinded, parallel two-arm randomized pilot trial.

University hospital.

20 patients undergoing robotic-assisted laparoscopic nephrectomy.

After institutional approval and informed consent, patients were randomized to receive O₂ titrated to arterial saturation (SpO₂): 90–94% (Conservative O₂, N =10), or to SpO₂ > 96% (Liberal O₂, N = 10) for up to 90 min after anesthesia. Continuous TcPCO₂, respiratory inductance plethysmography (RIP), and SpO₂, were recorded. We calculated the percentage of time at TcPCO₂ > 45 mmHg for each patient and compared the two groups using analysis of covariance, adjusting for sex, age, and body mass index. We also estimated the sample size required to detect the between-group difference observed in this pilot trial. RIP signals were used to calculate apnea/hypopnea index (AHI), which was then compared between two groups.

The mean percentage of time with a TcPCO₂ > 45 mmHg was 80.6% for the Conservative O₂ (N=9) and 61.2% for the Liberal O₂ (N=10) group [between-group difference of 19.4% (95% CI: −18.7% to 57.6%), P = 0.140]. With an observed effect size of 0.73, we estimated that 30 participants per group are required, to demonstrate this difference with a power of 80% at a two-sided alpha of 5%. Means SpO₂ were 94.5% and 99.9% for the Conservative O₂ and the Liberal O₂ groups, respectively. AHI was significantly higher in the Conservative O₂, compared with the Liberal O₂ group (median AHI: 16 vs. 3; P = 0.0014).

Hyperoxia in the post-anesthesia period reduced the time spent at TcPCO₂ > 45 mmHg and significantly decreased AHI, while mean SpO₂ ranged inside the a priori defined limits.

ClinicalTrials.gov identifier: NCT04723433

To compare 2 anesthetic techniques, general anesthesia or monitored anesthesia care, performed by the same cardiac anesthesiologists for transcatheter aortic valve implantation in the authors’ institution.

A retrospective study.

A single specialized cardiac surgery center.

Ninety-eight patients with severe aortic valve stenosis and a high logistic EuroSCORE considered not eligible to undergo conventional aortic valve replacement.

General anesthesia or monitored anesthesia care.

General anesthesia was used in 57 and monitored anesthesia care in 41 patients. The authors compared the following parameters: Duration of procedure, transfusion requirements, cardiac indices, superior vena cava saturation (ScVO₂) before and after the aortic valve implantation, hospital length of stay and 30-day mortality. The only significant differences between the groups concerned were the duration of anesthesia (p<0.001) and ScVO₂ values. Anesthesia duration was prolonged significantly when general anesthesia was administered, and ScVO₂ was significantly higher both before and after the valve implantation in the general anesthesia group. Thirty-day mortality was 5.3% in the general anesthesia group and 4.9% in the monitored anesthesia group.

It would appear that both anesthetic techniques may be used for patients with a high logistic EuroSCORE undergoing transcatheter aortic valve implantation.

Hyperoxaemia depresses the output of peripheral and central chemoreceptors. Patients treated with opioids often receive supplemental oxygen to avert possible decreases in oxygen saturation (Sp_O2). We examined the effect of a single dose of remifentanil in healthy volunteers inhaling room air vs air enriched with 50% oxygen.

Twenty healthy volunteers received i.v. 50 μg remifentanil (infused over 60 s) at anormoxic (N) or hyperoxic (FI_O2 0.5, H) background on separate occasions. Minute ventilation (Vi), respiratory rate (RR), end-tidal Pco₂, and Sp_O2 were collected on a breath-to-breath basis. The occurrence of apnoea was recorded.

During normoxia, remifentanil decreased Vi from 7.4 (1.3) [mean (sd)] to 2.2 (1.2) litre min⁻¹ (P<0.01), and during hyperoxia from 7.9 (1.0) to 1.2 (1.2) litre min⁻¹ (P<0.01; H vs N: P<0.001). RR decreased from 13.1 (2.9) to 6.1 (2.8) bpm during N (P<0.01) and from 13.2 (3.0) to 3.6 (4.0) bpm during H (P<0.01; H vs N: P<0.01). During normoxia, Sp_O2 decreased from 98.4 (1.5) to 88.6 (6.7)% (P<0.01), while during hyperoxia, Sp_O2 changed from 99.7 (0.7) to 98.7 (1.0)% (P<0.001). Apnoea developed in two subjects during normoxia and 10 during hyperoxia.

Respiratory depression from remifentanil is more pronounced in hyperoxia than normoxia as determined from minute ventilation, end-tidal Pco₂, and RR. During hyperoxia, respiratory depression may be masked when measuring Sp_O2 as pulse oximetry remains in normal values during the first minutes of respiratory depression.

Postoperative monitoring of ventilation is largely restricted to the measurement of haemoglobin-oxygen saturation (Sp_O2) and respiratory rate (RR) derived from the ECG. Sp_O2 measurement is inadequate when used with supplemental oxygen and ECG-derived RR is subject to artifacts. A new monitor measures RR by quantifying the humidity of exhaled air (respiR8^®).

The accuracy of the system was tested using a breathing simulator. In healthy volunteers, the respiR8^® monitor was compared with two other methods of measuring RR: capnometry and counting of thoracic breathing movements. The ability of the monitor to track changes in RR resulting from the infusion of 2.5 μg kg⁻¹ fentanyl was assessed and compared with RR measured from a validated flow measurement system. The RR in 50 postoperative patients measured with the respiR8^® was compared with that derived from the ECG. RR values were compared by population-based Bland–Altman analyses.

The respiR8^® monitor was accurate in the range required in clinical practice. There was a close agreement between RR from respiR8^®, capnometry, and manual counting of respiratory movements without bias (limits of agreement ±1 bpm). The respiR8^® monitor was well able to accurately track RR changes from fentanyl. In postoperative patients, RR from respiR8^® and ECG had a bias of 1.7 (5.7) bpm due to greater RR values observed from the ECG due to artifacts.

The respiR8^® gives an accurate measurement of RR and is useful in postoperative care.

Respiratory monitoring is standard after anaesthesia and surgery. Abnormal respiratory rate is a sensitive indicator of respiratory problems, even in patients receiving supplemental oxygen, but the best method for its continuous measurement in spontaneously breathing patients is unclear. This study compared respiratory rate assessment by capnometry using a new oxygen mask with a carbon dioxide sampling port (Capnomask^®) and thoracic impedance pneumography with clinical measurement (used as a reference method) in extubated patients receiving supplemental oxygen.

Adult males admitted to the post-anaesthesia care unit after general anaesthesia were studied. Immediately after extubation, a Capnomask^® connected to a capnometer was positioned appropriately. Respiratory rate was measured by visual inspection of chest movement for 1 min, by capnometry, and thoracic impedance pneumography. One set of measurements was obtained for every patient receiving supplemental oxygen at different flow rates.

Twenty men, mean (inter-quartile range) age 54 (23–66) yr and BMI 25 (21–31) kg m⁻², were studied. Compared with visual inspection, the bias and limits of agreement were 0.0 (1.0 to −1.0) bpm for the Capnomask^® and −2.2 (2.0 to −6.5) bpm for the impedance pneumography. The accuracy of respiratory rate assessment using Capnomask^® was not influenced by the supplemental oxygen flow rate.

In extubated patients, continuous assessment of respiratory rate with the Capnomask^® is more accurate than by thoracic impedance pneumography even when supplemental oxygen is delivered at a high flow rate.

This two-part study was designed to determine the effect of supplemental oxygen on the detection of hypoventilation, evidenced by a decline in oxygen saturation (Spo₂) with pulse oximetry.

Phase 1 was a prospective, patient-controlled, clinical trial. Phase 2 was a prospective, randomized, clinical trial.

Phase 1 took place in the operating room. Phase 2 took place in the postanesthesia care unit (PACU).

In phase 1, 45 patients underwent abdominal, gynecologic, urologic, and lower-extremity vascular operations. In phase 2, 288 patients were recovering from anesthesia.

In phase 1, modeling of deliberate hypoventilation entailed decreasing by 50% the minute ventilation of patients receiving general anesthesia. Patients breathing a fraction of inspired oxygen (Fio₂) of 0.21 (n = 25) underwent hypoventilation for up to 5 min. Patients with an Fio₂ of 0.25 (n = 10) or 0.30 (n = 10) underwent hypoventilation for 10 min. In phase 2, spontaneously breathing patients were randomized to breathe room air (n = 155) or to receive supplemental oxygen (n = 133) on arrival in the PACU.

In phase 1, end-tidal carbon dioxide and Spo₂ were measured during deliberate hypoventilation. A decrease in Spo₂ occurred only in patients who breathed room air. No decline occurred in patients with Fio₂ levels of 0.25 and 0.30. In phase 2, Spo₂ was recorded every min for up to 40 min in the PACU. Arterial desaturation (Spo₂ < 90%) was fourfold higher in patients who breathed room air than in patients who breathed supplemental oxygen (9.0% vs 2.3%, p = 0.02).

Hypoventilation can be detected reliably by pulse oximetry only when patients breathe room air. In patients with spontaneous ventilation, supplemental oxygen often masked the ability to detect abnormalities in respiratory function in the PACU. Without the need for capnography and arterial blood gas analysis, pulse oximetry is a useful tool to assess ventilatory abnormalities, but only in the absence of supplemental inspired oxygen.