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Apnea is one of the three cardinal findings in brain death (BD). Apnea testing (AT) is physiologically and practically complex. We sought to review described modifications of AT, safety and complication rates, monitoring techniques, performance of AT on extracorporeal membrane oxygenation (ECMO), and other relevant considerations regarding AT. We conducted a systematic scoping review to answer these questions by searching the literature on AT in English language available in PubMed or EMBASE since 1980. Pediatric or animal studies were excluded. A total of 87 articles matched our inclusion criteria and were qualitatively synthesized in this review. A large body of the literature on AT since its inception addresses a variety of modifications, monitoring techniques, complication rates, ways to perform AT on ECMO, and other considerations such as variability in protocols, lack of uniform awareness, and legal considerations. Only some modifications are widely used, especially methods to maintain oxygenation, and most are not standardized or endorsed by brain death guidelines. Future updates to AT protocols and strive for unification of such protocols are desirable.
The online version of this article (10.1007/s12028-020-01015-0) contains supplementary material, which is available to authorized users.
Keywords: Apnea test, Apnoea test, Apnea, Apnoea, Brain deathThe physiology of respiration is complex and beyond the scope of this review. The respiratory center with respiratory generators is located in the ventrolateral medulla oblongata [1]. If the function of these structures is irreversibly lost, a patient will not take any spontaneous breaths despite adequate stimulation. Therefore, AT assesses functionality of the lower brainstem by allowing the arterial carbon dioxide partial pressure (PaCO2) to increase and the CSF-pH to decrease to a level that maximally stimulates the respiratory centers in the medulla oblongata [2]. The exact carbon dioxide level at which the medullary chemoreceptors are maximally stimulated is unknown, but a PaCO2 value of 60 mm Hg (or 20 mm Hg above baseline) is generally considered an appropriate target for assessment of brain death [3].
Procedural flow for a standard AT in adults with oxygen insufflation according to the American Academy of Neurology (AAN) guidelines [4] is shown in Fig. 1 . Blood pressure can (and should) be supported with pressors, possibly preemptively, but the test should be aborted if the SBP drops < 90 mm Hg [4] despite adequate vasopressor support. The target duration of the apnea test (8–10 min) is derived from the traditional assumption that PaCO2 increases at a rate of 3 mmHg/min. However, in practice, the rate of PaCO2 increase is highly variable, from 0.5 to 10.5 mm Hg per minute, and neither linear nor predictable [5]. During normothermia and in the absence of pulmonary disease, even 5 min of apneic oxygenation may be enough to raise the PaCO2 from 40 to above 60 mm Hg [6]. If the PaCO2 has not reached the designated threshold, and the patient is stable, the test is continued and repeat gases are checked until the patient meets the goal [7]. After conclusion of the AT, the patient is reconnected to the ventilator. Alternatively, the test can be repeated for a longer period of time (10–15 min) after repeat pre-oxygenation on the ventilator [4]. If the AT has to be aborted, an ABG should be sent off immediately prior to reconnecting the patient to the ventilator; if the PaCO2 has risen adequately, the test can be considered positive, but if not, the is test is considered inconclusive [4]. If pulmonary status cannot be optimized to repeat the test, an ancillary test is required [4, 8].
Standard Apnea Test with Oxygen Insufflation according to the guidelines of the American Academy of Neurology [4]
Since inception of AT for the purpose of determination of brain death, a number of modifications, observations about safety and termination rates, practical considerations for AT conduction, new critical care technologies that affect AT and observations about AT in the context of worldwide application and society have been published.
In this scoping review, we set out to address the following questions:
Which modifications to conventional apnea testing have been described? What complications and termination rates have been documented for apnea testing? What monitoring techniques are available for apnea testing? How can apnea testing be performed on ECMO? Are there other important considerations pertaining to apnea testing?We conducted a database search (PubMed, EMBASE) and review of relevant medical literature from 1980 to 2019 on the topic of apnea testing for brain death determination. A search on registered work in progress studies similar to this topic on the International Prospective Register of Systematic Reviews (PROSPERO) revealed an ongoing review termed “Criteria for brain death: a worldwide comparison.” Hence, we decided to not focus on the details of worldwide differences in apnea testing; however, we included this topic as example for objective #5. Our review followed the Preferred Reporting Items for Scoping Reviews (PRISMA-ScR) guidelines [9]. Selected studies included English language articles concerning apnea testing using the search terms “brain death” with “apnea,” “apnea test,” “apnoea,” “apneoa test,” or “apnea/apnoea testing” (see Supplementary Material). Controlled trials, cohort studies, case series, case reports, and reviews on apnea testing for brain death determination in the adult population were included. Pediatric or animal studies were excluded. The search was most recently updated on Feb 12, 2020. Two reviewers independently screened the titles and abstracts and the selected full publications against the prespecified criteria. Disagreements regarding study selection were resolved by consensus. Selected studies were reviewed by both authors, and data were extracted for qualitative synthesis and grouped based on the prespecified objectives.
A total of 391 articles were initially retrieved and reviewed for relevance. Figure 2 shows the flowchart of retrieved and included literature. Ultimately, 87 articles met our inclusion criteria and were included in qualitative data synthesis.
Flow diagram of record identification, screening, eligibility and inclusion
A summary of described modifications to AT is displayed in Table 1 .
Modifications of Apnea testing
Similar mean duration of AT
Similar change in PaCO2, PaO2 or pH
Subgroups overweight patients and hanging injury: prevention of dramatic PaO2 reductions with MAT
AT apnea test, CO2 carbon dioxide, CPAP continuous positive airway pressure, FiO2 fraction of inspired oxygen, MAT modified apnea test, O2 oxygen, PaO2 partial pressure of oxygen, PEEP positive end-expiratory pressure
While O2 insufflation—the AAN guidelines recommend 6 L/min 3 , however, recommended rates vary considerably largely between 4 and > 6 L/min [10]—is aimed at preventing hypoxia, high flow O2 insufflation might result in a small degree of CO2 elimination, decreasing the rate of CO2 accumulation, and prolonging the test [11]. This can be addressed by lowering the O2 flow from the initial insufflation rate, commonly 4–6 L/min, by 1–2 L/min without inducing desaturation to increase the CO2 [12].
The modified apnea test (MAT) comprises delivery of 100% oxygen through the ETT and maintenance of positive end-expiratory pressure (PEEP) after disconnection from the ventilator, but does not interfere with the build-up of hypercapnia. This type of modification has been adopted widely and is included in the 2010 AAN guidelines as well as others (see below). Continuous positive airway pressure (CPAP) during AT can be generated in three ways: directly by the ventilator; through the use of a T-piece tube with a CPAP valve at the outflow end; and through the use of a traditional T-piece system with connection to a reservoir bag connected at one end with the distal outflow tubing immersed in water at a depth that can be adjusted depending on the target PEEP [13]. The first approach requires the use of a ventilator that allows disabling of rescue breaths for apnea. The 2010 AAN guidelines and the Austrian guidelines support AT with a T-piece and CPAP (second approach) if conventional AT resulted in poor oxygenation [4, 14]. Contrastingly, the Polish criteria recommend routine use of CPAP on a ventilator (CPAP-AT) [15]. These techniques can prevent the decruitment of lungs, reduce the risk of hypoxemia, and increase hemodynamic stability by prevention of blood pressure fluctuation compared to the traditional O2 insufflation method. This is especially true for patients on high PEEP and high doses of vasoconstrictor or inotropic medications [16, 30]. Various studies have shown success with using CPAP/PEEP during AT. In a Canadian study of 19 BD patients, the use of a 10-cm H2O PEEP valve attached to a T-piece with 12 L/min oxygen directly attached to the ETT was compared to the use of a simple T-piece (without CPAP valve, but with same 12 L/min oxygen flow) and to traditional oxygen insufflation with oxygen at 6 L/min. All patients had PaCO2 elevation > 60 mm Hg at the end of a 10 min trial, but the decrease in arterial oxygen partial pressure (PaO2) during AT was significantly less with the CPAP valve compared to the oxygen catheter or the plain T-piece [16]. In series of 4 and 26 BD patients by the same group, an Ambu bag with a PEEP valve was connected to the ETT with 10 L/min of 100% oxygen flow upon ventilator disconnection, resulting in stable PaO2/FiO2 (P/F) ratio when compared to prior to AT [17, 18]. In a recent comparison of the classic oxygen insufflation method with the CPAP-AT in 60 patients, a gradual decrease of P/F ratio was found throughout conventional AT, but not in the CPAP group [15]. In a Korean study comparing MAT to conventional AT, mean duration of AT was similar, without significant differences in change of PaCO2, PaO2 or pH. In overweight patients, however, MAT prevented dramatic PaO2 reductions, and in patients with hypoxic brain injury due to hanging, differences in PaO2 and SaO2 in the MAT group were significantly smaller than in the conventional AT group [19]. Less clear benefit was found in a multicenter Canadian study involving 14 ICUs with 77 patients comparing a group with O2 catheters placed inside the ETT to a group with an Ambu bag with a CPAP valve attached to the ETT, without significant difference in the degree of PO2 reduction, rate of PCO2 rise or pH decline [12]. Newer ventilators with the ability to switch the ventilator into complete apnea mode and keep the patient connected to the ventilator during the apnea test while using PEEP have shown success from a practical standpoint [20].
Carbon dioxide augmentation and elevation of the CO2 level prior to apnea testing are modifications that have been published as attempts to reach the same goal—proof that there is the absence of spontaneous respirations, but are neither widely adopted, standardized, nor supported by major guidelines. Carbon dioxide augmentation is limited by the need for steel CO2 containers, which are not readily available in most ICUs. Introduction of CO2 at a rate of 1 L/min into the circuit markedly reduced the observation time compared with conventional apneic oxygenation in a case series of 34 BD patients, allowing PaCO2 levels ≥ 60 mm Hg to be reached within 2 min [21]. A larger study from Argentina compared the same rate of CO2 administration for only 1 min duration followed by disconnection from the ventilator with the typical O2 insufflation method. Patients who received CO2 augmentation had less frequent serious adverse events (hypotension, cardiac arrest, arrhythmias, and hypoxemia). Pretest hypokalemia compared to normokalemia was a significant risk factor for complications in the group that received artificial CO2 augmentation. The authors recommended this CO2 augmentation method in patients who have low pretest PaO2/FiO2 ratio and higher risk of developing hypoxemia, but only after assuring normokalemia [22].
Intentional elevation of the baseline pre-apnea CO2 was reported in another study of 11 BD patients. Six patients who commenced AT at a CO2 of 40–45 mm Hg were compared to five patients who had a starting PCO2 level between 46 and 51 mm Hg. The group with higher baseline PCO2 reached a PaCO2 of 60 mm Hg in a shorter period. Both groups showed a PaCO2 rise of more than 20 mm Hg from baseline [23]. A similar modified apnea test, which allowed decrease in the minute ventilation to facilitate PaCO2 rise ≥ 20 mmHg from baseline, was also described in a case report of a patient who failed the regular apnea test and whose baseline PaCO2 level was 53 mm Hg. After 30 min of hypoventilation, the PaCO2 was 99 mmHg, no breaths were observed for 60 s and he was pronounced brain dead. The authors argue that the prior literature suggests that only 30–60 s are needed to confirm apnea. Furthermore, they discuss that a slower increase in PaCO2 compared to conventional AT does not result in CSF pH equilibrium based on animal data, indicating that if the hypoventilation process lasts less than 60 min, the CSF pH will not have enough time to equilibrate [24].
A combination of these two methods has also been described in 60 patients undergoing AT with CO2 augmentation, without disconnecting from the ventilator and with monitoring the end-tidal PCO2. The oxygen bayonet providing O2 to the ventilator from the wall O2 outlet was disconnected and reconnected to a Carbogen (97% O2/3% CO2) cylinder. The ventilator rate was set at 4 breaths/min and was gradually decreased by 1 breath/min until the predetermined end-tidal CO2 (PetCO2) was achieved, and then, adequate increase in PaCO2 was confirmed. The mean pre-apnea and post-apnea PaCO2 achieved by this method were 40 and 67 mmHg, respectively [25]. A similar process using Carbogen and capnometry, decreasing the ventilator set rate to 2–4 breaths/min and aiming for a calculated target PetCO2, has been reported in 24 BD patients. The mean post-apnea PetCO2 and PaCO2 were 63.2 and 71.5 mmHg, respectively, without major complications [26].
When considering these modifications that arguably infer a significant change when compared to conventional AT, it is important to view these modifications as a means to enable conduction of an AT—albeit significantly modified—as opposed to, in most instances, foregoing the AT altogether due to anticipated or manifest hemodynamic-respiratory instability.
Among the following methods of augmentation of oxygenation, recruitment maneuvers may be the most practical ones to attempt if needed. A study describing the use of recruitment maneuvers compared 27 patients treated with recruitment maneuvers after completing AT to 27 matched controls. In the recruitment group, immediately after reconnection to the ventilator, PEEP was increased from 5 to 35 cm H2O and the respiratory rate was decreased to 0.5 breaths per minute for 40 s before reapplying the initial ventilation settings. P/F ratio increased significantly in the recruitment group, although hypotension was noted in 55% during the recruitment maneuver [27]. A method described, but not widely used, with intention to avoid hypoxemia is the bulk diffusion method. This involves delivery of a large flow (40–60 L/min) of 100% FiO2 at the orifice of the ETT via the ventilator circuitry (with CPAP set at zero or by turning off the ventilator and disconnecting the expiratory humidity trap), with the goal of minimizing hypoxia during the assessment [28]. This method was described in a study of 24 BD patients, of which 23 completed the test. (In one patient, the test was aborted due to hypotension.) Only five (20.8%) patients had a post-apnea PaO2 of < 100 torr. Another method that has not gained wide use and is majorly limited by regulations and availability describes the use of artificial oxygen transporting solutions, such as perfluorocarbon-based oxygen carriers. In a study from Russia, the use of intravenous Perftoran (a solution of perfluordecaline and perfluormethylcyclohexylpiperidine 2:1 in emulsion) increased the pre-apnea and apnea-PaO2 compared to controls without affecting the PaCO2 levels [29].
Other, single case modifications aimed at improved oxygenation and/or ability to conduct and complete AT described a pre-AT recruitment maneuver with incremental titration of PEEP and maintenance of PEEP via a CPAP valve [30], prolonged AT for 110 min due to suspected cardiac ventilations resulting in a very slow increase in carbon dioxide levels [31] and successful use of prone positioning with CPAP and T-piece in a severely hypoxic patient [32].
While AT is generally considered safe, intentional induction of hypercarbia and respiratory acidosis can lead to cardiac arrhythmias, hypotension, or hypoxemia [3]. The patient should always be monitored for desaturation with an oxygen saturation [SpO2] monitor and for hypotension, which may develop due to dilatation of the peripheral arterioles and depression of the myocardial contractility in the setting of CO2 elevation [33]). Moreover, AT may de-recruit lungs and lead to decreased oxygenation and lung quality for potential transplantation. In patients on high PEEP or hemodynamic support, hemodynamic instability may be observed upon ventilator disconnection due to a sharp decrease in the intrathoracic pressure with immediate increase in the venous return to the heart [13]. Reported complications of AT are summarized in Table 2 .
Complications of Apnea testing and risk factors for completion failure
