Mean Event Detection Times
There was a main effect of Event for the event detection times (P < 0.001) but no main effect of Display. However, there was a significant interaction between Event and Display (P = 0.001, Fig. 7). Planned comparisons showed that participants detected Event 3 (tachypnea and hypertension) (16.8 s vs 29.7 s, P = 0.034) and Event 4 (hypotension) (13.9 s vs 36.3 s, P = 0.019) faster with the HMD than in the Control condition. However, participants were slower at detecting Event 2 (hypoventilation) with the HMD (77.9 s vs 25.7 s, P < 0.001) than in the Control condition. The difference in detection times between conditions for Event 1 (ST segment depression) was not significant (80.3 s vs 69.3 s).
Percentage of Time Looking Toward a Location
There was a main effect of Gaze Location (P < 0.001) for the percentage of time looking toward a location, no main effect of Display, and a significant interaction between the Gaze Location and Display (P < 0.001). Participants in the HMD conditions spent a larger percentage of time looking toward the Patient (99.0% vs 93.4%) and less time toward the Anesthesia machine (1.0% vs 6.5%) than they did in the Control condition.
Frequency of Changes in Gaze Location
There was a main effect of Display for the frequency of changes in Gaze Location (P < 0.001). Participants moved their gaze to the Anesthesia machine significantly less frequently in the HMD condition than in the Control condition (0.3 vs 1.5 changes/min).
Table 7 summarizes the participants’ responses to postexperiment questionnaires. They rated the scenario in which they used the HMD as being less busy (P = 0.039), easier to monitor the patient (P = 0.001), and they also believed they detected vital signs changes faster (P = 0.002) compared with the scenario with standard monitoring only. No participants reported that they detected the Excess sedation event more slowly with the HMD.
The attributes of monitoring with the HMD that participants liked were the ease of viewing (by not having to turn around), immediacy of vital signs, and being able to concentrate on the maze task better. Attributes they disliked were the weight and bulk of the HMD and associated equipment, headaches from wearing the HMD, not having all vital signs information available on the HMD, visual distraction from the maze task, and eye discomfort.
Participants reported that the HMD would be most useful when the anesthesiologist is performing tasks when the monitor is out of view, during busy cases or when no help is available, and during procedures using invasive monitoring.
We discuss the specific findings from the two experiments and the limitations of the research before turning to a discussion of broader issues raised by both experiments, and describe future areas of research.
Experiment 1—Inattentional Blindness
In contrast to a prior study, in which all unexpected events were detected faster with a HMD,6 this study did not indicate that participants detect unexpected events faster when they wear the HMD. However, the prior study only investigated events in the HMD + Monitor location. Participants in the present study detected events in the four locations at different speeds, possibly because of the relative urgency of the events. Events in the HMD + Monitor and Monitor only locations required corrective action by the anesthesiologist to prevent harm to the patient, whereas events in the other locations (Patient, OR) tended to carry a shared responsibility with other staff and less need for immediate corrective action. The interaction between Distractor Task Location and Event Location for detection times was caused by differences in the specific events used in the scenarios.
There were no significant differences between the two focus settings on the HMD in participants’ performance or their scanning patterns, but participants clearly preferred the HMD-Near condition. The best approach may be to let anesthesiologists self-adjust the focus to match their resting point of accommodation and to minimize eye strain18,19 by using the “hyperopic” focusing technique.19,20
The HMD changed participants’ monitor scanning patterns by letting them spend more time looking toward the patient, and less time toward the monitor. However, even though participants spent a larger percentage of time looking toward the patient and surgical field with the HMD, the fact that they did not notice events occurring at the patient or in the surgical field more effectively suggests that they may have been attending to information presented on the HMD and not to events occurring at the patient or in the surgical field.
There are other possible explanations for why the changes in participants’ scanning patterns did not translate into performance differences, such as differences in the likelihood of detecting unexpected events. One explanation is that anesthesiologists’ ability to detect unexpected events may be insensitive to changes in their scanning patterns. An alternative explanation is that anesthesiologists may have multiple means for maintaining vigilance that were not affected by the HMD in this experiment.
If the latter explanation is correct, and if anesthesiologists’ means for maintaining vigilance are compromised by physical or operational constraints in the OR, then a HMD may help them detect unexpected events. Supporting this view, participants in Experiment 1 reported that the HMD would be most useful when they were physically and operationally constrained by what they were doing.
Experiment 2—Constrained Context
The effects of the more physically and operationally constrained conditions of Experiment 2 are evident when the behavioral results are compared with those from the less constrained scenarios of Experiment 1. Both the frequency with which anesthesiologists turned to look at the patient monitor and the percentage of time they spent looking toward it were much lower in Experiment 2 (Table 6) than in Experiment 1 (Table 4). In Experiment 2, the benefit of the HMD for monitoring was evident with the dramatic fivefold reduction in how often participants looked toward the monitor when they used the HMD.
We expected that when participants worked under the more constrained conditions event detection should be faster with the HMD, regardless of clinical significance. Participants detected the tachypnea/hypertension and hypotension events more quickly with the HMD, which was as hypothesized. Participants’ slower detection of the hypoventilation event with the HMD was, however, not as hypothesized, and can be explained by three factors: inattentional blindness, perceptual differences, and overconfidence.
First, when people’s attention is diverted to one task among two or more, they can exhibit “inattentional blindness” to quite striking events,21 especially when using HMDs.11,12 Excess sedation is the only event of the four we investigated that produced results consistent with inattentional blindness. Therefore, inattentional blindness alone cannot explain the Excess sedation results; it must be viewed in the context of several factors at work in determining the effectiveness of HMDs in anesthesia contexts.
Second, the four events on the HMD had different perceptual characteristics because of the specific patient vital signs involved (Table 3). Although the ST segment depression during Event 1 could not be detected on the HMD, there was an audible pulse oximetry cue (an ectopic beat) at the event onset, which cued some participants and equalized the conditions. In Events 3 and 4, either systolic or diastolic noninvasive arterial blood pressure numbers increased from <100 to >100 (or vice versa), which created a noticeable “pop out” effect on the HMD. In contrast, Event 2 involved a slow change (decrease in RR) in the cycling of a wave form that was already moving with each breath cycle, so there was no visual “pop-out”.22 However, this explanation assumes that the participant did not regularly scan the display.
Third, participants may have been overconfident in their ability to monitor vital signs changes with the HMD while attending to the intubation task. The HMD presented information that was always in their field of view, which may have given them the impression that they would unfailingly notice any changes. In the postexperiment questionnaires, participants rated their event detection as being faster with a HMD, even though this was true only for two of the four events. Moreover, no participant reported detecting the hypotension event more slowly with the HMD. Participants may have scanned the HMD less often than needed to detect higher-order changes and may have been more dependent on visual pop-out than they realized.
The above three factors would rarely occur at the same time in clinical practice. Furthermore, the inattentional blindness and perceptual factors could be overcome by a combination of training and improved display design, and the overconfidence factor could be mitigated through training.
First, participants were unfamiliar with the HMD and may not have had sufficient time in Experiment 1 to develop new monitoring strategies to take advantage of its affordances. Second, the relatively high frequency of events in both experiments may have contributed to a degree of hypervigilance. Third, these studies only investigated the modality of information delivery and did not manipulate the displayed content or its presentation. Fourth, study participants were selected for 20/20 vision but anesthesiologists wearing large or multifocal glasses may have difficulties using the HMD. Fifth, auditory alarms were disabled in both experiments (although audible variable-tone pulse oximetry was present), which does not represent ideal or routine practice. Sixth, the immersive simulation environment may not replicate the behavior of participants using HMDs in the clinical environment; a clinical trial is needed. Finally, the physical form and weight of the HMD configuration used in the two experiments limited its practicality, but this will change as improvements in display technology will ultimately allow displays embedded in contact lenses.23
Our experiments indicate that HMDs may be most helpful to anesthesiologists during procedures where it is difficult to view a standard monitor, and as long as the display reliably attracts visual attention to important changes. In both experiments, anesthesiologists changed their scanning patterns when using the HMD, just as Ross et al.7 reported.
In Experiment 1, anesthesiologists detected unexpected events no faster with the HMD than with the standard monitor, contrary to the findings of Via et al.6 At the same time, the anesthesiologists missed unexpected events no more than when using the HMD than with the standard monitor, suggesting that inattentional blindness2,11 may not be a major cause for concern in anesthesia. There were no performance or behavioral differences as a result of the near versus far focus settings of the HMD: therefore, the focal distance should be set at the wearer’s resting point of accommodation to minimize eye strain.
In Experiment 2, participants detected some patient events faster with the HMD, as found by Via et al.6 The finding that anesthesiologists working in constrained conditions may sometimes detect an event more slowly with a HMD has not been reported in prior simulator-based studies of HMDs.6–8 The slower event detection is most probably due to a combination of inattentional blindness, perceptual characteristics of the displayed information, and overconfidence. Interestingly, this failure to detect a particular event was not reflected in subjective measures because participants believed that they detected events faster with the HMD (without being aware that the results showed otherwise for half of the events). If HMDs were to be routinely used for monitoring, it would be important to raise awareness of inadequate scanning and potential overconfidence through training.
Future areas of research include manipulating the content and layout of information presented on the HMD, investigating other types of HMDs, and evaluating the HMD across a variety of different clinical environments.
The authors thank Phil Cole and Matt Thompson (The University of Queensland) for help with designing and setting up the audiovisual systems; Tania Xiao and Dr Stas Krupenia (The University of Queensland) for their acting roles; Lucas Tomczak, Daniel Host, Dylan Campher, Andrea Thompson and Mike Wren (Queensland Health Skills Development Centre) for scenario ideas and rehearsals; and Dr Norris Green (Princess Alexandra Hospital) for early input into Experiment 1.
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© 2009 International Anesthesia Research Society
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