TABLE 4.
ΔSaO2 PREDICTION MODEL AND THE IMPACT OF CHANGING INDIVIDUAL VARIABLES ON AN EVENT WITH A PREDICTED ΔSaO2 OF 5.9% (THE PREDICTED ΔSaO2 FOR AN EVENT WITH THE CHARACTERISTICS INDICATED IN THE “BASELINE VALUE” COLUMN)
| Variable* | Coefficient† (SE) | Baseline Value | Comparison Value | Expected Difference in ΔSaO2 (%) | Expected new ΔSaO2 (%) |
|---|---|---|---|---|---|
| BMI‡ | 0.055 (0.014) | 30 kg/m2 | 40 kg/m2 | +0.55 | 6.4 |
| Sleep state‡ | 0.39 (0.05) | NREM sleep | REM sleep | +0.39 | 6.3 |
| ΔVt‡ | 0.030 (0.002) | ||||
| (ΔVt)2 | 0.00054 (0.00005) | 80% | 90% | +0.35 | 6.2 |
| Baseline SaO2 | 0.52 (0.04) | ||||
| (Baseline SaO2)2 | 0.025 (0.003) | 100% | 98%§ | −0.93 | 4.9 |
| Event type | 0.25 (0.07) | Hypopnea | Apnea‖ | +0.25 | 6.1 |
| Body position | 0.54 (0.07) | Lateral | Back | +0.54 | 6.4 |
| Event duration | 0.025 (0.002) | ||||
| (Event duration)2 | 0.00016 (0.00005) | 30 s | 60 s | +0.88 | 6.8 |
| Time of event | −0.022 (0.011) | 1st Event | 1 h After 1st event | −0.02 | 5.9 |
| Gender | −0.26 (0.08) | Male | Female | −0.26 | 5.6 |
| Age | 0.028 (0.006) | 50 yr | 60 yr | +0.28 | 6.2 |
| Smoking |
0.20 (0.07) |
Nonsmoking |
1 Pack/d |
+0.20 |
6.1 |
To aid interpretation of the model intercept (5.88; SE, 0.17), select variables (BMI, ΔVt, baseline SaO2, event duration, and age) were “centered” in fitting the model by subtracting constants from their observed values as indicated by the “Starting value” column (i.e., BMI was centered at 30 kg/m2 [“BMI” = BMI − 30]; ΔVt at 80% [“ΔVt” = ΔVt − 80%]; etc.).
All coefficients (including quadratic and interaction terms) are significantly different from 0 with a P < 0.001, except for smoking (P = 0.008) and time of event (P = 0.04).
There were also significant interactions of BMI × sleep state (interaction coefficient, 0.042; SE, 0.009) and BMI × ΔVt (interaction coefficient, 0.00082; SE, 0.00018), indicating that the association of BMI and ΔSaO2 is stronger in REM compared with NREM sleep and that the effect of ΔVt on ΔSaO2 increases with increasing BMI.
The fitted quadratic relationship between baseline SaO2 and ΔSaO2 suggested that baseline SaO2 < 80% was strongly associated with greater ΔSaO2 reductions but also that there are modest decreased expected reductions in ΔSaO2 with increasing baseline SaO2 for baseline SaO2 > 80%.
Comparing a hypopnea with ΔVt = 80% to an apnea with ΔVt = 100% would yield an expected 1.1% greater ΔSaO2 (i.e., 5.9 + 1.1 = 7.0% “expected new ΔSaO2”).