Figure 1.

Path diagram of the dependencies of the TST₉₀−PaO₂ feed back structural equations designed (*). The observed variables are enclosed in boxes, and the unobserved variables (error terms) are circled. An arrow from one variable to another indicates that the first variable has a direct influence on the second, two straight single-headed arrows connecting two variables indicate a reciprocal direct influence, and a curved two-headed arrow signifies a correlation between two variables. The path coefficients displayed on the arrows are the regression coefficients for the observed variables, and the residual variances/covariances for the unobserved errors.

(*) VC%: vital capacity in % of predicted; BMI: body mass index (Kg/m²); TST₉₀: percent sleep time spent with SaO₂ <90%; PaO₂%: partial oxygen arterial pressure in % of predicted; ODI: oxygen desaturation index; AHI: apnea-hypopnea index. The SEM is written as:

TST₉₀ = α₁ BMI + α₂ VC_% + α₃ PaO_2% + α₄ ODI + α₅ AHI + e₁

PaO_2% = β₁ BMI + β₂ VC_% + β₃ TST₉₀ + e₂

var (e₁)= σ₁; var (e₂)= σ₂; cov (e₁; e₂) = σ₁₂