The effect of lung-size mismatch on mechanical ventilation tidal volumes after bilateral lung transplantation

Abstract

Objectives: Mechanical ventilation tidal volumes are usually set according to an estimate of patient size in millilitres (ml) per kilogram (kg) body weight. We describe the relationship between donor-recipient lung-size mismatch and postoperative mechanical ventilation tidal volumes according to recipient- and donor-predicted body weights in a cohort of bilateral lung transplant patients.

Methods: A most-undersized (10 patients with lowest predicted total lung capacity [pTLC] ratio = pTLC-donor/pTLC-recipient), a most-oversized (10 patients with highest pTLC ratio) and best-matched subset (10 patients with predicted total lung capacity ratio closest to 1.0) were selected within a cohort of 70 patients. All tidal volumes during mechanical ventilation in the first 96 h after bilateral lung transplantation were recorded. Tidal volumes were expressed in ml and ml/kg-recipient-predicted body weights and ml/kg-donor-predicted body weights.

Results: Postoperative absolute tidal volumes (in ml) were comparable between subsets of patients with undersized, matched and oversized allografts (552 ± 103 vs 581 ± 107 vs 582 ± 104 ml), and tidal volumes in ml/kg-recipient-predicted body weights were also similar (8.8 ± 1.4 vs 9.3 ± 1.1 vs 9.8 ± 2.1). However, tidal volumes in ml/kg-donor-predicted body weights revealed significant differences between undersized, matched, and oversized subsets (11.4 ± 3.1 vs 9.4 ± 1.2 vs 8.1 ± 2.1, respectively; P < 0.05). Two patients developed primary graft dysfunction grade 3, both in the undersized subset. Four patients in the undersized group underwent tracheotomy (vs none in matched and one in oversized subset).

Conclusions: During mechanical ventilation after bilateral lung transplantation, undersized allografts received relatively higher tidal volumes compared with oversized allografts when the tidal volumes were related to donor-predicted body weights.

Figure 1:

Conceptual graphic on the possible effect of lung-size mismatch on mechanical ventilation tidal volumes expressed as ml/kg-predicted body weights of the donor.

Figure 2:

Scatter plot of tidal volumes in ml/kg against predicted total lung capacity (pTLC) ratio. For each patient (n = 30) the mean tidal volume is displayed. In (A) tidal volumes are expressed as ml/kg-predicted body weights of the donor. Tidal volumes in ml/kg [donor-predicted body weights] = 17.1 - (7.18 * pTLC ratio), P < 0.001. In (B) tidal volumes are expressed as ml/kg-predicted body weights of the recipient, P = 0.24. Red squares = most-undersized cohort (n = 10). Black triangles = best-matched cohort (n = 10). Green circles = most-oversized cohort (n = 10). Dotted lines represent regression lines. Solid lines represent mean tidal volumes for most-undersized cohort (red), best-matched cohort (black) and most-oversized cohort (green).

Figure 3:

Scatter plot of P_aO₂/F_iO₂ ratio and dynamic compliance against pTLC ratio. For each patient (n = 30), all available observations are displayed. (A) P_aO₂/F_iO₂ ratio. Dotted curve represents polynomial regression curve (degree = 2), P_aO₂/F_iO₂ ratio = 418 − (69 * pTLC ratio) − (462 * [pTLC ratio − 1.006]²), P = 0.0003. (B) Dynamic compliance. Dotted curve represents polynomial regression curve (degree = 2), P = 0.33. Red squares = most-undersized cohort (n = 86 observations). Black triangles = best-matched cohort (n = 71 observations). Green circles = most-oversized cohort (n = 81 observations).