The effects of hypercapnia on cerebral autoregulation in ventilated very low birth weight infants

Abstract

Permissive hypercapnia, a strategy allowing high Pa(CO2), is widely used by neonatologists to minimize lung damage in ventilated very low birth weight (VLBW) infants. While hypercapnia increases cerebral blood flow (CBF), its effects on cerebral autoregulation of VLBW infants are unknown. Monitoring of mean CBF velocity (mCBFv), Pa(CO2), and mean arterial blood pressure (MABP) from 43 ventilated VLBW infants during the first week of life was performed during and after 117 tracheal suctioning procedures. Autoregulation status was determined during tracheal suctioning because it perturbs cerebral and systemic hemodynamics. The slope of the relationship between mCBFv and MABP was estimated when Pa(CO2) was fixed at 30, 35, 40, 45, 50, 55, and 60 mm Hg. A slope near or equal to 0 suggests intact autoregulation, i.e. CBF is not influenced by MABP. Increasing values >0 indicate progressively impaired autoregulation. Infants weighed 905 +/- 259 g and were 26.9 +/- 2.3 wk gestation. The autoregulatory slope increased as Pa(CO2)) increased from 30 to 60 mm Hg. While the slopes for Pa(CO2) values of 30 to 40 mm Hg were not statistically different from 0, slopes for Pa(CO2) > or = 45 mm Hg indicated a progressive loss of cerebral autoregulation. The autoregulatory slope increases with increasing Pa(CO2), suggesting the cerebral circulation becomes progressively pressure passive with hypercapnia. These data raise concerns regarding the use of permissive hypercapnia in ventilated VLBW infants during the first week of life, as impaired autoregulation during this period may be associated with increased vulnerability to brain injury.

Figures

Figure 1

(A) Example of intact cerebral autoregulation. Adapted from van de Bor and Walther (4) and Papile et al. (5). (B) Example of impaired cerebral autoregulation. Adapted from Lou et al. (6).

Figure 2

The influence of increasing Paco2 values on the estimated slope of the autoregulatory plateau in 43 ventilated VLBW infants. The autoregulatory plateau (the relationship between mCBFv and MABP at 30–40 mm Hg) is demonstrated for seven Paco2* values. A slope near or equal to 0 suggests intact cerebral autoregulation whereas increasing values >0 indicate progressively impaired autoregulation.

Figure 3

Estimated mean slopes and 95% confidence intervals of the autoregulatory plateau for Paco2 values 30, 35, 40, 45, 50, 55, and 60 mm Hg for 43 VLBW infants. Bars indicate 95% confidence intervals for the mean slopes of the autoregulatory plateaus for Paco2 30 (n = 82), 35 (n = 94), 40 (n = 100), 45 (n = 103), 50 (n = 100), 55 (n = 90), and 60 mm Hg (n = 83). The horizontal line at slope 0 indicates intact autoregulation. The estimated means of the slope of the autoregulatory plateau (cm/s * mm Hg−1) increased as Paco2 increased (p = 0.004).