Hemodynamics and intrathoracic pressure transmission during controlled mechanical ventilation and positive end-expiratory pressure in normal and low compliant lungs

Effects of positive end-expiratory pressure on the predictability of fluid responsiveness in acute respiratory distress syndrome patients

The prediction accuracy of pulse pressure variation (PPV) for fluid responsiveness was suggested to be unreliable in low tidal volume (VT) ventilation. However, high PEEP can cause ARDS patients relatively hypovolemic and more fluid responsive. We hypothesized that high PEEP 15 cmH₂O can offset the disadvantage of low VT and improve the predictive performance of PPV. We prospectively enrolled 27 hypovolemic ARDS patients ventilated with low VT 6 ml/kg and three levels of PEEP (5, 10, 15 cmH₂O) randomly. Each stage lasted for at least 5 min to allow for equilibration of hemodynamics and pulmonary mechanics. Then, fluid expansion was given with 500 ml hydroxyethyl starch (Voluven 130/70). The hemodynamics and PPV were automatically measured with a PiCCO2 monitor. The PPV values were significantly higher during PEEP15 than those during PEEP5 and PEEP10. PPV during PEEP15 precisely predicts fluid responsiveness with a cutoff value 8.8% and AUC (area under the ROC curve) of ROC (receiver operating characteristic curve) 0.847, higher than the AUC during PEEP5 (0.81) and PEEP10 (0.668). Normalizing PPV with driving pressure (PPV/Driving-P) increased the AUC of PPV to 0.875 during PEEP15. In conclusions, high PEEP 15 cmH₂O can counteract the drawback of low VT and preserve the predicting accuracy of PPV in ARDS patients.

Cardiopulmonary effects of matching positive end-expiratory pressure to abdominal pressure in concomitant abdominal hypertension and acute lung injury

BACKGROUND

To evaluate the cardiopulmonary effects of positive end-expiratory pressure (PEEP) equalization to intra-abdominal pressure (IAP) in an experimental model of intra-abdominal hypertension (IAH) and acute lung injury (ALI).

METHODS

Eight anesthetized pigs were submitted to IAH of 20 mm Hg with a carbon dioxide insufflator for 30 minutes and then submitted to lung lavage with saline and Tween (2.5%). Pressure x volume curves of the respiratory system were performed by a low flow method during IAH and ALI, and PEEP was subsequently adjusted to 27 cm . H2O for 30 minutes.

RESULTS

IAH decreases pulmonary and respiratory system static compliances and increases airway resistance, alveolar-arterial oxygen gradient, and respiratory dead space. The presence of concomitant ALI exacerbates these findings. PEEP identical to AP moderately improved oxygenation and respiratory mechanics; however, an important decline in stroke index and right ventricle ejection fraction was observed.

CONCLUSIONS

Simultaneous IAH and ALI produce important impairments in the respiratory physiology. PEEP equalization to AP may improve the respiratory performance, nevertheless with a secondary hemodynamic derangement.

Estimating cardiac filling pressure in mechanically ventilated patients with hyperinflation

OBJECTIVE

When positive end-expiratory pressure (PEEP) is applied, the intracavitary left ventricular end-diastolic pressure (LVEDP) exceeds the LV filling pressure because pericardial pressure exceeds 0 at end-expiration. Under those conditions, the LV filling pressure is itself better reflected by the transmural LVEDP (tLVEDP) (LVEDP minus pericardial pressure). By extension, end-expiratory pulmonary artery occlusion pressure (eePAOP), as an estimate of end-expiratory LVEDP, overestimates LV filling pressure when pericardial pressure is >0, because it occurs when PEEP is present. We hypothesized that LV filling pressure could be measured from eePAOP by also knowing the proportional transmission of alveolar pressure to pulmonary vessels calculated as index of transmission = (end-inspiratory PAOP--eePAOP)/(plateau pressure--total PEEP). We calculated transmural pulmonary artery occlusion pressure (tPAOP) with this equation: tPAOP = eePAOP--(index of transmission x total PEEP). We compared tPAOP with airway disconnection nadir PAOP measured during rapid airway disconnection in subjects undergoing PEEP with and without evidence of dynamic pulmonary hyperinflation.

DESIGN

Prospective study.

SETTING

Medical intensive care unit of a university hospital.

PATIENTS

We studied 107 patients mechanically ventilated with PEEP for acute respiratory failure. Patients without dynamic pulmonary hyperinflation (group A; n = 58) were analyzed separately from patients with dynamic pulmonary hyperinflation (group B; n = 49).

INTERVENTION

Transient airway disconnection.

MEASUREMENTS AND MAIN RESULTS

In group A, tPAOP (8.5+/-6.0 mm Hg) and nadir PAOP (8.6+/-6.0 mm Hg) did not differ from each other but were lower than eePAOP (12.4+/-5.6 mm Hg; p < .05). The agreement between tPAOP and nadir PAOP was good (bias, 0.15 mm Hg; limits of agreement, -1.5-1.8 mm Hg). In group B, tPAOP (9.7+/-5.4 mm Hg) was lower than both nadir PAOP and eePAOP (12.1+/-5.4 and 13.9+/-5.2 mm Hg, respectively; p < .05 for both comparisons). The agreement between tPAOP and nadir PAOP was poor (bias, 2.3 mm Hg; limits of agreement, -0.2-4.8 mm Hg).

CONCLUSIONS

Indexing the transmission of proportional alveolar pressure to PAOP in the estimation of LV filling pressure is equivalent to the nadir method in patients without dynamic pulmonary hyperinflation and may be more reliable than the nadir PAOP method in patients with dynamic pulmonary hyperinflation.

[Effect of a lung contusion on the prognosis of severe head injury in the child]

OBJECTIVES

To assess the effects of a pulmonary contusion (PC) on the outcome of a severe head trauma (SHT) in children less than 15-year-old.

STUDY DESIGN

Retrospective study.

PATIENTS

The study included 30 severely head injured children with a Glasgow Coma Scale score (GCS) < or = 8, associated with a PC (PC+) diagnosed on a thoracic CT-scan and 30 severely head injured children without PC (PC-).

METHODS

Outcome was assessed using the Glasgow Outcome Scale (GOS), on discharge and six months later. Age, body weight, gender, GCS, PTS, ISS, hypoxaemia, arterial hypotension, the results of the cerebral CT-scan, the main treatment administered, complications, the duration of tracheal intubation as well as the duration of stay in the intensive care unit (ICU) and in the hospital were compared between groups.

RESULTS

GCS median was lower (6 vs 8, P = 0.001) and ISS median higher (25 vs 23, P = 0.0004) in the PC+ group. Hypoxaemia was more frequent in the PC+ group (n = 12 vs n = 0, P = 0.0001). There was no difference between groups regarding the results of cerebral CT scan. Blood transfusion was more frequently used in the PC+ group (n = 14 vs n = 5, P = 0.03). Median duration of tracheal intubation, and of stay in the ICU and in the hospital were shorter in the PC- group (respectively 8 vs 6 days, P = 0.03; 10 vs 7.5 days, P = 0.008; 13.5 vs 10.5 days, P = 0.01). No difference was observed regarding complications between groups. GOS on discharge was higher in the PC+ group (3 vs 2, P = 0.01). There was an increase in GOS at six months in the two groups, however GOS remained significantly higher in the PC+ group (median values 2 vs 1, P = 0.002). A favourable outcome occurred less frequently in the PC+ group on discharge and at six months (respectively n = 14 vs 25, P = 0.006; n = 20 vs 28, P = 0.02).

CONCLUSION

The association of a PC to a severe head trauma is responsible for a poorer outcome in children, probably because, at least in part, a higher incidence of hypoxaemia.

Cardiopulmonary effects of positive pressure ventilation during acute lung injury

STUDY OBJECTIVES

To assess the gas exchange and hemodynamic effects of pressure-limited ventilation (PLV) strategies in acute lung injury (ALI). We hypothesized that in ALI, the reduction of plateau airway pressure (Paw) would be associated with less alveolar overdistention and thus have better hemodynamic and gas exchange characteristics than larger tidal volume (Vr) ventilation.

SETTING

Laboratory.

DESIGN

Prospective time-controlled sequential animal study.

MEASUREMENTS

Right atrial, pulmonary artery, left atrial, arterial, lateral pleural (Ppl), and pericardial (Ppc) pressures, Paw, ventricular stroke volume, mean expired CO2, and arterial and mixed venous oxygen contents. Airway resistance and static lung compliance were also measured.

INTERVENTIONS

Intermittent positive pressure ventilation (IPPV) given before (control) and after induction of ALI by oleic acid infusion (0.1 mL/kg). IPPV at FIO2 of 1, VT of 12 mL/kg, and frequency adjusted to maintain normocarbia. ALI PLV was given during ALI and defined as that VT which gave a similar plateau Paw to that of control IPPV. High-frequency jet ventilation (HFJV) and ALI HFJV were also given and defined as frequency within 10% of heart rate and mean Paw similar to that during control IPPV.

RESULTS

After ALI, static lung compliance, PaO2, and pH decreased, whereas airway resistance and PaCO2 increased. For a constant lung volume, Ppl and Ppc were not different between control and ALI. Both absolute dead space (VD) and intrapulmonary shunt fraction increased after ALI, but absolute VD was lower with ALI PLV and ALI HFJV when compared with ALI IPPV. Ventilation did not alter hemodynamics during ALI.

CONCLUSIONS

Changes in lung volume determine Ppc and Ppl. PLV strategies do not alter hemodynamics but result in less of an increase in VD/VT than would be predicted from the obligatory decrease in VT.

Doppler evaluation of right ventricular outflow impedance during positive-pressure ventilation

Positive-pressure ventilation has often been advocated to increase oxygen delivery. This ventilation mode itself, however, can impair right ventricular ejection and, thus, diminish cardiac output. In this study, alterations of right ventricular outflow impedance were evaluated after stepwise increases of positive end-expiratory pressure (PEEP). Different pulmonary artery flow characteristics were evaluated with transesophageal echocardiography in mechanically ventilated postoperative coronary artery bypass surgery patients without pulmonary hypertension. A progressive decrease of pulmonary artery flow velocity and time velocity integrals was found with increasing PEEP levels. No changes in acceleration time or pre-ejection period were observed. In order to decrease the influence of heart rate, the ratios of the different pulmonary artery flow characteristics were calculated. At end-inspiration, both the ratio of acceleration time to right ventricular ejection period and the ratio of pre-ejection period to right ventricular ejection period showed progressive increases above 10 cmH2O positive end-expiratory pressure (13.3% at the level of 15 cmH2O and 8.5% at the level of 20 cmH2O). In this study, acceleration time appears not to be of importance in ventilated patients. These data strongly support the hypothesis that intermittent squeezing of the pulmonary arterial tree during inspiration, rather than positive end-expiratory pressure, creates an increase of right ventricular outflow impedance.

Central and regional blood flow during hyperventilation. An experimental study in the pig

Mechanical hyperventilation not only reduces brain oedema after neurotrauma but also affects the central and systemic circulation. We have, in pigs, measured blood flow in the pulmonary artery, the portal vein and in the femoral artery, as well as estimated the splanchnic blood flow and studied the relative perfusion using the microsphere technique in normo- and hypocarbia during intermittent positive pressure ventilation. A normoventilated control group did not change in cardiac output, portal vein blood flow, splanchnic blood flow and femoral arterial blood flow. Hyperventilation was performed to a PCO2 of 3.0 +/- 0.1 kPa. We found that in pigs ventilated with high tidal volume skeletal muscle blood flow did not change during the first 60 min of hyperventilation but gradually decreased thereafter. Blood flow to the cerebellum decreased soon after the induction of hyperventilation, whereas the cerebral blood flow did not decrease until the second hour of hyperventilation. Cardiac output, splanchnic perfusion and portal vein blood flow all decreased. Myocardial perfusion and arterial blood flow to spleen and kidney decreased while pancreatic and liver arterial blood flows were unaffected. It is concluded that mechanical hyperventilation with low frequency and large tidal volumes reduces the flow to most tissues, where the relative decrease according to microsphere measurements is most pronounced in skeletal muscles, heart muscle and cerebellum. However, the changes in cardiac output and splanchnic blood flow were not observed when hyperventilation was induced by increased frequency, keeping the tidal volume constant.