ANESTH ANALG 429 1984;63:429-32 Hemodynamic Consequences of PEEP in Seated Neurological Patients-Implications for Paradoxical Air Embolism Nancy A. K. Perkins, MD, and Robert F. Bedford, MD PERKINS NAK, BEDFORD RF. Hemodynamic 47% and right atrial pressure (RAP) increased from 3.6 +- consequences of PEEP in seated neurological patients- 0.7 SEM mm fig to 8.9 ? 0.9 SEM mm Hg (P < 0.05). implications for paradoxical air embolism. Anesth Analg Pulmona ry capillary wedge pressure (PCWP) did not in- 1984;63:429-32. crease significantly during PEEP. RAP exceeded PCWP in only two patients before PEEP, but RAP exceeded PCWP In order to better understand the hemodynamic conse- in seven patients during PEEP. Weconclude that PEEP is quences of the use of positive end-expiratory pressure (PEEP) potentially detrimental during operations in the seated po- in patients in the seated position, 22 patients undergoing sition because it not only impairs hemodynamic perform- neurosurgical operations were monitored with radial arterial ance, but might predispose patients with a probe-patent and thermistor-tipped Swan-Ganz catheters both before and foramen ovule to the risk of paradoxical air embolism. during 10-cm H20 PEEP. Significant (P < 0.05) reduc- tions in cardiac output (15%), stroke volume (15%), and Key Words: ANESTHESIA-neurosurgical. VENTI- mean arterial pressure (24%)occurred with the introduction LATION-positive end-expiratory pressure. HEART- of PEEP, while pulmonary vascular resistance increased myocardial function. EMBOLISM, AIR-paradoxical. Positive end-expiratory pressure (PEEP) has been ad- (9-ll), human volunteers (12), and patients with car- vocated for neurosurgical patients undergoing oper- diorespiratory disease (13-15), these studies have fo- ations in the seated position, both as a preventive cused primarily on changes in cardiac output and ven- measure to avoid air embolism (1,2) and as acute treat- tricular performance without regard to changes in ment when air embolism is suspected (3,4). In these interatrial pressure gradient. situations, it has been proposed that an increase in This study was undertaken to examine the hemo- venous pressure induced by PEEP might help to pre- dynamic impact of PEEP in anesthetized patients in vent entrainment of air into the circulation or to stop the seated position: a situation in which decreased air entrainment after air embolism has been detected. lung blood volume due to the effect of gravity (16) The use of PEEP in this setting, however, is contro- might have unanticipated effects on the cardiovas- versial, because it has been suggested that such an cular response to PEEP and in which changes in in- increase in venous pressure might promote paradox- teratrial pressure gradient due to posture may pre- ical (systemic) air embolism if the patient were one of dispose patients to paradoxical air embolism (17). the 25-35% of the population with an asymptomatic foramen ovale (5,6), thus tending to drive air bubbles from the right atrium into the left atrium and by- Methods passing the filtering function of the lungs (7,8). Al- The subjects of the study (N = 11) were informed, though the hemodynamic effects of PEEP recently consenting adult patients (ASA class I or 11) scheduled have been examined extensively in supine animals for elective cervical laminectomy at the C5-6 level. The protocol was reviewed and approved by the Hu- This study was supported in part by United States Public Health man Investigation Committee Of the University Of Vir- Services Grant T32-GM-075900-04. ginia. Anesthesia was induced with thiopental (4 Received from the Department of Anesthesiology, University mg/kg), and maintained with 70% N@ in o2and of Virginia Medical Center, Charlottesville, Virginia. Presented in part at the American Society of Anesthesiologists Annual Meeting, morphine (0.3-0.5 mg/kg Pan- Las Vegas, Nevada, October 1982. Accepted for publication De- curonium (0.1 mg/kg-- intravenously) was given to fa- cember-30, 1983. cilitate endotracheal intubation, and ventilation was Address correspondence to Dr. Bedford, Department of Anes- thesiology, University of Virginia Medical Center, Charlottesville, to maintain PaC02 between 30 and 35 mm VA 22908. Hg. Using pressure waveform control, radial and 0 1984 by the International Anesthesia Research Society ANESTH ANALG PERKINS AND BEDFORD 430 1984;63:429-32 thermistor-tipped quadruple-lumen pulmonary ar- 4PCWPl- 30 - tery catheters were placed percutaneously, and car- I diovascular pressures were transduced with Bentley PAP (mmHg) Model 800 transducers and were referenced to the I level of the 6th intercostal space anteriorly. End-tidal 0- CO, fraction (Beckman LB-2) and all cardiovascular 5- pressures were continuously recorded using a Brush I Model 440 strip-chart recorder. Placement of the pul- monary artery catheter tip in a perfused (zone 111) lung segment (16) was documented by observing a de- crease in end-tidal COz fraction during each pulmo- nary capillary wedge pressure (PCWP) determination " (Fig. 1) (18). If a measurable decrease in end-tidal CO, I+ 1- MIN~ did not occur during balloon inflation, the position of the pulmonary artery catheter was manipulated using Figure 1. Representative strip-chart record demonstrating a de- a sterile introducer sleeve until proper placement of crease in end-tidal CO, fraction (FEKO~) during PCWP determi- nation. Because the decrease in FEKo~ indicates a decrease in re- the catheter tip was verified (mean change in F,<o, gional pulmonary blood flow, the PA catheter tip must be located =4.2-+0.1%SEMtO3.9+-0.1%SEM,P<0.001).All in a perfused lung segment where PCWP would reflect left atrial patients received 1 L of 5% dextrose in lactated Ring- pressure. er's solution intravenously and then were placed in the semirecumbent seated position. Heart rate (HR), mean arterial pressure (MAP), right atrial pressure Discussion (RAP), pulmonary arterial pressure (PAP), pulmo- Patients undergoing operations in the seated position nary capillary wedge pressure (PCWP), and cardiac are at a considerable hernodynamic disadvantage with output (thermodilution technique; Edwards model 9528 regard to venous air embolism. Cardiac output and cardiac output computer) were recorded at the fol- stroke volume are decreased from values in the supine lowing times: during steady-state general anesthesia state (19) and many of these patients have a right atrial with constant surgical stimulation immediately before pressure that is higher than left atrial pressure (LAP), applying 10-cm H20 PEEP to the anesthesia circuit; thus possibly placing them at risk for paradoxical air and 5 min after 10-cm H20 PEEP (Boehringer valve) embolism (17). Our results indicate that application was added to the expiratory limb of the anesthesia of 10-cm H20 PEEP in this situation causes a further circuit. reduction in cardiac output and mean arterial pres- Derived hemodynamic variables were calculated sure while simultaneously resulting in a higher right- from appropriate formulae. Values measured during to-left interatrial pressure gradient. If PEEP is used PEEP were compared to those obtained without PEEP prophylactically to prevent air embolism, it appears using Student's t-test for paired data. The incidence that small, otherwise asymptomatic, air bubbles in the of RAP > PCWP was compared before and during venous circulation might be forced through a probe- PEEP using Fisher's exact test. P < 0.05 was regarded patent foramen ovale and into the arterial circulation as significant. where coronary or cerebral damage might occur. If used as treatment of suspected air embolism, PEEP tends to mimic the hemodynamic effects of air em- Results bolism (increased pulmonary vascular resistance, right During surgery in the seated position, institution of ventricular and right atrial pressures, and decreased 10-cm PEEP caused significant reductions in MAP cardiac output) (20), and therefore, might further im- (14%), stroke volume (15%), and CO (15%)without pair cardiovascular performance at a critical juncture. a change in HR or systemic vascular resistance (Table Furthermore, because recent studies indicate delayed 1). There was a 28% increase in pulmonary artery arrival of embolized air bubbles from above the su- pressure (PAP) and a 47% increase in pulmonary vas- perior vena cava (21), rapid institution of PEEP con- cular resistance (PVR). RAP increased significantly ceivably could deliver the critical increase in interatrial with PEEP, whereas PCWP did not increase signifi- pressure gradient required to produce paradoxical air cantly, and the mean interatrial pressure gradient embolus. Although the minimal pressure gradient re- (RAP-PCWP) changed from negative to positive (Fig. quired to cause right-to-left shunting across a probe- 2). Only two patients had RAP > PCWP before PEEP, patent foramen ovale is not known, we have seen whereas seven patients had RAP > PCWP during paradoxical air embolism in a patient with only a + 5 PEEP (P < 0.05). mm Hg RAP-PCWP gradient (17), and it has been PEEP IN THE SEATED POSITION ANESTH ANALG 431 1984;63:429-32 Table 1. PEEP-induced Hemodynamic Changes in Patients in the Seated Position 0-cm HzO PEEP 10-crn H,O PEEP Heart rate (beatsomin I) 78 2 5 78 2 5 Mean arterial pressure (mm Hg) 99.4 ? 9.4 85.6 2 3.P Mean pulmonary artery pressure (mm Hg) 14.8 2 0.7 19.0 2 1.1” Right atrial pressure (mm Hg) 3.6 2 0.9 8.9 2 O.Y Pulmonary capillary wedge pressure (mm Hg) 6.4 2 0.9 7.9 2 0.8 Cardiac output (Lamin I) 5.4 2 0.8 4.6 2 0.5” Stroke volume (ml) 69.2 8.3 59.0 2 11.2’ Systemic vascular resistance (dyne*sec*cm~ 5, 1418 ? 232 1333 2 109 Pulmonary vascular resistance (dyne*sec*cm 5, 124 2 27 192 26“ All values are mean -t scv “D < 0.05 vs 0-cm H20 PEEP shown that a + 4 mm Hg interatrial pressure gradient pressure (3).