Abdominal perfusion pressure and coronary arterial perfusion pressure in patients undergoing coronary artery bypass graft surgery

Abstract

Abdominal perfusion pressure (APP) is defined as the difference between the mean arterial pressure and the intra-abdominal pressure (IAP). IAP elevation results in various side effects, including a decrease in coronary arterial perfusion pressure (CoPP). The present study analyzed the relationship between APP and CoPP in patients undergoing extracorporeal circulation (ECC). The patient population selected for the present study comprised 45 adult patients with a mean (± SD) age of 65.9±7.21 years (range 42 to 80 years), undergoing coronary artery bypass grafting with ECC and normovolemic hemodilution under general anesthesia. CoPP was measured as the difference between mean arterial pressure and pulmonary capillary wedge pressure. APP and CoPP were measured at seven time points (TPs): before surgery after the induction of anesthesia (TP1), during internal mammary artery preparation (TP2), 10 min after the heart-lung machine disconnection (TP3), after completion of the procedure but before sending the patient to the postoperative intensive care unit (TP4), 1 h after surgery (TP5), 6 h after surgery (TP6) and 18 h after the procedure (TP7). TP1 was considered to be the baseline value. IAP increased from TP3 to TP7; APP decreased at TP3 and TP4; there were no significant changes in CoPP. Significant correlations between APP and CoPP were observed at all TPs. Moreover, IAP correlated with CoPP at TP2 and TP4. Additionally, there was a strong overall correlation between APP and CoPP (P<0.001, r=0.9598). The present study arrived at two major conclusions: that ECC resulted in IAP elevation and that APP was strongly correlated with CoPP.

Intra-abdominal pressure (IAP) is defined as the steady-state pressure contained within the abdominal cavity (1). The normal value of IAP ranges from 0 mmHg to 7 mmHg and depends on both the elasticity of the abdominal wall and abdominal capacity. Several pathologies result in IAP increases, including abdominal trauma with intra- or extra-peritoneal bleeding, ascites, intra-abdominal tumours, intestinal injury, ileus, pancreatitis and surgical ‘packing’. Moreover, increases in IAP may be observed after massive fluid resuscitation, poly-transfusion, hypothermia and severe coagulation disorders. IAP values higher than 12 mmHg represent intra-abdominal hypertension (IAH), and a significant IAP elevation reduces the microcirculatory blood flow in most organs of the abdominal cavity. The abdominal blood flow pressure is known as the abdominal perfusion pressure (APP) and can be calculated as the difference between mean arterial pressure (MAP) and IAP (ie, APP = MAP – IAP) (1,2). Therefore, the changes in APP are strongly dependent on changes in MAP and IAP.

Previous studies described changes to IAP and APP in patients undergoing cardiac surgery with extracorporeal circulation (ECC) and normovolemic hemodilution (NH) (3–6). According to these reports, ECC resulted in an increase in IAP; this elevation was dependent on the degree of NH, duration of anesthesia, surgery, ECC and aortic clamping. Nevertheless, none of the studies analyzed changes in APP or its effect on the function of extra-abdominal organs. Elevated IAP may result in impaired splanchnic blood flow and directly affects the circulatory, respiratory and, via changes in central venous pressure, central nervous system. Additionally, IAP and APP measurements can be crucial for the diagnosis of postoperative disturbances. However, such effects have not been well-documented in patients undergoing cardiac surgery.

Coronary arterial perfusion pressure (CoPP) is one of the most important measurements obtained for patients undergoing coronary artery bypass grafting (CABG). Traditionally, CoPP is measured using a Swan-Ganz catheter and calculated as the difference between MAP and the pulmonary capillary wedge pressure (PCWP). In addition, several authors have shown the usefulness of Doppler or positron emission tomography examination (7–9). The noninvasive nature of Doppler examination is a great advantage in clinical practice; however, CoPP cannot be accurately defined with this modality. CoPP may also be measured using a special coronary catheter, which is inserted through a systemic artery (10). This is a very accurate method of CoPP measurement, although its invasive character significantly reduces utility. All of the techniques mentioned are based on observations of coronary blood flow and pressure. Moreover, all of them require special equipment, extensive clinical experience and significant financial investment. From a clinical point of view, it is, therefore, important to find an easy, noninvasive method for CoPP measurement. Such a method would be very useful for clinicians who treat or anesthetize patients with serious cardiological disorders. The inexpensive, safe and easy APP measurement described above may be very promising and useful as a marker of CoPP for inexperienced clinicians. However, its clinical utility has not yet been documented. Therefore, the aim of the present study was to identify and analyze any correlations between IAP, APP and CoPP, and to determine the usefulness of these parameters in patients undergoing cardiac surgery.

METHODS

Patients

The study was approved by the Committee of Bioethics of the Medical University of Lublin (Lublin, Poland), and informed consent was obtained from all patients. Patients scheduled for elective CABG due to stable angina pectoris were examined. The exclusion criteria were clinically important mitral valve pathology, severe aortic valve insufficiency (more than first degree), neurological diseases, any chronic respiratory disease, serious endocrine diseases, unstable angina pectoris, history of abdominal surgery, chronic renal failure and a European system for cardiac operative risk evaluation score greater than 8.

One day before surgery, all patients received oral lorazepam 2 mg (Lorafen, Polfa, Poland). One hour before the induction of anesthesia, all patients received intramuscular morphine hydrochloride 0.1 mg/kg (Morphicum hydrochloricum, Polfa, Poland) with midazolam 0.01 mg/kg (Midanium, Polfa, Poland). Anesthesia was induced with fentanyl 0.01 mg/kg to 0.02 mg/kg (Fentanyl, Polfa, Poland), midazolam 0.05 mg/kg to 0.1 mg/kg and etomidate 0.1 mg/kg to 0.5 mg/kg (Hypnomidate, Janssen, Germany). Muscle relaxation was induced by injecting a single dose of pancuronium 0.08 mg/kg to 0.1 mg/kg (Pavulon, Organon-Teknica, The Netherlands). After orthotracheal intubation, mechanical ventilation with a mixture of air and oxygen (60% and 40%, respectively) was provided. All patients were ventilated using intermittent positive pressure ventilation, with monitoring of tidal volume (6 mL/kg to 7 mL/kg) and respiratory rate (9 breaths/min). The parameters were adjusted to achieve normocapnia, controlled by gas analysis. Anesthesia was maintained throughout the procedure using a midazolam-fentanyl infusion and fractionated doses of 0.5% to 1% inhaled forane (Isoflurane, Baxter, USA). After the induction of anesthesia and before surgery, a Swan-Ganz catheter (Arrow, USA) was inserted via the left internal jugular vein. A thermodilution technique using a 10 mL bolus of ice-cold saline was used for cardiac output measurements. In addition, pulmonary and systemic hemodynamic parameters were measured during surgery and the early postoperative period.

Before ECC, heparin (Heparinum sulfuricum, Polfa, Poland) was used at a dose of 3 mg/kg and the activated clotting time was controlled up to 400 s. For ECC, standard cannulation of the ascending aorta and inferior vena cava was performed through the right atrium. During ECC, circulation and ventilation were maintained with the S III heart-lung machine (Stöckert, Germany). The machine priming fluid consisted of 1000 mL of Ringer’s solution (Ringer, Polfa Poland), 500 mL of a 6% solution of hydroxyethylated starch (HAES, Fresenius-Kabi, Germany), 250 mL of 20% mannitol (Fresenius-Kabi, Germany), 20 mL of sodium hydroxycarbonate (Natrium bicarbonatum, Polfa, Poland) and 75 mg of heparinum sulfuricum. Cardiopulmonary bypass was initiated at a pulsatile flow rate of 2.4 L/min/m². After traditional aortic clamping, myocardial viability was preserved with antegrade hyperkalemic blood cardioplegia. During ECC, MAP, hematocrit, gasometric parameters, lactate, and sodium and potassium levels were measured. In all cases, disconnection of the heart-lung machine was uneventful and intra-aortal counterpulsation was not necessary. After the completion of ECC, some patients received an infusion of dopamine (dopamine hydrochloride, Polfa, Poland) or dobutamine hydrochloride (Dobutrex, Hexal, Germany) in doses adjusted according to their clinical condition (3 μg/kg/min to 15 μg/kg/min, or 3 μg/kg/min to 9 μg/kg/min, respectively). The effect of heparin was reversed by an adequate dose of protamine.

After surgery, the patients were moved to the postoperative intensive care unit and ventilated using synchronized intermittent mandatory ventilation with pressure support. Patients were subsequently evaluated for extubation for 6 h after surgery.

After the induction of anesthesia and until the beginning of ECC, 500 mL of gelatin preparation (Gelafundin Braun, Germay) was infused. After ECC, none of the patients required intensive fluid therapy; possible insufficiency of intravascular fluids in the early postoperative period was supplemented with gelatin preparations or electrolyte fluids (PWE, Polfa, Poland and Ringer’s), with hemodynamic and hematological parameters monitored.

According to the World Society of the Abdominal Compartment Syndrome (www.wsacs.org), intermittent IAP measurements were performed in the urinary bladder using a clipped Foley’s catheter through which sterile saline solution was administered earlier (Kron technique) (1,2). The added volume of saline was then subtracted from the urine volume when documenting fluid balance.

The observations were conducted at seven time points (TPs): after the induction of anesthesia and before surgery (TP1), during internal mammary artery preparation (TP2), 10 min after disconnection of the heart-lung machine (TP3), after procedure completion and before moving the patient to the postoperative intensive care unit (TP4), 1 h after surgery (TP5), 6 h after surgery (TP6) and 18 h after the procedure (TP7). TP1 was considered the baseline value.

Means and SDs were calculated, and statistics were analyzed using Wilcoxon’s signed-rank test and the Kruskal-Wallis ANOVA test for initial detection of differences. The Dunnett’s multiple comparison post hoc test and Spearman’s rank correlation tests were used for interpoint and intergroup comparisons. Additionally, the Spearman’s rank correlation test was used for overall analyses. P<0.05 was considered to be statistically significant.

RESULTS

From January 2006 to December 2007, IAP was measured in 200 adult patients. Only 45 patients (seven women and 38 men, mean [± SD] age 64.64±7.58 years) qualified for inclusion in the study because they did not have pulmonary hypertension or mitral or aortic valve pathologies, and underwent elective CABG. The remaining 135 patients were excluded. The mean body mass index of the included subjects was 27.17±4.22 kg/m². In all cases, disconnection of the heart-lung machine was uneventful and intra-aortal counterpulsation was not necessary. After ECC, none of the patients required aggressive fluid therapy. Nine patients did not require catecholamine infusion, 16 received dopamine and 20 received dobutamine infusion.

There were no significant changes in MAP, the mean values of which are presented in Table 1. PCWP increased at TP4 and decreased at TP7. In all patients, baseline IAP increased from TP3 to TP7 (Figure 1). Importantly, all patients with an IAP higher than 12 mmHg received dopamine or dobutamine infusions in doses concordant with their postoperative clinical condition. There were significant correlations between IAP and PCWP at TP1, TP4, TP5, TP6 and TP7 (P=0.0165, r=0.4855; P=0.0455, r=0.2995; P=0.0001, r=0.5449; P=0.0000, r=0.5610; and P=0.0030, r=0.4312, respectively). Moreover, there was a significant overall correlation between IAP and PCWP (P=0.0000, r=0.3865). APP decreased at TP3 and TP4, then returned to levels comparable to baseline (Figure 2). There were no significant changes in CoPP at any of the TPs measured; however, slight declines were observed at TP3 and TP4 (Figure 3).

TABLE 1.

Mean arterial pressure (MAP), pulmonary capillary wedge pressure (PCWP) and intra-abdominal pressure (IAP) values

Parameter, mmHg	TP
	1	2	3	4	5	6	7
IAP	6.64±1.87	6.77±1.75	9.92±2.72	11.65±2.99	11.17±3.81	10.01±4.01	9.08±3.93
MAP	88.05±16.62	90.4±16.37	84.18±11.69	85.98±10.89	88.91±11.40	85.86±11.29	84.68±11.78
PCWP	13.09±2.45	12.84±2.38	14.27±1.98	15.4±1.97	13.57±2.28	12.34±2.05	11.6±2.58

The observations were conducted at seven time points (TP): TP1 After the induction of anesthesia and before surgery; TP2 During internal mammary artery preparation; TP3 10 min after disconnection of the heart-lung machine; TP4 After procedure completion and before moving the patient to the postoperative intensive care unit; TP5 1 h after surgery; TP6 6 h after surgery; and TP7 18 h after the procedure. TP1 was regarded as the baseline value

Figure 1).

Changes in intra-abdominal pressure (IAP) in patients undergoing coronary artery bypass grafting (CABG). *P<0.05, ***P<0.001, compared with baseline values (ie, time point 1)

Figure 2).

Changes in abdominal perfusion pressure (APP) in patients undergoing coronary artery bypass grafting (CABG). *P<0.05 compared with baseline values (ie, time point 1)

Figure 3).

Changes in coronary arterial perfusion pressure (CoPP) in patients undergoing coronary artery bypass grafting (CABG). There were no significant changes throughout the examination period

There was a negative correlation between IAP and CoPP at TP3, TP4, TP5 and TP6 (P=0.0016, r=–0.4548; P=0.0015, r=–0.4591; P=0.0117, r=–0.3724; P=0.0024, r=–0.4410, respectively). APP strongly correlated with CoPP at all consecutive time points (Table 2). Additionally, there was a strong overall correlation between APP and CoPP (P<0.001); r=0.9598) (Figure 4).

TABLE 2.

Correlation between abdominal perfusion pressure (APP), intra-abdominal pressure (IAP) and coronary arterial perfusion pressure (CoPP) at several consecutive time points (TPs)

The observations were conducted at seven TPs: TP1 After the induction of anesthesia and before surgery; TP2 During internal mammary artery preparation; TP3 10 min after disconnection of the heart-lung machine; TP4 After procedure completion and before moving the patient to the postoperative intensive care unit; TP5 1 h after surgery; TP6 h after surgery; and TP7 18 h after the procedure; TP1 was regarded as the baseline value; NS Not statistically significant

Figure 4).

The overall correlation between abdominal perfusion pressure (APP) and coronary arterial perfusion pressure (CoPP) in patients undergoing coronary artery bypass grafting

DISCUSSION

The present study documented a strong overall correlation between APP and CoPP. Likewise, changes in APP correlated with CoPP at all consecutive TPs. Moreover, there were significant correlations between changes in IAP and CoPP, although these relations were not observed at all TPs. Nevertheless, these observations confirm that APP is a good marker of changes in CoPP.

Changes in APP have not been described in patients undergoing CABG. In the present study, APP strongly depended on changes in MAP and IAP. The effects of ECC on IAP levels were described previously (3–6). The analysis of changes in IAP according to the degree of NH showed that blood dilution had a significant impact on IAP levels. In addition, the volume of the priming solution in the heart-lung machine caused blood dilution, with the extent of dilution being dependent on the patient’s body weight. Therefore, markedly higher increases in IAP were noted in patients with lower body weight, as well as a higher degree of blood dilution (3). Moreover, the initiation of ECC results in decreased colloid osmotic pressure, which leads to an increase in microvascular permeability (11,12). Intestinal edema subsequently develops, which results in IAP elevation (13,14). Theoretically, post-ECC tissue edema may result from increased microvascular permeability (permeability edema), decreased lymph outflow (lymph edema), decreased colloid osmotic pressure (hypoproteinemic edema) and increased capillary pressure (stasis edema) (11,12). In the present study, the increase in IAP occurred immediately following surgery and this elevation persisted to the end of the study. Importantly, an increase in IAP was probably the main cause of the decrease in APP (APP significantly decreased at TP3 and TP4, while MAP did not change). Therefore, it can be assumed that lower values of APP resulted from IAP elevation immediately after ECC and surgery.

The Kron technique for IAP measurement is easy, relatively safe and slightly invasive. According to World Society of the Abdominal Compartment Syndrome protocol, 25 mL of sterile solution (eg, 0.9% NaCl) should be injected into the urinary bladder (1,2). Moreover, IAP should be expressed in mmHg and measured in the supine position during end expiration. The zero point should be at the level of the midaxillary line. Thus, the measurement of IAP in unconscious or sedated patients is not a complicated process.

There are many side effects associated with IAP elevation. In the present study, the cardiovascular disorders seemed to be the most important. An IAH higher than 20 mmHg decreased cardiac output, preload and ventricular end-diastolic volume (15,16). Moreover, IAH caused elevation of the diaphragm, which led to increased intrathoracic pressure, resulting in significantly increased central venous pressure, pulmonary blood pressure, pulmonary vascular resistance, right atrial pressure and PCWP. Animal studies (17) showed that 20% of IAH (higher than 20 mmHg) was transmitted to the chest cavity from upward bulging of the diaphragm. Additionally, an elevation in systemic vascular resistance, which results from arteriolar vasoconstriction caused by IAH, reduced stroke volume (17,18). These changes were particularly intense during hypovolemia. In the present study, the highest values of IAP were noted from TP3 to TP7 and extreme values ranged from 19 mmHg to 21 mmHg. There were no serious cardiovascular complications, although patients with an IAP higher than 12 mmHg required dopamine or dobutamine infusion.

Several studies have documented the advantages resulting from APP measurements (1,19–21). Elevated splanchnic perfusion pressure was a good predictor of patient outcome after gastroschisis repair (20). Moreover, increases in APP of greater than 50 mmHg optimized patient survival after major surgery or trauma (20,21). Additionally, APP monitoring was a strong predictive factor for the diagnosis of multiorgan dysfunction as well as mortality in severely ill patients (20,22,23). In the present study, APP caused changes in CoPP during and after CABG – a strong correlation between APP and CoPP was the most significant finding. Such correlations were noted in pairwise comparisons of consecutive TPs as well as in the overall analysis, and could have resulted from the influence of IAP elevation on changes in PCWP. Importantly, there were significant correlations between these parameters, but only in patients with closed chests. The opening of the chest significantly disturbed such correlations. Therefore, it seems that strong correlations between APP and CoPP result mainly from a correlation between IAP and PCWP.

Limitations

The exclusion criteria for the present study were strict – no patients had mitral or aortic valve disorders, or pulmonary vascular disease. The postoperative period for each individual was without serious hemodynamic complications. Moreover, the present study presented only general disruptions in CoPP; disturbances in each coronary artery’s blood flow were not evaluated. Additionally, CoPP and APP were noted only in uncomplicated cardiac surgery patients.

Mathematically, CoPP and APP are related to changes in MAP as well as PCWP and IAP (ie, CoPP = MAP – PCWP and APP = MAP – IAP). In our study, we showed a significant correlation between CoPP and APP as well as between IAP and PCWP. From a mathematical point of view, it can be concluded that APP may be a predictor of CoPP; however, the correlation between APP and CoPP in different clinical disorders requires further research, particularly in patients who have not undergone noncardiac surgery and are critically ill.

CONCLUSION

APP strongly correlated with CoPP. Moreover, ECC results in IAP elevation, which leads to a decrease in APP. IAP measurement is, therefore, useful for the diagnosis of CoPP disturbances in patients undergoing CABG.