ORIGINAL ARTICLE

TITLE: “THE EFFECT OF POSITIVE END EXPIRATORY PRESSURE (PEEP) ON

CENTRAL VENOUS PRESSURE (CVP) IN POSTSURGICAL MECHANICALLY
VENTILATED PATIENTS”.

AUTHORS: S. Moied Ahmed, Rahul Gupta, Abu Nadeem, Sabihul Islam, Shiwani Agarwal,
Asif Siddiqui

NAME QUALIFICATION DEISGNATION

Dr S Moied Ahmed MD (Anaesth), Associate Professor (Anaesth)
Fellow in Critical Care Medicine
FCCP, FIMSA
Dr Rahul Gupta MD (Anaesthesia) Senior Resident (Anaesth)
Dr Abu Nadeem MD (Anaesthesia) Assistant Professor (Anaesth)
Dr Sabihul Islam MBBS Junior Resident (Anaesth)
Dr Shiwani Agarwal MBBS Junior Resident (Anaesth)
Dr Asif Siddiqui MBBS Junior Resident ( Anaesth)
Affiliation: Department of Anaesthesiology, JN Medical College, AMU, Aligarh, India
Research credited to: Intensive Care Unit, Department of Anaesthesiology, JN Medical
College, AMU, Aligarh, India
Number of Pages: 14
Number of words: Abstract – 291, Full Text – 1664, Introduction – 276, Discussion – 556

Address for Correspondence:

Department of Anaesthesiology,
JN Medical College,
AMU, Aligarh, India
Email: sma99@rediffmail.com

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Abstract:
Positive end expiratory pressure (PEEP) is usually applied during mechanical ventilation to
improve oxygenation. Administration of PEEP would raise the intra-thoracic pressure and
influence the central venous pressure (CVP), which is a thoracic structure. A prospective study
was conducted to observe the effect of PEEP on CVP, on thirty adult post operative patients
undergoing emergency laparotomy, requiring elective mechanical ventilation. The patients in
whom application of PEEP was contraindicated were not included in the study. All the patients were
paralysed with vecuronium bromide and sedation and analgesia was obtained with propofol and
fentanyl respectively. The variables CVP, Mean Arterial Blood Pressure (MABP), arterial
oxygen saturation (SpO2), Static lung compliance (Cst) and Mean airway pressure (Map) were
obtained at zero PEEP. The PEEP was subsequently increased to 2 cm, 4 cm, 6 cm, 8 cm, 10 cm
of H2O and the respective values of the variables were observed after 10 minutes in each level of
PEEP. The CVP gradually increased from 8.3 ± 0.3 to 12.3 ± 1.4 cm of H2O (48.2%) with the
increase in PEEP from 0 to 10 cmH2O. The rise in CVP was statistically significant (p < 0.05)
from 4 cmH2O PEEP onwards. PEEP showed a significant linear correlation (Pearson
correlation coefficient, R = 0.957, R2 = 0.917). The linear regression equation was Y (CVP) =
0.402x (PEEP) + 0.732, (p < 0.05). The MABP and SpO2 did not increase significantly with
increase in PEEP. However, the Map and Cst increased significantly (p < 0.05). We therefore
conclude that CVP increases with PEEP and the increase was statistically significant after 4 cm
H2O PEEP. There was a linear and positive correlation between PEEP and CVP. The
compliance of the lung and the intra-abdominal pressure probably influenced this relationship.

Key words: CVP, PEEP

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INTRODUCTION

Central Venous Pressure (CVP) measurement is one of the essential and commonly used
hemodynamic monitor in critically ill patients 1. It measures the pressure at right atrium, reflects
the preload of the right ventricle and ultimately the overall output of the heart 2. It therefore
indicates fluid or volume status in any patient and guide appropriate therapy 3, 4. However, there
are various factors, for example, zeroing, levelling, transmural pressure, cardiac function etc, that
can influence the CVP and lead to misinterpretation of the actual value5.
Positive end expiratory pressure (PEEP) is applied during controlled mechanical ventilation to
treat refractory hypoxemia6. The pressure is transmitted to the intra-thoracic structure according
to the compliance of the lung. Accordingly, the central veins being a thoracic structure can be
influenced by PEEP. This would inadvertently cause a false higher CVP reading2.
Patroniti, et al7 observed that with increase in PEEP there was a significant decrease in cardiac
function and an increase in CVP. The increase in CVP may be erroneously interpreted as
improvement in cardiac function. Therefore to avoid such misinterpretation it is advisable to
measure the CVP only when the PEEP has been removed. However, by doing so, the beneficial
effect of PEEP on gas exchange would be lost leading to increased chances of hypoxemia5.
There are various studies that have observed that increasing level of PEEP also increases the
CVP7, 8, 9, 10, 11. However, controversy exists on the levels at which it raise and the rate at which it
rises ( ). Hence the aim of the present study was to observe the level of PEEP at which the CVP
starts rising and the correlation of PEEP with CVP

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MATERIALS AND METHODS

A prospective observational study was conducted in the Intensive Care Unit of the department of
Anaesthesiology. The study was approved by the departmental board of studies. Thirty adult
patients (age ranging between 20 – 50 years) of either sex admitted to the ICU for mechanical
ventilation were selected. Written informed consent was obtained from the patients relatives
before the commencement of the study.
Patient with major cardiac disorder, respiratory disorder, pregnancy, post thoracic surgery and
with septic shock or in patients in whom application of PEEP was contraindicated were not included in
the study.

Before the commencement of the study all the patients were relaxed with a bolus dose of
0.1mg/kg of vecuronium bromide, sedated with a bolus dose of 2.0 mg/kg of propofol followed
by infusion at the rate of 2.0 mg/kg/hour throughout the observation period. Analgesia was
obtained with a bolus dose of fentanyl of 2mcg/kg followed by infusion of 1-2 mcg/Kg/hr.
Patients were infused with ringer’s lactate at the rate of 2 ml/kg/hour throughout the study. No
colloids and blood were transfused to the patient during the study period. All the observations
were done in supine position.

All the patients with normal CVP (6-8 cm of H 2O) were mechanically ventilated on VELA
VIASYS T BIRD SERIES in controlled mode, with a respiratory rate of 12 breaths per minute,
tidal volume of 8 ml per kg and inspiratory to expiratory ratio of 1 : 2.5 and a plateau pressure
of < 30 cm of H2O. The patients were ventilated with these ventilatory settings for at least 30
minutes before the initiation of observation.

At zero PEEP the CVP, Mean Arterial Blood Pressure (MABP) and SpO 2 (arterial oxygen
saturation) Cst (Static lung compliance) and Mean airway pressure (Map) were recorded
followed by successive observations at 10 minutes (S. Masayuki, O. Masaharu, I. Osamu, et al,
2004)10. Observations at zero PEEP were taken as baseline values. Subsequently the PEEP was

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increased to 2 cm, 4 cm, 6 cm, 8 cm, 10 cm of H 2O and the respective values of above
mentioned variables were observed after 10 minutes in each level of PEEP.

The gradual increase in PEEP was stopped when -

1. The BP fell by greater than 20% of baseline values OR

2. SpO2 fell below 90% OR
3. CVP increased to > 20 cm of H2O, whichever was earlier and the case was
excluded from the study and a new case was included in its place.
CVP, MABP and SpO2 measurements were done through the multichannel monitor DATEX
OHMEDA MULTICHANNEL MONITOR S/5.

All the statistical analysis was done using SPSS version 17.
Pearson correlation test
It was used to calculate relationship between the variables. Pearson correlation coefficient is a
measure of linear association between two variables. The values of Pearson correlation
coefficient range from -1 to +1. The sign of the correlation coefficient indicates the direction of
the relationship (positive or negative). The absolute value of the correlation coefficient indicates
the strength, with larger absolute values indicating stronger relationships. The significance of
each correlation coefficient is also displayed in the correlation table. The significance level (or p-
value) is the probability of obtaining results as extreme as the one observed. It is considered to be
significant when it is < 0.05.

Paired samples T test

Paired‘t’ test was used to compare two variables that represent the same group at different times
(e.g. before and after an event) or related groups. All through the study, a ‘p’ value of < 0.05 was
taken as indicative of statistical significant.

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OBSERVATIONS AND RESULTS

The demographic profile of the patients is shown in Table – 1.

Table 1: Demographic profile of the patients

No. of patients Percentage
Sex
Male 22 73 %
Female 8 27 %
Age
20 - 30 yrs. 12 40 %
31- 40 yrs. 11 36.7 %
41- 50 yrs. 7 23.3 %
Indication
Perforation peritonitis 18 60 %
Acute intestinal obstruction 6 20 %
Blunt trauma abdomen 3 10 %
Gunshot abdomen 2 6.7 %
Stab injury abdomen 1 3.3 %

Table 2: Mean ± SD values of CVP at 10 minutes of observation against the

increasing values of PEEP.
PEEP N Mean value of CVP Std.
(cm of H2O) (cm of H2O) deviation

STARTING AFTER 10 p-
POINT MIN. VALUE

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0 30 8.3 8.4 0.3

2 30 8.4 8.5 0.11 0.4

4 30 8.5 8.9 0.08 0.4

6 30 8.9 9.8 0.03 1.6

8 30 9.8 11.1 0.01 1.6

10 30 11.1 12.3 0.001 1.4

 N = total no. of patients observed against the each values of PEEP.

 p- value is significant at < 0.05 level.
 n = total no. of observations observed against the each values of PEEP.

Table – 2 shows the mean values of CVP after 10 minutes of application of PEEP. The Baseline
value of CVP was 8.3 ± 0.3 cm of H2O after 10 minutes of observation. Subsequent observations
were calculated against the increasing values of PEEP. The mean value of CVP was found to be
9.9 cm ± 2.1 cm of H2O. The p-value was generated using paired‘t’ test. The p- value was
calculated between the baseline value of CVP and the different values of CVP observed against
the increasing values of PEEP. The mean values of CVP rose significantly from 4 cmH2O of
PEEP and thereafter compared to its baseline value (p < 0.05).

There was an increase in CVP by 48.2% from 0 to 10 cm H2O PEEP with a maximum CVP of
12.3 ± 1.4 cm of H2O at 10 cm H2O PEEP (p < 0.001). The rate of rise in CVP was regular from
4 cmH2O PEEP onwards (Figure – 1).

Table – 3: Correlation between CVP at 10 minutes and PEEP.

CVP PEEP

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CVP Pearson correlation coefficient (r) 1 0.957

Significance (2 tailed) . 0.003

N 6

PEEP Pearson correlation coefficient (r) 0.957 1

Significance (2 tailed) 0.003 .

N 6

 p- value is significant at < 0.05 level.

 N = total no. of observations of CVP for 30 patients observed against different values of
PEEP at 10 minutes.

Table – 3 demonstrates the Pearson correlation test between PEEP and CVP. PEEP showed a
significant linear correlation (Pearson correlation coefficient R = 0.957. R 2 = 0.917). The
relationship between the PEEP and CVP was found to be highly significant (p<0.05) from 4 cm
of H2O PEEP onwards. The linear regression equation was Y (CVP) = 0.402x (PEEP) + 0.732,
(p < 0.05)

Table – 4: Changes in MABP, SpO2, Map, Cst after administration of PEEP

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Parameters At 0 cm H2O PEEP At 10 cm H2O PEEP.

MABP 72.7 ± 5.6 mmHg 70.5 ± 3.4 mmHg

SpO2 95.7 ± 2.1 % 96.1 ± 1.9 %

Map 8.1 ± 2.9 cm of H2O 13.6 ± 2.7 cm of H2O*

Cst 49.8 ± 9.5 mL/cm of H2O 56.2 ±10.3 mL/cm of

H2O*
* p < 0.05 – the change in Map and Cst was statistically significant

There was no statistically significant change in MABP and SpO2 with increase in PEEP.
However, the Map and Cst increased significantly (p < 0.05)

Figure 1: Figure showing the relationship of mean values of CVP at 10 minutes of

observation against the increasing values of PEEP.

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Figure – 2 showing correlation of CVP with PEEP

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11
DISCUSSION

The present study was conducted in 30 adult patients of either sex undergoing emergency
abdominal surgery, requiring postoperative elective mechanical ventilation. PEEP was applied
during mechanical ventilation, which was progressively increased from 0 to 10 cmH2O. The
effect of application of PEEP on CVP was observed in these patients. The mean CVP levels
started rising significantly from 4 cmH2O of PEEP. The rate of rise was linear and the
correlation was positive (Pearson correlation coefficient = 0.957). This observation was in
accordance to the previous authors 7-11.
Usually, during controlled mechanical ventilation, application of PEEP increases the intra-
thoracic pressure. In normal compliant lung approximately half of the PEEP is transmitted to the
pleural space and increase the pleural pressure. Since CVP is a thoracic structure application of
PEEP should increase the CVP depending upon the compliance of the lung. Accordingly
application of 10 cmH2O of PEEP should raise the CVP roughly by 4 to 5 cmH2O in a
compliant lung2, 5. In our series the CVP rose from 8.3 ± 0.3 to 12.3 ± 1.4 cm of H2O after
application of 10 cmH2O of PEEP. The rise in CVP was approximately 4 to 5 cmH2O from the
baseline. This was in accordance with the opinion of the previous authors2, 5 (Table – 4).
However, the pressure transmitted to the pleural space is much less in patients with stiff or non
compliant lung. Further, in such patients there would be no significant rise in CVP till a PEEP of
7.5 to 10 cmH2O5, 7, 8. In the present study the CVP started rising significantly (p < 0.05) after 4
cmH2O PEEP (Table – 2). This was contrary to the opinion of the previous authors. The early
effect of PEEP on CVP in our study could be primarily due to two reasons. First, it could be due
to the normal compliance of the lung in our patients (Table – 4). Second, all the patients in our
study underwent abdominal surgery. This probably lead to increase in intra-abdominal pressure,
diaphragmatic stenting, and decrease in diaphragm compliance12, 13, 14. All these effects ultimately
lead to increase in intrathoracic pressure. The application of PEEP along with the increase in
intrathoracic pressure probably influenced our results. However, Nunn15 was of the opinion that
even in presence of normal lung compliance, PEEP > 10 cm of H 2O makes measurement of
pulmonary capillary wedge pressure (another measure of central blood volume like CVP)
unreliable.

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Usually most study has some element of bias. Our study probably was free from bias since all the
data were recorded from the ventilator and multi channel monitor of same make. They were
recorded by the same observer. Further, all the data were recorded after 10 minutes when the
effect of the PEEP was stabilized. Hence there was no scope of observer variation. However, our
study could not be double blinded since the data was obvious on the ventilator and monitor
screen. This was one of the limitations of the study.
We therefore conclude that CVP increases with PEEP. However, the increase is statistically
significant after 4 cm H2O PEEP. Further, there is a linear and positive correlation between
PEEP and CVP. The compliance of the lung and the intra-abdominal pressure can influence this
relationship. However, a further study should be conducted to measure the individual effect of
intra-abdominal pressure and lung compliance on CVP.

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 References:

1. Boldt J, Lenz M, Kumle B, Papsdorf M. Volume replacement strategies on intensive care

units: results from a postal survey. Int Care Med 1998; 24:147–151
2. Magder S. How to use central venous pressure measurements? Curr Opin Crit Care 2005;
11:264–270
3. Michard F, Teboul JL. Predicting fluid response in ICU patients: a critical analysis of the
evidence. Chest 2002; 121:2000—2008.
4. Michard F, Martin GS, Ely EW. Underutilized tools for the assessment of intravascular
volume status. Chest 2003; 124:414—416.
5. Magder S. Central venous pressure monitoring. Current Opinion in Critical Care 2006,
12:000–000
6. Levy MM. PEEP in ARDS — How Much Is Enough? N Engl J Med 2004; 351:389-391
7. Patroniti N, Manfio A, Cortinovis B, Maggioni E, Bellani G, Sala F, Consonni F and
Pesenti A. Effects of PEEP on intrathoracic blood volumes and cardiac function in ARDS
patients assessed by a double indicator dilution technique. Critical Care, 2003, 7 (supple
2): P 168.
8. Al-Shanableh. The correlation between PEEP and CVP. The Middle East Journal of
Anesthesiology, 1985 Feb; 8 (1): 65-9.
9. Sakai T, Takaori M, Fukui A, Endoh E, Kimura Kl. The effect of IPPV, with or without
PEEP, on central venous pressure. Japanese Journal of anesthesiology, 1989; 38 (6): 730-
35.
10. Masayuki Santo, M.D., Masaharu Okahara, M.D., M.S., Osamu Iwata, M.D., Tomohiro
Sawa, M.D., Ph.D., Yoshinori Nakata. Peripheral venous pressure during intermittent
positive pressure ventilation with or without positive end expiratory pressure.
Anesthesiology, 2004; 101: A560.
11. Jellinek H, Krenn H, Oczenski W, et al. Influence of positive airway pressure on the
pressure gradient for venous return in humans. J Appl Physiol 2000; 88:926–32.

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12. Chang MC, Miller PR, D'Agostino R Jr, Meredith JW: Effects of abdominal
decompression on cardiopulmonary function and visceral perfusion in patients with intra-
abdominal hypertension. J Trauma 1998; 44: 440-5.

13. Dabrowski W. Changes in intra-abdominal pressure and central venous and brain venous
blood pressure in patients during extracorporeal circulation. Med Sci Monit. 2007
Dec;13(12):CR548-54.

14. Bloomfield GL, Ridings PC, Blocher CR, Marmarou A, Sugerman HJ. A proposed
relationship between increased intra-abdominal, intrathoracic, and intracranial pressure.
Crit Care Med. 1997 Mar;25(3):496-503

15. Nunn. Positive end expiratory pressure. Anesthesiology clinics, 1984; 22: 149.

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