Sensitivity of a disposable end-tidal carbon dioxide detector

A small disposable carbon dioxide detector that can be used to provide evidence of correct endotracheal tube placement is now commercially available (FEF). The device contains an indicator that changes color when exposed to carbon dioxide. This study measured the lowest concentration of carbon dioxide causing a perceivable color change in the device. Ten volunteers were blinded to the concentrations of carbon dioxide in an airway circuit/lung model, and the minimal concentration of carbon dioxide that caused a perceivable color change was recorded. The mean minimum concentration required for detection of a color change was 0.54% (4.1 mm Hg) and ranged from 0.25 to 0.60% (1.9 to 4.6 mm Hg). We conclude that this device should produce a detectable color change even in patients with low end-tidal carbon dioxide, as might be observed during cardiopulmonary resuscitation.Presented in part at the annual meeting of the American Society of Anesthesiologists, New Orleans, October 1989.     

End-tidal carbon dioxide as a measure of arterial carbon dioxide during intermittent mandatory ventilation

To determine if end-tidal carbon dioxide tension (PetCO₂) is a clinically reliable indicator of arterial carbon dioxide tension (PaCO₂) under conditions of heterogeneous tidal volumes and ventilation-perfusion inequality, we examined the expiratory gases of 25 postcardiotomy patients being weaned from ventilator support with intermittent mandatory ventilation. Using a computerized system that automatically sampled airway flow, pressure, and expired carbon dioxide tension, we were able to distinguish spontaneous ventilatory efforts from mechanical ventilatory efforts. ThePetCO₂ values varied widely from breath to breath, and the arterial to end-tidal carbon dioxide tension gradient was appreciably altered during the course of several hours. About two-thirds of the time, thePetCO₂ of spontaneous breaths was greater than that of ventilator breaths during the same 70-second sample period. The most accurate indicator of PaCO₂ was the maximalPetCO₂ value in each sample period, the correlation coefficient being 0.768 (P < 0.001) and the arterial to end-tidal gradient being 4.24 ± 4.42 mm Hg (P < 0.01 compared with all other measures). When all values from an 8-minute period were averaged, stability was significantly improved without sacrificing accuracy. We conclude that monitoring the maximalPetCO₂, independent of breathing pattern, provides a clinically useful indicator of PaCO₂ in postcardiotomy patients receiving intermittent mandatory ventilation.     

A carbon dioxide monitor that does not show the waveform is worthless

The author suggests that the carbon dioxide waveform should be displayed, as are the electrocardiogram and arterial pressure waveforms. He argues that a carbon dioxide analyzer that does not provide a waveform is not of value, as subtle changes in the carbon dioxide waveform can reflect impending problems. Only when a plateau is present in the capnogram can one be certain that end-tidal gas is being measured, and the author asserts that the presence or absence of this plateau can be detected only by visually inspecting the waveform.     

Reduction of fresh gas flow requirements by a circle-modified Bain breathing circuit

We modified a Bain circuit by placing the circuit into the Y piece of a standard carbon dioxide absorber circle, connecting the fresh gas hose on the anesthetic machine to the Bain's fresh gas inlet, and occluding the circle's fresh gas inlet. This circle-modified Bain breathing circuit was studied to evaluate whether it reduces fresh gas flow requirements. The Bain and modified Bain steady states were analyzed by mechanical and computer modeling. The mechanical model consisted of an artificial lung ventilated to steady state. Carbon dioxide was measured with capnography. Computer modeling was by compartmental analysis calculated with spreadsheet software. Steady-state solutions were obtained by numeric analysis. The circle-modified Bain greatly reduced retention of carbon dioxide. For example, with 1-liter tidal volumes, 10-liter minute volumes (10 breaths per minute), and a 2.1 L/min fresh gas flow, the steady-state end-tidal carbon dioxide values of the Bain and modified Bain were 9.3 and 4.6%, respectively, in the physical model (carbon dioxide inflow of 230 ml/min). Results from the mechanical model helped validate the computer model.Presented at the University of Nebraska Medical Center Student Research Forum, Omaha, NE, November 1988, and at the University of Wisconsin Department of Anesthesiology Peer Review-Quality Assurance Clinical Case Conference, Madison, WI, February 1989.     

Accuracy of expiratory carbon dioxide measurements using the coaxial and circle breathing circuits in small subjects

Mass spcctrometry is widely used to measure the end-tidal concentrations of inhalation anesthetics and other gases during surgery in order to estimate their arterial concentrations. When certain breathing circuits are used in newborns, however, fresh gas or ambient air may contaminate the expired sample, introducing a systematic error in the measurement of any end-tidal gas concentration. We estimated this error in newborn piglets using carbon dioxide as an indicator substance of expired gas. The capnograms and the difference between arterial carbon dioxide tension (PaCO₂) and peakexpired carbon dioxide tension (PeCO₂) were compared when either a coaxial (Bain) or circle breathing circuit was used. Gas was sampled from the proximal airway and distal trachea. No combination of circuit and sampling site produced a flat alveolar phase until the circle circuit was modified with diversion valves to reduce gas mixing. The mean PaCO₂-PeCO₂ gradients using the coaxial/proximal sampling, coaxial/distal sampling, and modified circle/proximal sampling circuits were 12.4, 9.2, and 8.8 mm Hg, respectively. The mean PeCO₂ in each of these combinations was significantly different from the corresponding mean PaCO₂ (p<0.05). Using the modified circle circuit with distal sampling, mean PeCO₂ was not significantly different from mean PaCO₂: the mean PaCO₂-PcCO₂ gradient was 2.2 ± 0.2 mm Hg (SEM), range, 0 to 6 mm Hg, with 95% confidence limits ⩽ 8 mm Hg. When a coaxial breathing circuit is used in small subjects, PaCO₂ may be significantly underestimated regardless of sampling site, although the circle breathing circuit with distal tracheal sampling yields accurate results. Supported in part by BRS Grant SO RR05507-20 from the Biomedical Research Support Grant Program, Division of Research Resources, National Institutes of Health, and by the American Heart Association, Lancaster, PA Chapter. The authors thank Robert Hirsch, PhD, for his statistical advice, and Greg Harris and Perkin-Elmer, Inc for loaning the mass spectrometer.     

Effects of reduced rebreathing time, in spontaneously breathing patients, on respiratory effort and accuracy in cardiac output measurement when using a partial carbon dioxide rebreathing technique: a prospective observational study

Introduction

New technology using partial carbon dioxide rebreathing has been developed to measure cardiac output. Because rebreathing increases respiratory effort, we investigated whether a newly developed system with 35 s rebreathing causes a lesser increase in respiratory effort under partial ventilatory support than does the conventional system with 50 s rebreathing. We also investigated whether the shorter rebreathing period affects the accuracy of cardiac output measurement.

Method

Once a total of 13 consecutive post-cardiac-surgery patients had recovered spontaneous breathing under pressure support ventilation, we applied a partial carbon dioxide rebreathing technique with rebreathing of 35 s and 50 s in a random order. We measured minute ventilation, and arterial and mixed venous carbon dioxide tension at the end of the normal breathing period and at the end of the rebreathing periods. We then measured cardiac output using the partial carbon dioxide rebreathing technique with the two rebreathing periods and using thermodilution.

Results

With both rebreathing systems, minute ventilation increased during rebreathing, as did arterial and mixed venous carbon dioxide tensions. The increases in minute ventilation and arterial carbon dioxide tension were less with 35 s rebreathing than with 50 s rebreathing. The cardiac output measures with both systems correlated acceptably with values obtained with thermodilution.

Conclusion

When patients breathe spontaneously the partial carbon dioxide rebreathing technique increases minute ventilation and arterial carbon dioxide tension, but the effect is less with a shorter rebreathing period. The 35 s rebreathing period yielded cardiac output measurements similar in accuracy to those with 50 s rebreathing.     

On-line computer estimation of carbon dioxide response curves

Anesthesiologists are concerned with the effect of various anesthetics on a patient's central nervous ventilatory control. The most widely accepted method of determining the effect of a drug is to compare carbon dioxide response curves ( e/PetCO₂, where e = minute ventilation [in L/min] andPetCO₂ = end-tidal carbon dioxide [in mm Hg]) measured before and after administration of the drug. Additional information concerning neuromechanical control can be obtained by also including a measure of the airway occlusion pressure (generally measured 100 ms after occlusion, i.e., P₁₀₀).To facilitate these measurements we have developed a portable, computer-controlled data acquisition system. It includes an Apple II+ computer and measures e,PetCO₂, and P₁₀₀. Each subject rebreathes exhaled carbon dioxide through a two-way breathing valve attached to a 9-liter reservoir, which is initially filled with 5% carbon dioxide and balance oxygen. Exhaled carbon dioxide concentrations are measured with an infrared medical gas analyzer on samples taken through a catheter connected at the mouthpiece. The exhaled flow is measured with a pneumotachograph in conjunction with a differential pressure transducer, and P₁₀₀ is determined with a Validyne MP45 pressure transducer.     

Relationship between arterial carbon dioxide and end-tidal carbon dioxide when a nasal sampling port is used

End-tidal carbon dioxide (ETCO₂) values obtained from awake nonintubated patients may prove to be useful in estimating a patient’s ventilatory status. This study examined the relationship between arterial carbon dioxide tension (PaCO₂) and ETCO₂ during the preoperative period in 20 premedicated patients undergoing various surgical procedures. ETCO₂ was sampled from a 16-gauge intravenous catheter pierced through one of the two nasal oxygen prongs and measured at various oxygen flow rates (2, 4, and 6 L/min) by an on-line ETCO₂ monitor with analog display. Both peak and time-averaged values for ETCO₂ were recorded. The results showed that the peak ETCO₂ values (mean = 38.8 mm Hg) correlated more closely with the PaCO₂ values (mean = 38.8 mm Hg; correlation coefficient r = 0.76) than did the average ETCO₂ values irrespective of the oxygen flow rates. The time-averaged PaCO₂-ETCO₂ difference was significantly greater than the PaCO₂-peak ETCO₂ difference (P < 0.001). Values for subgroups within the patient population were also analyzed, and it was shown that patients with minute respiratory rates greater than 20 but less than 30 and patients age 65 years or older did not differ from the overall studied patient population with regard to PaCO₂-ETCO₂ difference. A small subset of patients with respiratory rates of 30/ min or greater (n = 30) did show a significant increase in the PaCO₂-ETCO₂ difference (P < 0.001). It was concluded that under the conditions of this study, peak ETCO₂ values did correlate with PaCO₂ values and were not significantly affected by oxygen flow rate. However, obtaining peak ETCO₂ values is clinically more difficult, especially when partial air-way obstruction is present.     

Measurement of end-tidal carbon dioxide in spontaneously breathing patients in the pre-hospital setting. A prospective evaluation of 350 patients

OBJECTIVE: Monitoring of end-tidal carbon dioxide (EtCO(2)) is good clinical practice in the patient who is intubated and ventilated. This study investigated the EtCO(2) values in spontaneously breathing patients treated in a physician-staffed mobile intensive care unit (MICU). This article also discusses whether EtCO(2) monitoring may have an influence on therapeutic decisions by emergency physicians by providing additional information. METHODS: Over a period of 6 months, 350 spontaneously breathing patients (162 males, 137 females) were treated and transported in our MICU and monitored using a LifePak 12 monitor (EtCO(2), respiratory rate, pO(2), blood pressure, heart rate). Only 299 were enrolled in the study. RESULTS: Pathological EtCO(2) values were detected in 19 patients (6.3%). EtCO(2) levels of >55 mmHg (7.3 kPa) were found in nine of 12 (75%) patients with asthma, in one of 23 patients with hypoglycaemia (4.3%), and in all patients with subarachnoid hemorrhage, acute seizures and drug intoxications. With the exception of the asthma patients, all patients had an initial Glasgow Coma Score <8. EtCO(2) levels <20 mmHg (2.7 kPa) were found in all patients with hyperventilation or shock due to volume deficiency. Errors in EtCO(2) measurement occurred in 5% of cases. CONCLUSION: Although EtCO(2) monitoring may be a useful additional variable in spontaneously breathing patients. Consideration of the respective disease and the cost to benefit ratio suggests that this method should only be used for selected indications.     

Stroke volumes and end-tidal carbon dioxide generated by precordial compression during ventricular fibrillation

OBJECTIVE: The objective of this study was to measure stroke volumes produced by precordial compression during cardiopulmonary resuscitation and to quantitate relationships of stroke volume to measurements of end-tidal carbon dioxide. DESIGN: A prospective, observational animal study. SETTING: Medical research laboratory in a university-affiliated research and educational foundation. SUBJECTS: Domestic pigs. INTERVENTIONS: Eighteen anesthetized male, domestic pigs weighing between 40 and 45 kg were investigated. Ventricular fibrillation was electrically induced and continued for intervals ranging from 4 to 10 mins. Precordial compression was maintained at 80 per minute together with mechanical ventilation after endotracheal intubation. MEASUREMENTS AND MAIN RESULTS: Stroke volumes were measured with the aid of transesophageal echocardiographic imaging. End-tidal carbon dioxide was quantitated with conventional capnography. Baseline values of thermodilution cardiac output were highly correlated with echocardiographic measurements (r =.92). The stroke volume index produced by precordial compression averaged 0.45 mL/kg or approximately 37% of the average prearrest value of 1.22 mL/kg. The end-tidal carbon dioxide was highly predictive of stroke volume index (r =.88, p <.001) with a mean bias of 0.003 mL/kg. CONCLUSIONS: We confirmed that precordial compression produces approximately one third of prearrest stroke volumes during cardiopulmonary resuscitation and demonstrated that end-tidal carbon dioxide was quantitatively predictive of stroke volume index estimated by transesophageal echocardiographic imaging.     

Resting fluctuations in arterial carbon dioxide induce significant low frequency variations in BOLD signal

Carbon dioxide is a potent cerebral vasodilator. We have identified a significant source of low-frequency variation in blood oxygen level-dependent (BOLD) magnetic resonance imaging (MRI) signal at 3 T arising from spontaneous fluctuations in arterial carbon dioxide level in volunteers at rest. Fluctuations in the partial pressure of end-tidal carbon dioxide (Pet(CO(2))) of +/-1.1 mm Hg in the frequency range 0-0.05 Hz were observed in a cohort of nine volunteers. Correlating with these fluctuations were significant generalized grey and white matter BOLD signal fluctuations. We observed a mean (+/-standard error) regression coefficient across the group of 0.110 +/- 0.033% BOLD signal change per mm Hg CO(2) for grey matter and 0.049 +/- 0.022% per mm Hg in white matter. Pet(CO(2))-related BOLD signal fluctuations showed regional differences across the grey matter, suggesting variability of the responsiveness to carbon dioxide at rest. Functional magnetic resonance imaging (fMRI) results were corroborated by transcranial Doppler (TCD) ultrasound measurements of the middle cerebral artery (MCA) blood velocity in a cohort of four volunteers. Significant Pet(CO(2))-correlated fluctuations in MCA blood velocity were observed with a lag of 6.3 +/- 1.2 s (mean +/- standard error) with respect to Pet(CO(2)) changes. This haemodynamic lag was adopted in the analysis of the BOLD signal. Doppler ultrasound suggests that a component of low-frequency BOLD signal fluctuations is mediated by CO(2)-induced changes in cerebral blood flow (CBF). These fluctuations are a source of physiological noise and a potentially important confounding factor in fMRI paradigms that modify breathing. However, they can also be used for mapping regional vascular responsiveness to CO(2).     

Patients alter power of breathing as the primary response to changes in pressure support ventilation

IntroductionThe patient-ventilator relationship is dynamic as the patient's health fluctuates and the ventilator settings are modified. Spontaneously breathing patients respond to mechanical ventilation by changing their patterns of breathing. This study measured the physiologic response when pressure support (PS) settings were modified during mechanical ventilation.MethodsSubjects were instrumented with a non-invasive pressure, flow, and carbon dioxide airway sensor to estimate tidal volume, respiratory rate, minute ventilation, and end-tidal CO₂. Additionally, a catheter was used to measure esophageal pressure and estimate effort exerted during breathing. Respiratory function measurements were obtained while PS settings were adjusted 569 times between 5 and 25 cmH₂O.ResultsData was collected on 248 patients. The primary patient response to changes in PS was to adjusting effort (power of breathing) followed by adjusting tidal volume. Changes in respiratory rate were less definite while changes in minute ventilation and end-tidal CO₂ appeared unrelated to the change in PS.ConclusionThe data indicates that patients maintain a set minute ventilation by adjusting their breathing rate, volume, and power. The data indicates that the subjects regulate their Ve and PetCO₂ by adjusting power of breathing and breathing pattern.     

A carbon dioxide monitor that does not show the waveform has value

The author argues that a simple analog needle display can provide the anesthesiologist with the essential information he or she needs when monitoring carbon dioxide in the patient airway. He argues that essentially the most important information is virtually a binary, or all or none, phenomenon; in other words, carbon dioxide is either continuously present in the breathing circuit or is absent. Thus, circuit disconnects and undesirable endotracheal tube locations are readily identified. He relates the analog display of information to that of an automobile speedometer or the hands of a standard wrist watch. The author also compares analog meters with those used by pilots in aviation. He concludes with the argument that the carbon dioxide analyzer provides necessary information without the need to resort to expensive microprocessed displays that would include the waveform and trending, but would substantially increase the cost of the instrument.     

Oxygen uptake and mean blood pressure as indicators of induced hyperthermia

This dog study was designed to identify which of two measurements (oxygen consumption, mean blood pressure) tracked the onset of hyperthermia as reflected by rectal temperature. The animals were anesthetized, paralyzed, and mechanically ventilated. Hyperthermia was induced with 2,4-dinitrophenol (5 mg/kg) injected intravenously in 5 dogs. It was found that the best and earliest predictor of approaching hyperthermia was the increase in oxygen consumption, which increased 10% in 1.72 min. Mean blood pressure was an insensitive indicator of approaching hyperthermia. Rectal temperature, not surprisingly, was found to be a late and undependable early indicator of developing hyperthermia, requiring about 15 minutes to exhibit a definite increase. It is concluded that among these indicators, monitoring oxygen consumption (ml/min) is the most reliable way to identify a metabolic change such as incipient hyperthermia.     

The effects of passive humidifier dead space on respiratory variables in paralyzed and spontaneously breathing patients

BACKGROUND: Passive humidifiers have gained acceptance in the intensive care unit because of their low cost, simple operation, and elimination of condensate from the breathing circuit. However, the additional dead space of these devices may adversely affect respiratory function in certain patients. This study evaluates the effects of passive humidifier dead space on respiratory function. METHODS: Two groups of patients were studied. The first group consisted of patients recovering from acute lung injury and breathing spontaneously on pressure support ventilation. The second group consisted of patients who were receiving controlled mechanical ventilation and were chemically paralyzed following operative procedures. All patients used 3 humidification devices in random order for one hour each. The devices were a heated humidifier (HH), a hygroscopic heat and moisture exchanger (HHME) with a dead space of 28 mL, and a heat and moisture exchanger (HME) with a dead space of 90 mL. During each measurement period the following were recorded: tidal volume, minute volume, respiratory frequency, oxygen consumption, carbon dioxide production, ratio of dead space volume to tidal volume (VD/VT), and blood gases. In the second group, intrinsic positive end-expiratory pressure was also measured. RESULTS: Addition of either of the passive humidifiers was associated with increased VD/VT. In spontaneously breathing patients, VD/VT increased from 59 +/- 13 (HH) to 62 +/- 13 (HHME) to 68 +/- 11% (HME) (p < 0.05). In these patients, constant alveolar ventilation was maintained as a result of increased respiratory frequency, from 22.1 +/- 6.6 breaths/min (HH) to 24.5 +/- 6.9 breaths/min (HHME) to 27.7 +/- 7.4 breaths/min (HME) (p < 0.05), and increased minute volume, from 9.1 +/- 3.5 L/min (HH) to 9.9 +/- 3.6 L/min (HHME) to 11.7 +/- 4.2 L/min (HME) (p < 0.05). There were no changes in blood gases or carbon dioxide production. In the paralyzed patient group, VD/VT increased from 54 +/- 12% (HH) to 56 +/- 10% (HHME) to 59 +/- 11% (HME) (p < 0.05) and arterial partial pressure of carbon dioxide (PaCO2) increased from 43.2 +/- 8.5 mm Hg (HH) to 43.9 +/- 8.7 mm Hg (HHME) to 46.8 +/- 11 mm Hg (HME) (p < 0.05). There were no changes in respiratory frequency, tidal volume, minute volume, carbon dioxide production, or intrinsic positive end-expiratory pressure. DISCUSSION: These findings suggest that use of passive humidifiers with increased dead space is associated with increased VD/VT. In spontaneously breathing patients this is associated with an increase in respiratory rate and minute volume to maintain constant alveolar ventilation. In paralyzed patients this is associated with a small but statistically significant increase in PaCO2. CONCLUSION: Clinicians should be aware that each type of passive humidifier has inherent dead space characteristics. Passive humidifiers with high dead space may negatively impact the respiratory function of spontaneously breathing patients or carbon dioxide retention in paralyzed patients. When choosing a passive humidifier, the device with the smallest dead space, but which meets the desired moisture output requirements, should be selected.     

Concordance between transcutaneous and arterial measurements of carbon dioxide in an ED

Background

Transcutaneous carbon dioxide pressure (PtcCO₂) has been suggested as a noninvasive surrogate of arterial carbon dioxide pressure (PaCO₂). Our study evaluates the reliability of this method in spontaneously breathing patients in an emergency department.

Patients and methods

A prospective, observational study was performed in nonintubated dyspneic patients who required measurement of arterial blood gases. Simultaneously and blindly to the physicians in charge, PtcCO₂ was measured using a TOSCA 500 monitor (Radiometer, Villeurbanne, France). Agreement between PaCO₂ and PtcCO₂ was assessed using the Bland-Altman method.

Results

Forty-eight patients (mean age, 65 years) were included, and 50 measurements were done. Eleven (23%) had acute heart failure; 10 (21%), pneumonia; 7 (15%), acute asthma; and 7 (15%), exacerbation of chronic obstructive pulmonary disease. Median PaCO₂ was 42 mm Hg (range, 17-109). Mean difference between PaCO₂ and PtcCO₂ was 1 mm Hg with 95% limits of agreement of − 3.4 to + 5.6 mm Hg. All measurement differences were within 5 mm Hg, and 32 (64%) were within 2 mm Hg.

Conclusion

Transcutaneous carbon dioxide pressure accurately predicts PaCO₂ in spontaneously breathing patients.     

Evaluation of two commercially available carbon dioxide sampling nasal cannulae

Our study compared two commercially available carbon dioxide sampling nasal cannulae for efficacy of oxygenation and relationship of end-tidal carbon dioxide (Petco ₂) to arterial carbon dioxide (Paco₂). The two-prong nasal cannula (2PNC) has one prong dedicated to delivering O₂ via one naris and the second prong dedicated to sampling exhaled gases via the other naris. The four-prong nasal cannula (4PNC) delivers O₂ via a prong in each naris, and samples exhaled gases via another set of prongs in each naris. Forty six patients were divided into three groups, which received either 2 (n = 15), 3 (n = 16), or 4(n = 15) L/min O₂, respectively, and were studied sequentially with standard nasal cannula (SNC), the 2PNC, and then the 4PNC. At each O₂ flow rate, Pao₂ was equivalent regardless of whether the SNC, 2PNC, or 4PNC was used. Seventy-four percent (34/46) of the 2PNC and 0% (0/46) of the 4PNCPetco2 values were within ±4 torr of the Paco₂ value. The authors conclude that the 2PNC and 4PNC are equally effective compared with an SNC in oxygenating patients, but thePetco2 measured by the 2PNC provides a superior quantitative estimate of the Paco₂ than that obtained by the 4PNC.     

Measurement of end-tidal carbon dioxide concentration during cardiopulmonary resuscitation.

End-tidal carbon dioxide concentrations were measured prospectively in 12 cardiac arrest patients undergoing cardiopulmonary resuscitation (CPR) in an accident and emergency department. The end-tidal carbon dioxide (CO2) concentration decreased from a mean (+/- SD) of 4.55 +/- 0.88% 1 min after chest compression and ventilation was established, to values ranging from 2.29 +/- 0.84% at 2 min to 1.56 +/- 0.66% following 8 min of CPR. Spontaneous circulation was restored in five patients. This was accompanied by a rapid rise in end-tidal CO2 which peaked at 2 min (3.7 +/- 1.08%). Changes in end-tidal CO2 values were often the first indication of return of spontaneous cardiac output. There was a significant difference in the end-tidal CO2 in patients undergoing CPR before return of spontaneous circulation (2.63 +/- 0.32%) and patients who failed to develop spontaneous output (1.64 +/- 0.89%) (p < 0.001). We conclude that measurement of end-tidal CO2 concentration provides a simple and non-invasive method of measuring blood flow during CPR and can indicate return of spontaneous circulation.     

Ventilatory responses to exercise and to carbon dioxide in mitral stenosis before and after valvulotomy: causes of tachypnoea

1. The ventilation and cardiac frequency during progressive exercise and the respiratory responses to breathing carbon dioxide have been measured in 33 female patients with mitral stenosis and in 31 control subjects. Compared with the control subjects, the patients' exercise ventilation and cardiac frequency were increased; the exercise tidal volume at standard minute volume, the vital capacity and the ventilatory response to carbon dioxide were reduced. The extent to which the standardized tidal volume was lower during exercise than during breathing carbon dioxide was correlated with the severity of the stenosis, as gauged by the increase in exercise cardiac frequency above the level predicted from anthropometric measurements. 2. Twenty patients were studied postoperatively. In the 12 who showed clinical improvement the exercise ventilation and cardiac frequency were reduced and the exercise tidal volume at a given minute ventilation was increased. The latter change occurred despite a reduction in vital capacity, which was probably a residual effect of thoractomy. There was no significant change in the response to breathing carbon dioxide. No material change in function was observed in the patients whose condition was not improved by the operation. 3. It is suggested that in mitral stenosis the tachypnoea which occurs during exercise, whilst mainly a mechanical consequence of the reduced vital capacity, is also partly due to pulmonary congestion stimulating intrapulmonary receptors.     

Cardiac output and end-tidal carbon dioxide

Previous studies demonstrated selective increases in mixed venous carbon dioxide tension (PvCO2) during CPR in a porcine model of cardiac arrest. This was associated with a decrease in end-tidal carbon dioxide concentration (ETCO2), possibly due to a critical reduction in cardiac output and therefore pulmonary blood flow during CPR. We investigated the relationship between ETco2 and cardiac output before cardiac arrest and during CPR. Observations in 19 minipigs confirmed a high linear correlation between ETco2 and cardiac output. We conclude that the increase in Pvco2 and the concurrent decrease in ETco2 reflect a critical reduction in cardiac output, which reduces alveolar blood flow to the extent that carbon dioxide clearance by the lung fails to keep pace with systemic CO2 production.