Anatomic Dead Space Cannot Be Predicted by Body Weight

Lara M. Brewer, M.S. and Joseph A. Orr, Ph.D. Department of Anesthesiology, University of Utah Health Sciences Center

Abstract

Anatomic, airway, or tracheal, dead space is the part of the tidal volume that does not participate in gas exchange. Knowledge of the size of the dead space is important for proper mechanical ventilation, especially if small tidal volumes are used. Respiratory and medical textbooks state that anatomic dead space can be estimated from the patient’s body weight. Specifically, these references suggest dead space can be predicted using a relationship of one milliliter per pound of body weight. Using a volumetric capnography monitor that incorporates on-airway flow and CO 2 monitoring (NICO 2, Respironics, Wallingford CT), anatomic dead space can be automatically and directly measured using Fowler’s method in which dead space equals the exhaled volume up to the point when CO 2 rises above a threshold [4]. We retrospectively analyzed data collected in 58 (43 male, 15 female) patients to assess the accuracy of weight-based estimation of anatomic dead space. It appears that the average anatomic dead space roughly corresponds to the average body weight for the overall population; however, the poor correlation between individual patient weight and dead space contradicts the suggestion that dead space can be estimated from body weight.

Introduction inspiration, posture, position of the neck and jaw, drugs acting on the bronchiolar Anatomic dead space volume is the part musculature, tracheal intubation, of the tidal volume that remains in the tracheotomy, and tidal volume and conducting passages at the end of inspiration respiratory rate 4. and therefore does not participate in gas exchange. Upon expiration, the gas from the Many current text books 4-7 suggest a conducting passages has the same simple estimate of anatomic dead space composition as it did in inspiration; it is based on the patient’s body weight or commonly referred to as wasted ventilation. predicted body weight. Specifically, these Anatomic dead space is also called airway, references suggest anatomic dead space can tracheal or series dead space. Anatomic dead be approximated by one milliliter per pound space was first measured using a fast (or 2.2 ml per kg) of body weight. Because nitrogen analyzer by Fowler 1 in 1948. By this dead space estimation technique has 1952, DuBois 2 had described anatomic dead been so widely disseminated, many space measurement technique using a rapid clinicians apply the 1 lb = 1 ml rule in 3 CO 2 analyzer, and by 1954, Bartels had clinical practice. shown that several indicator gases including The observation that anatomic dead space oxygen and carbon dioxide all gave the in ml is roughly correlated with body weight same value for anatomic dead space and in lbs seems to have been first put forth by could therefore be used interchangeably. Radford 8 in 1955. In his article, Radford Anatomic dead space is not a fixed value described ventilation standards he had for each individual, as it is known to be developed to predict an individual’s required influenced by several factors, most notably: ventilation based on their body weight. He anesthesia, lung volume at the end of presented a summary of anatomic dead

1 space data from eleven patient groups patients in the operating room and ICU. obtained from several researchers that These patients were monitored using a included a total of 131 subjects aged volumetric CO 2 monitor that utilizes a newborn to 59.6 ± 6.3 years and having combination CO 2/flow sensor (NICO 2, mean body weights ranging from about 8 to Respironics, Wallingford CT). This monitor 170 pounds. Radford plotted the mean calculates anatomic dead space on a breath- values of dead space against the mean to-breath basis by analyzing the expiratory values of body weight for each group. He volume at which the CO 2 signal transitions observed a “remarkable, but approximate, from anatomic to alveolar CO 2 by rule that the respiratory dead space in implementing the method described by milliliters (BTPS) equals the body weight in Fowler 1. For each patient, the average pounds”. This approximation served anatomic dead space was measured using Radford’s needs well since he proposed tidal data collected during the first 10 minutes of volumes that were relative to any error in monitoring and compared to the values dead space estimation. predicted using five published prediction methods, which were based on patient body Contemporary ventilation protocols such weight, height, and ideal body weight. The as the ARDS network 9, which call for the difference, standard deviation of the use of smaller tidal volumes as part of a lung difference and correlation between the protection strategy for some patient measured and estimated values were populations, result in a larger percentage of calculated for each of the published each breath being wasted in the anatomic prediction methods. dead space volume. When weight-based estimates of anatomic dead space are For 21 patients, there was an elbow incorrect, assumed alveolar minute placed in the breathing circuit between the ventilation may be much different from endotracheal tube and the volumetric actual alveolar minute volume for patients capnometry sensor. For those patients, we ventilated with smaller tidal volumes and subtracted a volume of 6 ml from the higher respiratory rates. This leads to measured anatomic dead space to unintentional hyperventilation or compensate for the extra dead space added hypoventilation. The case of hypoventilation by the elbow. For all other patients, the could be made worse in breathing circuits endotracheal tube was connected directly to that include excessive apparatus dead the volumetric capnometry sensor and no space 10, 11 . compensations were required. Anatomic dead space can be directly The first, most common published measured using Fowler’s equal area method, anatomic dead space prediction equation is which is based on volumetric capnometry 1. cited in many general and respiratory We analyzed data collected using a physiology texts 4-7. This method simply respiratory profile monitor that includes states that anatomic dead space in ml is 8 volumetric CO 2 analysis to retrospectively equal to body weight in pounds, as Radford study how well estimated anatomic dead recognized. Alternatively, this can be stated space predicts measured anatomic dead as body weight in kg multiplied by 2.2 is space for a set of mechanically ventilated equal to anatomic dead space in ml. A patients. second method commonly in use 12 uses the ideal body weight (lbs) based on the

patient’s height to predict the anatomic dead Methods space (ml). A refinement 13 of the 1 lb = 1 ml We retrospectively analyzed data method states that estimated anatomic dead collected in 58 (43 male, 15 female) space should be decreased by 72 ml when tracheally intubated, mechanically ventilated patients are intubated to account for the extrathoracic volume bypassed by the

2 endotracheal tube. Others 13, 14 proposed Measured Anatomic Dead Space and Ideal Body Weight reducing the estimate of 1 lb = 1 ml by 50% to account for the volume bypassed by the 350 15 airway maintenance devices. The Suwa y = 0.3587x + 74.61 2 method is a similar but related approach that 300 R = 0.0578 estimates dead space (ml) as 2/3 of the patient weight (lbs). 250

200 Results The mean patient age was 63.2 ± 13.8 150 years (range 14-81 yrs.). The mean patient 100 body weight was 85.3 ± 19.1 kg (188 ± 42

lbs) (range 49.9 - 136.5 kg). The mean MeasuredAnatomic Dead Space (ml) 50 height was 172.9 ± 9.8 cm (range 149-198 cm), the mean predicted ideal body weight 0 was 67.6 kg (149 lbs) and the mean BSA 0 50 100 150 200 250 300 350 2 was 2.01 ± 0.26 m . Figures 1 and 2 Ideal Body Weight (Lbs) illustrate the correlation of measured anatomic dead space with body weight and Fig. 2: Regression analysis of measured ideal body weight anatomic dead space and ideal body weight.

Measured Anatomic Dead Space and Body Weight Table 1 reports the correlation, average 350 difference and standard deviation of the y = 0.0102x + 126.12 difference when comparing each of the 300 2 R = 0.0002 estimation methods described above to the

250 measured anatomic dead space.

200 Ave SD differ- differ- 150 Refer- ence ence ence 2 Method r (ml) (ml) 100 a 8 0.0002 59.9 53.9 MeasuredAnatomic Dead Space (ml) 50 b 12 0.058 20.9 35.9 0 a - 72 13 0 50 100 150 200 250 300 350 ml 0.0002 -12.1 53.9 Body Weight (Lbs) 1/2a 14 0.0002 -34.1 39.7 Fig 1: Correlation between measured 2/3a 15 0.0002 -2.7 43.8 anatomic dead space and body weight. Table 1: Results for each of the standard methods analyzed: method “a” (weight in pounds = anatomic dead space in milliliters) 8, method “b” (ideal weight in pounds = anatomic dead space in milliliters) 12 , method “a” – 72 ml 13 , 50% of “a” 14 , 66% of “a” 15 .

3 If the ideal body weight was used in each error bars indicate the standard deviation of of the last three equations instead of the his anatomic dead space predictions were actual body weight, the results would be similar to those we observed. Radford those reported in Table 2. emphasized that the rule of 1 ml dead space SD for every pound of body weight gives only a Ave differ- rough approximation of anatomic dead differ- ence space, as evidenced by the large standard Method r2 ence (ml) (ml) deviations of the data he presented. He warned that it is probably not justifiable to extend the dead space-to-body weight b - 72 ml 0.058 -51.1 35.9 relationship in patients weighting more than 200 pounds (91 kg). Radford also elected to 1/2b 0.058 -53.6 33.0 ignore the evidence that anatomic dead space increased with age for the purpose of his ventilation guidelines since it was a 2/3b 0.058 -28.7 33.6 small effect and was offset by a fall in VCO 2 with age. In fact, Radford did not advocate the use of a dead space estimate for anything Table 2: Results for each of the standard but a way to simplify the ventilation methods when ideal body weight is used guidelines he was proposing. It appears that rather than actual weight: method “b” – 72 the practice of estimating dead space from ml, 50% of “b”, 2/3 of “b”. body weight has become a matter of The ratios of mean anatomic dead space convenience, but it was not Radford’s to mean predicted dead space were 1:1.10 intended message. His proposed ventilation for “weight - 72”, Nunn’s classic method, guidelines, on the other hand, have stood the and 1:1.7 for “ideal body weight - 72”. The test of time and are still in wide use today as ratios that were the closest to 1:1 were from a starting point for setting automatic support the Suwa method: 1:1.02 (weight) and ventilation and weaning protocols 16, 17 . 1:1.29 (ideal body weight). Radford’s ventilation nomogram, which was based on body weight, sex and breathing frequency, required adjustment for Discussion changes in anatomic dead space associated The poor correlation in this data set with endotracheal intubation. He between patient weight and measured recommended a rough correction, which was anatomic dead space appears to contradict defined by subtracting a volume equal to the common practice of estimating anatomic one-half the body weight from the total tidal dead space from body weight. It appears the volume. He based this recommendation on average anatomic dead space in milliliters the observation that the volume of the oro- corresponds to the average body weight in nasal dead space and upper part of the pounds for the overall population since the trachea are approximately 50% of the total line of identity passes through the data anatomic dead space 18 . Clearly, the cluster. However, based on the variability of contemporary use of Radford’s 1:1 rule for the actual value observed in our data, there estimating anatomic dead space was not is no basis for estimating an individual intended by Radford to be used as an patient’s anatomic dead space volume from independent estimate of an intubated the body weight or ideal body weight. patient’s anatomic dead space. The 1 pound = 1 ml rule was first Precise knowledge of the anatomic dead proposed by Radford 8. In Radford’s original space becomes more important when a paper, he plotted anatomic dead space patient is ventilated using smaller tidal versus body weight in lbs. On his plot, the volumes as suggested by the ARDSnet 9

4 ventilation recommendations. The (V D/V T) is independently associated with percentage of each breath lost to anatomic mortality in ARDS patients 19 . Unfortunately, dead space ventilation increases as the tidal in their study, Nuckton and colleagues only volume decreases. As an example, consider reported the total pulmonary dead space, so the average patient weighing 85.3 kg in our it is not possible to reanalyze their results data set. With the ARDSnet tidal volume such that anatomic dead space and alveolar suggestion of 6 ml/kg, the tidal volume dead space are separated. In a subsequent would be set to 512 ml; since the average paper, Kallet et al 20 . found that the ARDS measured anatomic dead space is 128 ml, patients with lower V D/V T had better 25% of every breath is lost to dead space survival rates. They found that the ventilation. If tidal volume were set using a difference in V D/V T between survivors and rule of 10 ml/kg, only 15% of each breath non-survivors was about 0.1. A large portion would be lost to dead space; at 12 ml/kg, of total dead space is anatomic dead space. only 12.5% of the breath is wasted. Our data show that when the contribution of the variability in the anatomic dead space is In our average patient with an assumed ± ventilation of 6 ml/kg, the predicted alveolar considered, the V D/V T can change by 0.13 tidal volume (tidal volume – predicted based solely on patient-to-patient differences anatomic dead space) is 324 ml based on in anatomic dead space. This means that the body weight. The measured range of dead variability in anatomic dead space contributes to V /V measurements by a space volumes (mean ±2 standard D T similar magnitude as the difference observed deviations) for this patient pool was 60 to between survivors and non-survivors. It is 196 ml, which is a change in expected likely that the prognostic value of V /V alveolar volume of ±21%. The measured D T measurements is related to ventilation range of alveolar tidal volumes observed for perfusion mismatch and not to the percent of this group of patients is 316 to 452 ml, a - each breath lost in anatomic dead space. 3% to +40% change from the assumed However, if anatomic dead space variability alveolar tidal volume. These average is not considered, then the relationship numbers reveal that the effective ventilation between V /V and V/Q mismatch is delivered to patients on the ARDSnet D T weakened. Consider a patient with a low protocol can be greater or less than the V /V and an abnormally small anatomic expected value if the individual to individual D T dead space. Based on the V /V , this patient variation in anatomic dead space is not D T might be considered to have a favorable considered. prognosis when in fact serious V/Q The alveolar tidal volume predicted by mismatch problems are masked by a small ideal body weight (363 ml) would lead to an anatomic dead space. The solution, as erroneous estimate of alveolar minute proposed by Moppett 21 , is to calculate the ventilation of between -13% and +25% ratio of alveolar dead space to alveolar tidal compared to the assumed value. Even the volume rather than the total V D/V T. That is, more complicated (and less common) one should measure the anatomic dead method of body weight minus 72 ml gives space, then subtract the anatomic dead space poor estimation of actual alveolar from both the total dead space and the tidal ventilation: -20% to 14%. Given these data, volume before calculating the ratio. The direct measurement of an individual’s resulting V D/V T would be a ratio of alveolar anatomic dead space appears to be the only dead space to alveolar tidal volume. reliable method of assessing true dead space Moppett et al. speculated that the association and therefore true alveolar ventilation. Nuckton and Kallet observed between dead space ratio and mortality was likely due to Quantification of physiologic dead space disturbed VQ matching, and that the is clinically important. Nuckton observed alveolar dead space ratio would be even that an increased dead space fraction more strongly associated with mortality.

5 Drummond 22 pointed out that right-left effect of PEEP on anatomic dead space and shunting (intra-pulmonary or intra-cardiac) found a strong correlation between increased affects the total dead space measurement, PEEP and increased measured anatomic but not the anatomic dead space dead space 27 . measurement. The idea of measuring anatomic dead space in order to estimate the uniformity of alveolar ventilation goes back Conclusion 23-25 to 1944 . Anatomic dead space volume All these issues point to the need to use was also used to evaluate alveolar direct measurements of anatomic dead space ventilation-perfusion relationships in 26 rather than estimation for proper mechanical patients with pulmonary disease in 1949 . ventilation. The errors associated with We suggest the use of direct anatomic dead estimations were less significant when larger space measurement in future studies in order tidal volumes were used; however, when to develop better descriptions of the changes smaller tidal volumes are used, the that occur in the alveolar dead space with percentage of each breath lost to anatomic lung injury. dead space ventilation becomes greater. It is important to ensure patients receive With volumetric capnography, it is simple to adequate tidal volume to overcome the directly measure anatomic dead space under apparatus dead space 10-11 . Apparatus dead every condition and use its measure to space affects both the alveolar tidal volume inform treatment. and V D/V T, and Nuckton and Kallet ensured their V D/V T analyses were carried out using minimal apparatus dead space. Correct References assessment of the effect of all series dead space (anatomic and apparatus) requires a 1. Fowler WS. Lung Function Studies II: The calculation of the apparatus dead space and Respiratory Dead Space. Am J Physiol 1948: addition of this volume to an estimate of 154:405. anatomic dead space. Direct measurement 2. DuBois A B, Fowler RC, Soffer A and Fenn using volumetric capnography should WO. Alveolar CO2 Measured by Expiration into combine both anatomic and apparatus dead the Rapid Infrared Gas Analyzer. J Appl Physiol volume into a single volume. 1952:4:526-534. As stated previously, the anatomic dead 3. Bartels J, Severinghaus W, Forster RE, space is known to change with the size of Briscoe WA and Bates DV. The Respiratory the tidal volume. We made no effort in this Dead Space Measured by Single Breath Analysis analysis to control for the tidal volume of Oxygen, Carbon Dioxide, Nitrogen or effect. In fact, tidal volumes can be Helium. J Clin Invest 1954:33(1):41–48. generally assumed to vary widely from 4. Nunn’s Applied Respiratory Physiology, 6th patient to patient, so it is not reasonable to Edition. Elsevier: Philadelphia, PA; 2005. pp. assume a specific tidal volume to anatomic 118-120. dead space relationship. A need for 5. Respiratory Physiology – the Essentials, 2nd assumptions about this relationship points Ed. Lippincott Williams and Wilkins: Baltimore, out another significant drawback of using MD; 1979. p 19. weight-based estimates of dead space rather than the actual measured value. Since the 6. Hlastala MP and Berger AJ. Physiology of Respiration. Oxford University Press:England; conducting airways are somewhat compliant 1996. pp. 73-74. the anatomic dead volume can be expected to change with time in a single individual, 7. Ganong WF. Review of Medical Physiology, especially in the presence of changed 19th Ed. Appleton & Lange: Stamford, CT; ventilator settings, inhaled anesthetics, 1999. change in posture 4 and PEEP. We tested the

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